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]

Originally Posted by 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

Ray,
I'm not sure I even have this vss buffer module. I wonder if I left it in the donor car? I hooked A10 up to the speedo and it didn't work. I don't know if that is the wrong wire or if I have a bad speedo? back to the drawing board

[raycow]

Originally Posted by mcbassin

Ray,
I'm not sure I even have this vss buffer module. I wonder if I left it in the donor car? I hooked A10 up to the speedo and it didn't work. I don't know if that is the wrong wire or if I have a bad speedo? back to the drawing board

No, I think A10 is an input, not an output, so you aren't going to see any signal there. To be honest, I am as much in the dark on this as you are. The only speedos I have ever run off the VSS signal were aftermarket, and those have their own internal signal processor.

If you are really brave, you could try connecting the speedo directly to the VSS output. The reason I said brave is that I don't know if the unprocessed VSS signal could damage the speedo. What are the chances of your getting a more complete wiring diagram for the burb so you could find out where the other end of the wire on B3 is connected?

Ray

[ChevyTech]

Do a search on DRAC module and another on vehicle speed buffer.

The instrument panel in the photo does not look like the earlier one on C/K trucks that had the DRAC/ buffer built into them.

I agree with raycow. The ECM does not have output on the VSS terminal. It is an input to the ECM

The DRAC takes the alternating currant wave from the VSS and converts it to a square wave and has a ratio adaptor to condition the signal.
Here are typical DRAC outputs for C/K trucks of that age. The signal it is sent at the rate of:
4000 Pulses per mile to the cruise control, speedometer, and odometer.
2000 pulses per mile ECM
128,000 PPM to the RWAL module

[raycow]

Great! We finally got someone here who knows about this stuff. Is the DRAC a standalone unit in its own separate package with input/output connectors, or is it a part of some larger assembly? And if it is separate, what information would a GM parts dept need to have before they could sell you one?

Ray

[ChevyTech]

Here is what the DRAC models looks like from a TBI truck after they stopped building the DRAC into the instrument panel. I think they began using this part in mid year 1990, so some 1990s were built the old way and some the new way.



Link to photo source if photo won’t show
http://www.bigfloridacountry.com/drac1.JPG

I don’t believe you can buy a DRAC that is ready to use from a dealer an longer. Do to the number of DRAC modules because all the axle ratio and tire sizes making a difference in the signal output, they changed to a one fits all module that had to be sent out to get calibrated to fit the vehicle it was to be used in.

There are many articles on installing dip switches in the DRAC modules that were separate from the instrument panel. With dip switches you don't need to use a soldering iron to change the calibration.

Here some articles and sites look at:

http://www.tbichips.com/drac/

http://614streets.com/drac.html

http://www.asashop.org/autoinc/feb2004/mech.cfm

[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]

BTW, I think I have seen these
at the salvage.

So if I get one of these units, do I need to connect A10 to this and bring a signal wire from there
to the speedo?

[ChevyTech]

I guess I will look at the junkyard for one of these drac modules.

On C/K trucks:
Look in the center of the dash, just to the left of the glove box door, Just below the equalizer for the radio (if it has one) or just below the cubby hole if no equalizer. It is mounted very close to the inside surface of the dash.

On G vans;
The DRAC is located by the parking brake mechanism.

Open the DRAC and look inside to see if it has one set of jumper or two sets of jumpers. I think it was 1994 when the DRAC module started having two sets of jumpers inside them. You are using an instrument panel that was built when one set of jumpers was used in the DRAC, so find a DRAC with one set of jumpers (7 jumpers in a row)

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?

Keep in mind the year of equipment you are using makes a big difference.
If you had a 1989 instrument panal the DRAC would be built into the dash.
Mid 1990 - 1993? The DRAC had one set (7jumpers) and then two set of jumper (two sets of 7 jumpers for a total of 14) was used starting in 1994. Later on (I can’t remember what year) the DRAC was incorporated into the PCM.

If someone tells you what you need or what you should do without know the exact model year of all your parts you better question the quality of the advice they are giving.

WARNING:
Jumpers are numbered right to left in Gm information and some sites are confusing in regard to which jumper is which on the chart.

[ChevyTech]

So if I get one of these units, do I need to connect A10 to this and bring a signal wire from there
to the speedo?

Two wires from the VSS go to the DRAC
A separate signal wire goes to the ECM
A separate wire goes to the instrument panel for the speedometer and odometer.
Your 1986 does not have RWAL (antilock) so you don’t need that wire.

I suggest getting a wiring diagram for a 1991 to get all the connection to the DRAC.

There is a wire diagram in the post below. This is the best I can find at the moment. I lost my computer and all my files and links that I had saved.

Richards copy of a post that can no longer be viewed by non members at another board.
http://www.someotherplace.com/guides...p/cluster.html

[ChevyTech]

Here is a schematic that is not detailed enough but gives and idea of the circuits.
http://www.wedgeparts.com/images/747ecm5.jpg

[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.

[ChevyTech]

Nice schematics.

Originally Posted by clinebarger

R/V trucks show to use C14 for the VSS output to the cluster.

That makes sense and fits with this.

Originally Posted by ChevyTech

C14 – Blank - used on M & G vans

[mcbassin]

Any ideas of what wires beside ground, cluster and vss signal that must be connected?

Here is the 91 Suburban diagram from my good friend Cableguy:

[mcbassin]

I,m not sure what gear ratio the truck has? It originally had a 350turbo I believe. As far as tires go, we are using 255-55-18 which is nearly the same as the oem 235 75 15.

Here are the pics of the DRAC inside: