1947 Panel Truck Frame swap with ’88 k2500
 

This is my 1947 chevy panel truck. I plan on making it a reliable daily driver.
After a couple years of searching I found this 1 ton panel truck. It was listed in the greensheet for $800.
I responded to the ad and the seller told me it was in a storage yard. I rushed down there to have a look, the yard was locked, so I went through some woods to the side and saw the truck. I knew it was right when I saw it. In my excitement and haste I tripped and fell on some mesquite, jabbed a thorn into my palm.
I arrange a time to meet and showed up with a AAA towtruck and my weeks pay in my pocket, $200. Mike was sad to let it go, but he saw my enthusiasm. He had big plans and also had a 350 torn apart. He wanted to go through everything and build it right. He hadn’t done much in the 10 years he owned it. Some boyscouts had recently broken the windows, so he decided to it was time to sell. I handed him the $200 and said I would pay the rest each week with every dollar I earned. He just said to go ahead and don’t worry about it. I had it towed to the driveway of the house I was renting where I replaced the windshield and door windows. Later I had it towed to a storage space where I rebuilt the brake cylinders.
After two years, I went to a junk yard and asked if they had an engine for me. Recently a ’58 DelRey with a pickup camper mounted to the back half had arrived. This was a dream machine that a mechanic had built to live in during his battle with cancer. I purchased the 235 and had them remove it and deliver it to a mechanic. At the time I was living in a ’58 Viking bus RV in a trailer park. There was a mechanic next door that agreed to take on my project. They set up the engine and told me to get “the Beast” out of their yard. I stored it for another year at the neighbors. I found another mechanic to continue. I went back to the junkyard and found a dumptruck with an SM420 transmission. I had to go back several times to wrestle that out. The mechanic was making slow progress. I had gotten my truck stuck in the mud, so I went to the mechanic and told them my panel truck was ready. I started driving it to work until I got my truck unstuck.
I had to get it legal, so I had a patrolman come out and verify the numbers. They took the number off the engine, I wasn’t there at the time but my neighbor in the trailer park said he had trouble finding the number. Titled and insured I worked on it now and then and drove it occasionally. Off to college I would drive between Waco and Austin for the weekends. My F100 threw a rod, so I started making the 2 hour trip in the panel truck. I could go 60 mph floored. It was loud, shakey, and walked a couple feet when hitting a bump. There was an old F250 with 3.55 gears behind my girlfriends place in the woods. I had the panel truck jacked up on stumps in the mud swapping that in. Pulled some U-bolts from a junk yard and made an offset by drilling a couple holes in metal plate. I didn’t like it. Yes it was faster, but behind the 235/SM420, I had to start in the compound low every time unless going downhill. If I rolled up to a light that just changed from red to green, the engine would struggle in 2nd gear, or I would have to stop and shift to 1st. I had float shifted in the f100 on the highway, so I knew it could be done. Might be o.k. on a country road, but I needed consistent drivability in the busy city. That and one axle was bent.
I found a rear end from an 86 suburban with 3.73 gears. Now I could go 80 mph floored, loud shakey and walking a foot or two when hitting a bump. It would also make a sound at higher speeds, maybe the angles of the driveshaft. I would tell passengers it’s balking. Mostly a weekender, driving around town or short trips vehicle. One day there was a wretched sound from the engine compartment, a loud rattle. I pulled to the side of the road and shut it off. When I started it again, the valve that had snapped, wedged between the head and cylinder, engine siezed. I had another 235 installed.
I trailered it to Seattle and was commuting across town for a few years. I had the knee action shocks replaced with cylider shocks, and also added shocks on the rear as well as get the rear end re-installed correctly. The shocks helped eliminate the walking sideways when hitting a bump at high speed. I decided to get the numbers on the title squared away since the vin was from the blown engine I left in Texas. Washington had issued me a title when I surrendered the Texas title. I got the appropriate documents and drove to the state patrol where they looked at the ID plate and agreed to issue a title based on the number on the ID plate.
I really had to stand on the brake pedal when stopped on a hill, no fun. It mostly just sits. I start it every month and drive around a little. I want this to be my main vehicle. I could drive it around running errands, but after it gets hot, it is difficult to start. I have to not touch the gas pedal, and maybe it will start, or I’ll have to wait awhile. I need to be able to get in and go! A few years ago I picked up a 1988 GMC k2500 to use as a donor. Recently I found a shop to take on this project. I have been looking at options and haven’t found anything close to home. I went by one custom shop and they said there is a two year backlog. They also said they don’t know how to NOT take everything apart, blast everything, paint everything, and put it all back together. I just happened to stumble across a compound with several shops and walked into the body shop. At first they thought I was crazy and said they do $10,000 paint jobs, but as we talked, he said they don’t usually do that kind of work, but it would be fun. It’s a shop with a floor and tools and resources, I have none of that so this may be the place.
TLDR: Frame swap ’47 panel truck with ’88 k2500. I’ve had the panel truck for over 20 years and daily driver for 10.

