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Old 02-25-2021, 02:44 PM   #1
Second Series
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Keep the original speedometer.

Has anyone here made a solution to keep their original speedometer when changing to a modern transmission?
I wanted to keep my original speedometer so I looked at several solutions. Iíll be using a 5LM60 transmission with NP241 transfer case so the speedo signal is not cable driven. I could have the transfer case modified to accept a cable drive. I could get a speedometer drive module, a couple companies make them. I could modify my speedometer with guts from a modern speedo, stepper motor. I decided to DIY a solution. I had read about mounting an electric motor to the back of the speedometer, but how to do that. I thought it was a dumb idea to put the motor directly on the speedo, remotely mounted with a cable is a better idea. I read some more and found few examples. I finally decided to try mounting a motor directly to the speedo. I have been working on this project for a year and thought I would share my test run here. Here is a link to a video I made: https://youtu.be/K6YOm1WTpqg
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Old 02-25-2021, 08:09 PM   #2
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Re: Keep the original speedometer.

ive got an m21 trans so this doesnt really apply. i would think that a speedo drive module that would change your electrical signal to a cable drive would be the easiest route.
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Old 02-25-2021, 08:22 PM   #3
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Re: Keep the original speedometer.

At the price of the aftermarket units I can see why you are going to the effort.

That said, it is way beyond my skill level.

I'd just have to dig Broomhilda (the bossy gps) out and plug her in and go as I did on my 77 that the speedometer doesn't work on. I've driven a lot of miles in different rigs with that thing as my speedometer.
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Old 02-25-2021, 10:59 PM   #4
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Re: Keep the original speedometer.

If the speed/odometer is off, contact your local speedometer shop to see if they sell a sending unit that screws into the gearbox where the speedometer cable connects to, which has modified gears to meet the needs. You then connect the speedometer cable to the sending unit. My local shop just asks me what speed is my speedo reading when driving at say 50 mph? They tweaked the sending unit accordingly and now mine reads perfectly.

This was necessary when I changed the rearend gears to 3.55.

I assume you have a speedometer ap? My favorite is Speedbox. Very simple and extremely accurate.

Cheers
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Old 02-26-2021, 02:59 AM   #5
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Re: Keep the original speedometer.

Quote:
Originally Posted by Second Series View Post
Has anyone here made a solution to keep their original speedometer when changing to a modern transmission?
I finally decided to try mounting a motor directly to the speedo. I have been working on this project for a year and thought I would share my test run here. Here is a link to a video I made: https://youtu.be/K6YOm1WTpqg
Looks like you got it pretty close. Is this your final version or just a trial run? Is there adequate room behind the dash?

BTW, my favorite phone speedo app is DigiHUD
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Old 02-26-2021, 02:20 PM   #6
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Re: Keep the original speedometer.

I’m still tweaking the code, but if I did nothing more, I would be happy with it the way it is. I’m still learning how to adjust the PID code. I set the starting speed with Bias, and the top speed is set with kp. The sweep in non-linear so I use an equation based on the motor torque curve. There are a couple more values in the PID code to adjust reaction time and amount.The needle is pretty steady, but it does jump more than I would like when it moves. It also looked to be reading a little low at some points, but I’m not sure how acurate the ’90 speedometer is. I do need to run a speedometer app the next time I do a road test, thanks for the suggestion.
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Old 02-26-2021, 10:06 PM   #7
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Re: Keep the original speedometer.

NP241J (Jeep) uses speedometer drive gears and cable. THM400 output shaft is same size and number of splines as NP241 output shaft. I would start by checking into using parts from one of these applications.
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Old 03-01-2021, 03:39 PM   #8
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Re: Keep the original speedometer.

Thanks for the suggestions 1project2many! I found a post where someone changed their í89 Blazer from cable to electric, so I think the í89 Blazer speedo housing might also work, although I donít know if that is an NP241? I really wanted to go with a hardware solution. There are passthrough devices to generate an electronic signal for ECM that attach in-line to the speedo cable, I would need to add that and also a DRAC since Iím doing a frame swap and keeping pretty much everything. My donor rig has the DRAC incorporated into the gauge cluster. I didnít know what a DRAC was or does, so I started reading. By the time I wrapped my mind around the subject I was too deep in the rabbit hole to get out. I decided to try to build a motor drive. It has been a fun and challenging project.
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Old 03-01-2021, 05:34 PM   #9
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Re: Keep the original speedometer.

Hmm... Can you post links? I will try to help make sure you get a good parts list.

Fullsize square body Blazers and Suburbans used an NP241 from 1989 to 1991. These vehicles still used a cable driven speedometer. Finding parts from these vehicle can be challenging. S10 Blazer uses NP231 transfer case. S10 Blazer up to 1988 used speedometer cable. From '89 up the speedometer is electronic.
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Old 03-02-2021, 01:55 PM   #10
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Re: Keep the original speedometer.

I really like how my speedometer drive motor turned out, so I’ll stick with that. If my transfer case ever goes bad, I’ll look into modifying then. I’ll keep my eyes open for the NP241 housing for cable until then. But thanks for the offer anyway 1 project2many.
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Mechanical Speedometer Drive Solution
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Old 03-02-2021, 02:04 PM   #11
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Re: Keep the original speedometer.

It might be a wild haired idea but I wonder if a person could use this speedometer with the stock face in a modified stock housing. https://www.amazon.com/ELING-Speedom.../dp/B0755D8XNW
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My ongoing truck projects:
48 Chev 3100 that will run a 292 Six.
71 GMC 2500 that is getting a Cad 500 transplant.
77 C 30 dualie, 454, 4 speed with a 10 foot flatbed and hoist. It does the heavy work and hauls the projects around.
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Old 03-02-2021, 02:39 PM   #12
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Re: Keep the original speedometer.

Quote:
Originally Posted by mr48chev View Post
It might be a wild haired idea but I wonder if a person could use this speedometer with the stock face in a modified stock housing. https://www.amazon.com/ELING-Speedom.../dp/B0755D8XNW
I just bought this one to try https://www.amazon.ca/gp/product/B07...?ie=UTF8&psc=1

It has turn signal and high beam indicators built in. I believe there is also an MPH version available.
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Old 03-05-2021, 05:18 AM   #13
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Re: Keep the original speedometer.

Look carefully at my gauges

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