Tbi swap build thread

[skidder111]

OK Thanks, man. I will check it out. Will check back here tomorrow.

[rfmaster]

Originally Posted by cjracing15

Well I would re-trace the wires to see what they are doing.

A-10 is a brown wire from the ecm, and B-10 is a org wire with a blk strip on it. let me know what you find, I am just staring my swap as you can see.

P/N line Orange/black wire is grounded by a P/N switch when in park or neutral position. P/N switch ECM input is used by ECM to compensate idle speed when additional engine loads (AC, PS pressure, etc) are present while in drive and within idle speed range. ECM commands IAC to add additional 50 to 75 RPM over commanded idle speed under such conditions. In your case (manual tranny) leave p/n wire open (i.e always in 'drive')

VSS is extremely important for complete EFI operation. It is unfortunate that many omit VSS during conversion. In your case, having 4 speed manual, it is imperative to have a functioning VSS. Since you have 1227747 ECM you'll need to have 2-pulse square pulse VSS inline unit. These are readily available (JTR, others). VSS provides vehicle speed to ECM which uses this input to accurately set fuel mixture and timing. One common problem with absent VSS and manual tranny is a severe drive train jerking during deceleration. This due to inability of ECM, which uses TPS, MAP and VSS inputs, to enter DE/DFCO mode (Deceleration Enleanment / Deceleration Fuel Cut Off).

When the ECM detects a sudden drop in throttle position and engine load, it reduces the injector pulse width while continuing to monitor engine load and vehicle speed. The purpose of DE is to produce a lean fuel mixture by reducing the amount of fuel gong to the engine. DE mode conditions are determined by ECM on a continuous bases provided that 1) Vehicle speed is above XXMPH (this depends on EPROM calibration value), 2) engine load decrease is below a limit value (this parameter also depends on EPROM calibration value), and 3) Filtered %TPS drop is <1%.

The difference between DE and DFCO parameters is that DFCO parameters are based on fixed values stored in EPROM. The DFCO fuel mode is entered after moderate to heavy acceleration then followed by abruptly releasing the accelerator. Usually ECM will transition into DE prior to entering into DFCO mode as the deceleration continues. Interestingly enough operating conditions may or may not cause ECM to command DE exit by itself. However when ECM enters DFCO mode DE is automatically disabled. Usually DFCO is a 'long' term (self resetting) duration event - coasting down hill in gear for example.

Both DE and DFCO are used for emission control, (keeps CATS from being burned up due to excess fuel during deceleration), but the two will effect drive train and exhaust longevity. There are many other ECM controlled parameters that are derived from VSS input.

//RF

[cjracing15]

Cool.

[D13]

Why not put in a later model speedo with VSS? Pretty cheap on eflay. Looks to be pretty straightforward.

Is the under dash harness involved in feeding the ECM? One would think this is where the VSS and fused ECM power would come from.

[cjracing15]

What he said.

[DirtyLarry]

Rfmaster,

What you said above about VSS sounds right for 1996 and later OBD II sequential multiport EFI systems but I can’t find anything in any of my older GM STG service manuals for TBI years that state the VSS has anything to do with Deceleration Enleanment or Deceleration Fuel Cut Off. It could be one of the functions that takes place behind the scenes that isn’t really called out in the service manual.

From what I understood, the main sensor to control fuel on OBD I such as TBI is the MAP as it constantly monitors engine load and adjusts fuel pulses accordingly. Once you get into OBD II on sequential multiport things get a lot more complicated and more inline with your description above.

The only function I can find for the VSS in TBI years is to provide a pulse signal to the DRAC for the speedometer, RWAL ABS module, and cruise control module. The VSS isn’t even mentioned anywhere in any of the drivability symptom flowcharts or anything other than inop speedometers, cruise, ABS, etc.