Re: 1947 Panel Truck Frame swap with ’88 k2500
 

I’ll farm out the heavy lifting and work on what I can. Before I get too far will it work? Here is what I have mocked up.

Re: 1947 Panel Truck Frame swap with ’88 k2500
 

thats gonna be a ambitious project...as you can tell from your drawing ,the panel will have to sit really high above the frame...unless you cut the floor out...also the width will be a issue....k20 is considerably wider than a AD..

Re: 1947 Panel Truck Frame swap with ’88 k2500
 

Cool background story, cool truck. You're not the guy who bought my 1-ton panel rear bumper and brackets, are you?

Your chassis swap sounds like one of those projects that will never be finished. Better idea: completely rebuild the original front and rear suspension, disc brakes up front, all new drums in the rear. New carburetor and ignition system. Overdrive transmission to run at modern freeway speeds. Electric wipers, interior sound deadening, new window channels and weatherstrip all around. It will be like a new vehicle.

Re: 1947 Panel Truck Frame swap with ’88 k2500
 

“thats gonna be a ambitious project...” - Mongo...aka Greg
Agreed. Since I have both trucks, I took a tape measure to them.

“Cool background story, cool truck. You're not the guy who bought my 1-ton panel rear bumper and brackets, are you? “ -MiraclePieCo
Thanks! This truck was near complete when I got it, minus engine and transmission. I did install an electric wiper, but it is just as slow as the vacuum was. I am able to use the later wiper arms with springs to hold the blades on the glass, but it’s not fast enough for a torrentioal downpour.

The wheelbase is the first thing I considered when determining a donor frame. The 1947.2 wheelbase is 137". The 1988 K2500 single cab long bed wheelbase is 131.5".
The frame width is going to have to work. ’47 1 ton is about 35", the ’88 K2500 is 45". There needs to be 5" of clearance outside of the ’47 frame for this to work.

Re: 1947 Panel Truck Frame swap with ’88 k2500
 

There is about 10" between the frame and running board.

Re: 1947 Panel Truck Frame swap with ’88 k2500
 

The ’88 frame drops 8" under the cab. The ’47 battery tray will need to be trimmed. I’ll replace the battery tray with a box for storage or electronics.

Re: 1947 Panel Truck Frame swap with ’88 k2500
 

I was concerned with ride height, so I measured from the top of the frame to the bottom of the springs. The ’47 is 10", the ’88 is 13". If I place the ’47 body on the ’88 frame, the relative position of the wheel to the wheel well will be 3" greater than the stock ’47.

Re: 1947 Panel Truck Frame swap with ’88 k2500
 

The ’47 has 2" wood mounting blocks. There are rails across the bed underside. The bed can be mounted closer to the frame by removing the wood blocks and modifying the rails. It looks like I can set the ’47 body on the ’88 frame and maintain the stock ’47 stance.

Re: 1947 Panel Truck Frame swap with ’88 k2500
 

I have LT245/75R16 tires on the rear currently. Those are about 30" tall. If I maintain this stance, the space between the top of the rear tire and the fender will be 4". The fender opening is 31" so I won’t run a taller tire without modification.

Re: 1947 Panel Truck Frame swap with ’88 k2500
 

This rear end is from an ’86 suburban. I chose rims for clearance of the tie rod ends, so 3.5" back spacing. There is some room to move the wheels in with greater back spacing. I’ll get exact measurements for that. I plan on keeping the 8 lug rear axle, and modifying the front of the donor rig to 8 lug.

Re: 1947 Panel Truck Frame swap with ’88 k2500
 

The rear tires are 74" wide and the front tires are 63" wide on the ’47.

Re: 1947 Panel Truck Frame swap with ’88 k2500
 

The ’88 tires are 71". This will move the tires out 3.5" on the ’47. With an adjustment to the wheel back spacing, the tires should fit inside the fender.

