Using an electronic speedo with TBI and 700R4?

[mcbassin]

Anyone done this upgrade? I installed a motor and transmission from a 93 Roadmaster in my son's 86, now I am trying to get the speedometer from a 91 suburban to work. I don't know which wire should run to the speedo from the ECM or if I need new connector? any help or suggestion? I somehow lost my wiring diagram too...doh! HERE is a link to the swap. thanks!

[mcbassin]

Here is a picture of the cluster that I am using:

[mcbassin]

I went back through all my old emails from this account and found the diagrams. Still not sure what all needs to be connected for the speedo to work? here is a snapshot of the diagram:

[cableguy0]

It looks to me like B3 is the speed sensor itself. The other wires look to be shared so they will be ground power and possible for the dash light.

[cableguy0]

maybe this will help

[mcbassin]

Originally Posted by cableguy0

maybe this will help

I'm confused, the diagram above shows B3 going to the speedo, but the original diagram I used to connect the wiring to the truck HERE shows A10 as the vss signal? I should be able to tie a10 to the new speedo and make it work?
Best I can tell this trace is the one I need to connect to the brown vss wire?

[raycow]

I am thinking the speed sensor signal is used for other stuff besides the speedometer. It would certainly be needed for the transmission, and possibly also for some of the engine control functions. My guess is that the signal goes to the computer first, and the computer outputs a signal to the speedo. At the very least, you would need some kind of calibration unit between the speedo and the VSS even if nothing else were connected.

Well, I take some of that back. You said 700R4, so no electronics there, assuming this is the transmission that came with the 93 engine, and I honestly don't know if the TBI needs a road speed signal. However, the speedo still a calibration circuit if it is to work from the raw VSS output.

Ok I just now looked at your hand-drawn diagram and the VSS signal definitely goes to the ECM as I suspected. The ECM appears to control the torque converter lockup, so this makes sense. The part that is puzzling me is that there doesn't seem to be any output from the ECM to the speedo. So if you connect the VSS signal directly to the speedo, where is the calibration circuit?

Found it finally. Looking at the hand-drawn diagram again, the VSS does NOT connect directly to the ECM. If you trace the wire from A10, it goes to the "VSS buffer module". I'll bet money that's where the speedo signal comes off. Now your only problem is to find the VSS buffer module.

Ray

[mcbassin]

Thanks for the info about the drac modules. I'm glad there are guys like you fellows that willing to help ignorant people like me. I guess I will look at the junkyard for one of these drac modules.

So, how are the guys connecting directly to the pcms on their ls swaps to make this type of speedo work? Do the newer pcms have a built signal converter?

[mcbassin]

Thanks Chevytech! for all the technical data and links. I think it will help a bunch. I am going to pull a part tomorrow, hopefully I can find a drac module and a few other parts.

[ChevyTech]

You’re welcome.

If you are using a stock tire size that was available in 1988, and know that axle ratio, I have a GM pamphlet that shows that DRAC calibration / “divide ratio” of factory equipped vehicles.

[mcbassin]

Well, I found a DRAC module at the pull a part. It is from a 91 G20 van. Does anyone know if the this is the same as the 91 Suburban module?

Here is a pic:

[ChevyTech]

It is a DRAC.

Open it, and take a photo of the jumpers, and I can tell you the divide ratio which will determine if you need to recalibrate it or not. Or maybe you are just going to get a speedo working before you worry about if it reads the correct speed or not.

A 1991 DRAC should only have 7 jumpers.

***

This is the wiring for a 1993 DRAC which has an electronic shifted transmission but it may help you with wiring your in.
DRAC wiring

From 1993 C/K Schematic

C15 - Speedometer output - Light Blue with black tracer
C14 – Blank - used on M & G vans
C13 – Trns output speed to ECM/PCM – Brown
C12 - Vehicle speed sensor input – purple with white tracer
C11 - Vehicle speed to ECM/PCM (M30) – (Dark Blue M/L,C/K, G)
C10 - Vehicle speed to RWAL – White
C9 - 12 volt ignition – (Brown C/K) (pink & black M/L,G,S/T)
C8 - Ground – Black with white tracer
C7 - Vehicle speed sensor input – light green with black tracer

Small connector:
Blank
Blank
Blank
Speed signal to Cruise control – Red with white tracer


M30 - TRANSMISSION AUTO 4 SPD, HMD, 4L60-E, ELECTRONIC
C11 is brown & white on S/T trucks

[clinebarger]

Heres some 93 B-body & 91 R/V truck DRAC diagrams. R/V trucks show to use C14 for the VSS output to the cluster.

[mcbassin]

I hooked everything up just like the diagrams and the speedo doesn't work. I did notice for the first time yesterday that the needle moves just a little when the key is turned over. Also, since my son has been driving it the trip odometer has accumulated 15 miles. So, is it safe to assume, I have a bad speedo or wrong signal from the DRAC? any help?