For what it is worth, my 8.1L running multiport EFI on a Delphi MEFI-4 controller (similar to a GMPP RamJet engines and GM Marine engines) doesn’t have any VSS inputs either to run, and run well even with a manual trans. The whole MEFI-4 system is an entirely weird deal anyway.

I know you are an engineer and know this stuff a lot better than any of us but are you sure about the VSS function on TBI?

[cjracing15]

I thoght that on OBD II they did not use a vss sensor, but the ecm got it's signal from the abs computer.?

[DirtyLarry]

Originally Posted by cjracing15

I thoght that on OBD II they did not use a vss sensor, but the ecm got it's signal from the abs computer.?

VSS is still used today to provide data to the PCM/ECM for engine controls, speedo, cruise, trans shifting scheduling, torque converter lockup, etc but not ABS. These days, there is actually a wheel speed sensor at each wheel to provide input to the ABS module to control 4 wheel ABS system and traction control if equipped. In the TBI days the trucks only had rear wheel ABS which the rear wheel speed input to the ABS module was read from the VSS sensor on the tail shaft of the transmission.

[GlennBrittain]

Many thanks for taking the time to post your info and progress. This will be a great help to myself and I am sure many others when I get to my drive train. Right now I have an 88 GMC for the donor but may look for the last year of TBI.

[cjracing15]

my bad

[rfmaster]

My apologies - 7747 does not have DFCO - MPH threshold, which is used in 8746 ECM found in TBI passenger cars. Also, DE is mapped to MAP and RPM threshold in 8746.

Now that I looked back at my notes I know where I got confused - in my conversion I used 16136965 from 1991 B-Body which just happens to be almost pin compatible with 1227747 (there are couple more control outputs and MAT input) which in its stock form uses speed threshold for DFCO. In the end it did not matter to me at all since I have converted to EBL which provides full control over DE and DFCO.

VSS in 7747 is used primarily for TCC control, Knock retard, Highway SA, EGR, & Idle speed compensation.


//RF

[cjracing15]

This thread is turning out to be what I wanted, A bunch of info on this swap that you don't have to hunt and hunt for on the net at a ton of different sites.

Keep the info comming. Please share what you have done and if you have a question, please ask it.

I am going to have some more progress pics on my swap tomarrow so stay tune.

[BigrobDog]

Originally Posted by cjracing15

This thread is turning out to be what I wanted, A bunch of info on this swap that you don't have to hunt and hunt for on the net at a ton of different sites.

Keep the info comming. Please share what you have done and if you have a question, please ask it.

I am going to have some more progress pics on my swap tomarrow so stay tune.

so if my vss wires from tranny plugs right in to harness it should be hooked up ,and no need to wire any thing.or ground them,

[DirtyLarry]

Rf,

Don’t worry, it easy to get confused with all this EFI stuff as there is a lot of information out there with different operation systems. Every time I think I just about got it all figured out….you learn something new.

[skidder111]

So, rf & Dirty Larry, is there any benefit to installing a VSS in my ride? As mentioned above, it is an 89 setup, so there is no DRAC in this setup. I'm still using the mech speedo and have no cruise. THANKS FOR YOUR HELP!

[cjracing15]

Originally Posted by skidder111

So, rf & Dirty Larry, is there any benefit to installing a VSS in my ride? As mentioned above, it is an 89 setup, so there is no DRAC in this setup. I'm still using the mech speedo and have no cruise. THANKS FOR YOUR HELP!

What did you find out on your truck about the wiring?

[skidder111]

cj- I haven't had a chance to look at it yet. I'm gonna have to spend some time this weekend tracing it out. When this swap was done, I paid a GM technician in beer after hours to hook it up for me so I have limited knowledge on the details of the system. Is there something in particular you want info on? The dash is currently apart because I am just finishing off a frame off restoration, so it would be easy to trace out what was done. LMK

[cjracing15]

Nothing really I was wondering if the vss is acctually hooked up or not.