Re: 1947 Panel Truck Frame swap with ’88 k2500
 

Looks like your doing your homework...

Re: 1947 Panel Truck Frame swap with ’88 k2500
 

I know you already have the one but an X cab frame is slightly longer. About 141.5" or so. I started this with my son's truck once upon a time but he decided to stay straight axle. There is a seam under the 88's doors that takes the front cradle from boxed to the rear C channel. You can grind the welds out and cut the frame to length in 10 minutes. It is the same as doing an S10 swap using an X cab S10 frame. The 1 ton wheelbase is actually about 1.5" shorter than it should be so adding that back in you'd be about 140" for a rear centering wheel base.

There are several guys that have done this swap with pickups and they don't come out too bad. Late model HD rims look pretty sweet on these trucks, especially if powdercoated in a gray/silver color rather than the polished finish. 2008+ centercaps on them don't look so bad with them. I'll get you some photos from his truck as it sits.

If you are really concerned about width, you can swap a 1.5+ ton nose on and look at widening rear fenders. The one thing I would caution is make sure the damn rear wheels will come off with the fenders on. We have a set of rims with a deeper offset but couldn't get them onto the stock axle with spacers. We will swap to an 88+ 3.73 rear end here this summer but had to go back to the HDs because we just couldn't fit them onto the rear without dropping an axle because the fender hits!

Good luck. I'll keep an eye on this one. I've had 2 of those 3800s run through here and I dig them. 2 3800 pickups as well. I walked past Eli's 49 panel last night and thought "my suburban looks like a baby".

Re: 1947 Panel Truck Frame swap with ’88 k2500
 

Thanks for the feedback. I'm not too concerned with the wheels sticking out of the wheel wells, just addressing the issue beforehand. I'm kind of dragging my feet at this point, waiting until fall to start. I'm going through the different systems, planning how each will work. I'll use the stock gauges, so a bit of design work there. I figured out how to keep the stock ignition switch and foot starter. I'll look at everything else on the wiring diagram, I'd like to eliminate the bulkhead connector if I can. I'll look into building a drive for the speedometer too.

Re: 1947 Panel Truck Frame swap with ’88 k2500
 

I think it's Bob's Speedometer in Michigan that can do the electric to mechanical speedo conversion. I've talked to them a bit but not gone any further than that. It's around $250 IIRC but something I don't know if I could make on my own without some study!

Re: 1947 Panel Truck Frame swap with ’88 k2500
 

I just looked on Bob’s website, they show the Cable-X, Cable-X site lists it at $340. Dakota Digital is $285, Speedhut is $360. There may be others, this is just from a quick search this morning. I’ll look into reviews of what’s available.
I read a post about converting a ’89 Blazer transfer case mechanical to ’90-’91 electrical speedo, so it may be possible to go the other way. I also read that Dodge used a mechanical speedo on the NV4500 at some point. I have a 5LM60 now, and I understand that is identical to the NV3500 on the outside. If I can get it set up with a mechanical speedo drive, I’ll go that route. I’ll still need the electronic signal. If I have to remove the tone ring, I may be able to get an in-line VSS. Stealth Conversions made one for the 4-pulse system, there are bound to be others. This is partly speculation, I’ll take my time and create a detailed document.
Dan, how about a pic of your son’s truck?
My k2500 is the 6-lug. Does the 8-lug k2500 have a wider tread? I read where someone swapped the 6-lug to 8-lug spindle and modified the upper A-arm, that’s my plan. The diffs are 3.73, the ’88 8-lug rear in the ’47 is also 3.73. I was concerned about how the rear works with the RWAL (Rear Wheel Anti-Lock) system. I don’t see any wires to the rear axle. The schematic show the VSS supplies the signal for that system, so it should be good to swap the rear.

Re: 1947 Panel Truck Frame swap with ’88 k2500
 

I intend to keep the original key switch in the ’47, and modify the foot button to work with the ’88 system. My list of objectives:
1. Retain the original ignition key switch and foot starter.
2. Adapt donor circuit to function from original key and footbutton
3. Modify foot button bracket to actuate momentary switch, relocate switch-originally on starter.
Is there room between the lower firewall and the rear of the engine?
First I’ll identify how the donor key switch functions.

From the manual:
STEERING COLUMN HARNESS CONNECTIONS
The steering column harness branches from the instrument panel harness and then to the turn signal or multifunction lever switch connector, the windshield wiper connector, ignition switch connector, the brake pedal ' 5 switch, connector, the dimmer switch connector, the , back-up switch connector, the position sensor connector ’ and the horn switch. Refer to figure 24 and to ACCESSORIES (SEC. 9) for the multi-function lever switch.

This image shows the Ignition switch connector, 232.

Re: 1947 Panel Truck Frame swap with ’88 k2500
 

From the manual:

Re: 1947 Panel Truck Frame swap with ’88 k2500
 

There is also a switch in the lock cylinder that triggers the buzzer if the door opens with the key still in. I won’t have that.


Here is a diagram of the ’88 ignition switch.

Re: 1947 Panel Truck Frame swap with ’88 k2500
 

I’ll create a spreadsheet, a truth table of how the switch works based on this diagram and the schematics.
This shows the connector and pins 2, 3, 6/5.

Re: 1947 Panel Truck Frame swap with ’88 k2500
 

This includes pin 4.

Re: 1947 Panel Truck Frame swap with ’88 k2500
 

And this identifies pin 33.

Re: 1947 Panel Truck Frame swap with ’88 k2500
 

How does this relate to my truck? Heres the connector:

Re: 1947 Panel Truck Frame swap with ’88 k2500
 

The only discrepancy I see is my truck uses pin 36 instead of 33, but they appear to have the same function.
Here is the truth table:

Re: 1947 Panel Truck Frame swap with ’88 k2500
 

From this I came up with this diagram.
I think it will work.

Re: 1947 Panel Truck Frame swap with ’88 k2500
 

My local guy swapped his completely over to 8 lug with little or no effort. Swapped spindles and the axle. I don't think there is a significant width difference. Anti-lock I think just was a speed sensor in trans for 88. Nothing high tech there and you can by-pass it by ripping out the ABS box and running brakes direct. Those things are probably failed by now anyway!

I'll get pics of Eli's truck soon. He's tearing stuff apart again working on some body work right now. It's been up and down over the last 10 years. A giant wheelbarrow full of storage for at least 6. The nice thing was getting a set of 93-94 extended cab bucket seats. They sit almost perfect on the riser and give you a flip forward/slide forward option on the passenger seat for rear access if you need. No major mounting issues, either because the frames are about square and flat unlike about every other front seat you run into!

Re: 1947 Panel Truck Frame swap with ’88 k2500
 

I have been reading build threads here and found a few full size frame swaps very inspirational.
I signed a contract and put a deposit down at the shop, now I’m waiting for space to open up so I can drive the trucks there. Once the build starts, I’ll need parts. I don’t know what all I will need, but some of it is obvious.
I created a spreadsheet of available parts. I started at one on-line parts catalog, and looked through each category. I picked what I think I will need and put it on the spreadsheet. I looked at a few sites, and was able to compare prices. I placed an order today for some parts with one company that is having a holiday sale.
I realize some things won’t be needed for awhile, like window gaskets, but I got some anyway. This was a small order, I may be more cautious in the future. Some parts are common to the suppliers, other parts seem to be at one or the other. It’s random. I am also looking through discussion threads about what parts to avoid, or who sells the best parts. This helped me choose the right outer rubber window seals.
There is a sale this holiday weekend at the wrecking yard. I’ll look for seats, a steering column, spindles etc. I’m committed to this build now. At some point I’ll have boxes of parts. I don’t intend to have boxes of parts waiting for years. I plan on going full speed ahead!

Re: 1947 Panel Truck Frame swap with ’88 k2500
 

I received one box of parts minus a few on back order. The back ordered parts arrived this week, but one part was off by a couple digits so I’ll have to wait until they get the right part in stock before I can return this. I spent some time in the wrecking yard, scored a couple front seats out of a Astro van for $40. I tried to get the front spindles off a k2500, but got burned out after two hours. I didn’t have the right tools, my time was running out, but what I mostly needed was a cheeseburger. On the way out I saw a barebones steering column in a jeep. I’ll try to get back there after work soon. Yesterday I ordered some bullet tail lights.
Two weeks ago I drove my panel truck for the last time, as it is, to the resto shop. This week I drove the ’88 GMC there, the panel truck was being dismantled and they hit me with a healthy bill. Today I got a call that the GMC bed has been removed and most of the panel truck parts are ready to get blasted. I thought I could recoup a little by hauling the extra parts off myself, but I don’t have the time, don’t want to pay to store them, so it’s off to the scrappers with the ’88 GMC bed.
I’m still trying to decide on the color. I had wanted a very dark purple that looks black, when I started driving the panel truck over 20 years ago. I still think that would be cool, but something subdued might be better for a daily driver. I’m considering two-tone grey, or grey and midnight purple. The resto guy said he likes to use the vintage colors on builds like this, but that it’s my decision. I have been looking at the vintage colors, but in 1947 the Forester Green was the standard. There were a handful of options, and two-tone wasn’t available until ’54 then it was just white for the top half. I recently went on a road trip a saw a couple vehicles with a light blue on the lower half and cream on top, that I like. So now I’ll think about the two-tone grey, and the blue and cream, and I have some model paint to play around with a dark purple.
My parts spreadsheet is growing and I need to order all the hard parts before any paint. I’ll add windshield washer nozzles for example, and whatever bits and pieces I can think of. I also need to continue engineering the ’88 wiring to the ’47 controls. I just read that the brake light switch has multiple contacts, so that will all have to function one way or another. Moving right along!