[skidder111]

The VSS is not hooked up. I have the wire ready but I still need to buy a VSS which will give an electronic signal from a mechanical speedo drive. I would still like to know if this is necessary. I'm kinda waiting on rf or DirtyLarry to see what they say.

[cjracing15]

Well on a auto trans I am sure you will need it, but on a 4 speed I do not know.

[DirtyLarry]

Skidder,

I would venture to say you are good to go without VSS on a manual transmission. With an automatic, yes, you would need it for the torque converter lockup. Your earlier post mentioned a concern with going into closed loop. Closed loop is achieved by the engine coolant temperature sensor (ECT) and O2 sensor inputs not vehicle speed.

My 8.1L with a manual trans does not have a VSS at all. Even though the 8.1L is multiport EFI, mine is running an OBD I speed density calibration system through a Marine MEFI-4 engine controller with a protocol that is very similar to the old TBI and TPI systems of the late 80’s and early 90’s. As far as fuel delivery, the main difference is my 8.1L fires 8 injectors under a higher fuel pressure in batches where as TBI is firing 2 at a lower fuel pressure. Firing the 8 individual coil packs on the 8.1L is what is unique about the MEFI-4 controllers as they are very versatile in programming what they can control….but that is a whole nuther story.

This is one of the reasons I have chose to go with aftermarket harnesses opposed to reworking a donor vehicle harness. First, a donor vehicle TBI harnesses are getting old (last TBI truck was built 15 years ago!) and the harnesses tend to be very brittle and stiff, end connector break at the touch, etc. Second, a creditable aftermarket harness vendor such as Howell or Street and Performance, etc has already put a lot of research into knowing how to properly wire these things. Most of those companies have gone to the manufactures and purchased the original engineering layout drawings to create stand alone harnesses without all the unnecessary circuitry. For $325 a nice fresh harness is hard to beat vs. pulling your hair out trying to rework an old harness only to still have a 15+ year old harness when you are done.

[skidder111]

Dirty Larry- Thanks a million! This swap was actually done about 12 years ago. I drove it for a couple years then tore it down for restoration so it has been sitting for a while. The wiring is in nice condition, but if problems ever start, I will look at updating. It is difficult to make the '89 wiring look like it is supposed to be there tho. I am going to check on that p/n wire and try your suggestion. Once again, THANKS!

[cjracing15]

Larry you have a very good point about the old harness on the fact that it is old and the wires are brittle, (mine has been for the most part very managable though) and me and you talked about me buying a harness from Howell. I am right now off work untill the first of the year and I want to get this swap done before I go back to work. Untill I go back to work I really do not need to spend any more money than I have to. So I had the stock harness and decitted to use it. When I decited to do that I thougt while I was searching the net for info on this swap that the info is out there but you really have to look every where for it. So I decited to start this build thread to maybe help other people out that want to do this swap and use salvage parts in the process to save a buck or two.

I really do appreciate the input that people are putting in this thread ( like you and rf).

Got to go and work on my truck now.

[cjracing15]

Well I did not get to work on it as much as I wanted to to day but did get a little done.

First two is a before and after on one side of the motor. This is where the ESC, TPS, MAP, IAC, EGR , and temp plugs are located. You can also see where the wires enter the cab

The others are the wires that I will have to hook up in the cab. They are

B-10 park/neutral
A-5 SES light
A-8 ALDL
A-9 ALDL

[cjracing15]

Here are some of the wires that I will have to hook up that do not have factory plugs.

The first one turned out fuzzy but it is the 12 V feed wire (I am going to put it to the big post on the starter with a inline fuse).

The second is C-9 this wire goes to the sm post on the stater.

The Third is the switched 12 volt wire. ( I am going to put it on the smaller wire that pluged into the orginal HEI BAT terminal).

The Foruth is the ground wire for the system.

The Fifth pic is the wire that will power the dist. ( I am going to tie it to the big HEI wire that is on the truck now.