Re: 1947 Panel Truck Frame swap with ’88 k2500
 

One of the stipulations of the contract is that we meet once a week to go over progress, keep motivated, and settle up the bill. The panel truck body is almost free, and the donor truck is being picked apart.

Re: 1947 Panel Truck Frame swap with ’88 k2500
 

This is an exciting build. I really dig those 1-ton panels. I'll be watching!

Re: 1947 Panel Truck Frame swap with ’88 k2500
 

Each week I visit the shop, a little progress has been made. Last week the body was on a cart, and the two frames were side by side. Comparing body mounting points and deciding how to modify the donor frame. The ’47 gas filler is on the passenger side. 1988 the filler is on the driver side, tank is inside the frame rail. Pre ’88 the tanks were outside the frame and an auxilliary tank could be added to the passenger side. That doesn’t appear to be an option here, the ’88 auxiliary tank was behind the axle and I want the spare tire there. It could be done, re-route the exhaust to the driver side and find a tank with the passenger side filler. There is the driver side transfer case, so exhaust would go below that. I may go with moving the filler to the driver side on the panel. Here is a picture from a couple weeks ago…

Re: 1947 Panel Truck Frame swap with ’88 k2500
 

Little progress has been made since last week, but a little progress each week will get the job done. Fuel tank removed and exhaust cut. I have been researching how to use the stock instrument cluster and speedometer. I took the instrument cluster to hook it up to my ’90 chevy so I’ll have that working when this project is ready. I may be able to replace the 40 tooth reluctor in the transfer case tailhousing with a mechanical speedometer drive, and use a mechanical to electric pass through adapter for the DRAC. I’ll either build my own DRAC, or get one from a ’91-’94 truck. The ’88 DRAC is integrated in the display pcb…

Re: 1947 Panel Truck Frame swap with ’88 k2500
 

I really like what you've got going. Like others have already mentioned, due to the newer frame configuration, you will either need to raise the body mounts or raise the floor of the truck to recess the frame in to it. Personally, I would build the body mounts up to work with the stock cab floor. That is what I did. I can't stand a truck with a bed that is only 6" deep. I use my bed and don't like the look of the "non-functional" bed, but that is just personal preference.

The other issue will be track width. Your tires will stick out past the fenders to some extent. Some of this can be minimized with wheel offset, but unless you go to lengths to widen the body or flare the fenders, the tires will stick out. With tall/skinnys on stock offset wheels, mine stick out about 1.5" per side. Again, this is personal preference.

Getting the wheelbase to fit the body is the biggest part of the battle and it looks like you have a good start on that. You may need to extend the front and rear of the frame for your body mounts, but that can be done with some 10 gauge steel.

Since you have the original frame, you can replicate the profile of the frame rails using square tube laid across the new frame and then build your body mounts from there. Again, this was how I chose to do it.

I think this will be a great daily driver when you are done. Unique, reliable and easy to work on and enjoy. None of that is wrong. I'm looking forward to see where this goes.

The red truck is a local. I don't know what specific chassis he used but it is 1973-1987 GM 4x4 K10.

-Joe

Re: 1947 Panel Truck Frame swap with ’88 k2500
 

Thanks Joe, I feel the same about the functionality of the bed. The panel truck bed will be as close to the frame as possible. The cab floor is lower than the bed and the donor frame is lower under the cab, so it should work good. It looks like the cab floor will be near the level of the top of the transmission, so no hump needed. I dig that red truck, I don’t think mine will be that high, one of the reasons I will not use a solid front axle. Not much progress this week, but the plan is to remove the rear end and build a body mount template out of 1” L-stock. You may notice in the picture in my previous post, the frame appears to be higher in the rear driver side, we’ll check that at the alignment shop. We’ll use the brackets and frame ends from the ’47 frame. The original frame had extensions on the rear.

Re: 1947 Panel Truck Frame swap with ’88 k2500
 

Looking good so far...I like the one ton panel, a little longer then the 1/2 ton that I have. Keep up the good work, you will figure out the wires.....

Re: 1947 Panel Truck Frame swap with ’88 k2500
 

Thanks olcarguy. The frame bracket template is set in place, it needs a few tweaks to clear the front end, and one of the frame cross member will be replaced. The torsion bars are relaxed and the rear or the frame is now even, so the tension was causing the frame to twist. We will beef up the frame. The engine /transmission has been set aside to gain access to the frame work.

Re: 1947 Panel Truck Frame swap with ’88 k2500
 

Modifications have been done to the frame bracket template. It now clears the front end and the Z-axis points have been determined. I want the body to ride at or near stock height, so the 2” wood blocks won’t be used. The 2” hat channel cross sills will be replaced with 1x2 tubing layed flat. This will put the bed as close as possible to the frame. All other mounting points are lowered about 3” relative to the bed mounts. Now all the brackets can be tacked in place, and the body will be test fit.

Re: 1947 Panel Truck Frame swap with ’88 k2500
 

Progress is going slow this time of year, but little by little we’re moving right along. The ends of the ’47 frame are cut and being grafted to the donor frame. The rear frame extension is offset lower for two reasons. The body will be closer to the frame so the rear bumper will be lower relative to the frame, and there is a rear cross sill attached to the body that mounts on top of the frame. The front frame ends are narrower on the ‘47, we’ll move those out to match the donor and modify the bumper bracket. The bed wood is out and is in great condition, so I’ll probably keep it.
I have been researching how to run the original speedometer with the electronic system. I have a couple options. I could replace the tailhousing of the transfer case to use a speedometer cable. I would then use a mechanical to electric pass through adapter to get either 2000 pulse per mile, or 4000 pulse per mile. I would need to build a DRAC that works opposite of the original and get the 128ppm for the anti-lock, and 2kppm for ECM or 4kppm for cruise, but I don’t think I have cruise control, so 4kppm isn’t needed. The other option is to keep the system stock, DRAC is integral with the gauge cluster, so I’ll get a DRAC module from a ’92 to ‘95 and get the 2kppm signal to run a 12v motor to drive the mechanical speedometer, sure I could buy an electric to mechanical speedo drive, where’s the fun in that? I have found 3 different off the shelf solutions in the $300 range.
The mechanical speedometer can be adjusted to work with the rear gear ratio and the tire size. There are a few driven gear choices available for the np241, actually the 700R4 driven gear is used. There are 12 options with a range between 15 and 45 teeth. I may make a chart to compare driven gear, axle ratio, and tire size, unless anybody knows if one exists? If this isn’t right on and the ECM signal is being generated from it, there could be engine performance issues.
The electric speedometer is adjusted in the DRAC(digital ratio adapter controller). The tailshaft has a reluctor ring that generates 40 pulses per driveshaft revolution. The DRAC converts the 40 pulses to 128,000 pulses per mile, that’s 1:3,200 seems like a small margin for error. Before the 128k is generated, there is a circuit to set the diff/tire ratio. I’ll have to look at the charts on that. The 128k is divided by 32 to get 4kppm, and again ÷2 to get the 2kppm for the ECM. This just seems like a more precise way to generate the signal for the ECM, but I don’t know if it really matters or not how acurate the actual speed is compared to what the computer thinks?
The off the shelf electric to mechanical speedometer adapters have a way to calibrate the speedometer. This may be a redundant function, as it should be done in the DRAC. If the speed is not adjusted in the DRAC, the ECM may be getting the wrong information.
I took the motor out of a cordless drill, had some random transistors on hand and built a darlington pair for the supply. I have been playing around with motor controllers for awhile and have an arduino set up already, so I used that to for the speed control. The motor can run somewhat slow, but not slow enough. It works out to be at 2MPH the speedometer cable spins at 33.3rpm, and this motor looks like it is spinning faster than a record player. I’m looking for gears, or a different motor. Once I get the speed dialed in, I’ll work on the code to convert 2kppm to 1krpm.