I've got a '91 RS that was V6, now a carbed 350 V8. Engine swap was done by previous owner, so I'm not really sure about all the wiring that was redone. I do know the ECM was disabled and as a result the speedometer quit working.

I've been searching the forum and found references to people getting the speedo to work after similar - but not exactly - the same circumstances. I've hooked up an aftermarket speedo on the dash, but it's just ugly up there and I wanted to put a tach in it's place; but I know the sending unit in the tranny works as a result.

Any ideas how I can get this working? Thanks in advance!

 

Bump for the day crew.

Anyone?

 

Okay, how about this? Since I've got connections to the aftermarket speedometer already, can I simply splice these into the dash cluster and make the in-dash speedometer go? If so, which wires do I splice into?

I can get into the cluster with no problem, but picking the wires from the harness would be a crap shoot. Please, if anybody can help let me know!

 

I have no personal experience with electronic speedometers, but from what I understand reading on this Board, there is a speedometer module separate from the ECM under the dash. Removing the computer shouldn't affect that unless too much was removed.

 

Do you know what it looks like or how many/what color wires go into it? The ECM is still there and hooked up, but I don't see another module... maybe it's jammed pretty far up there.

 

BigRuss
I'm having the same problem you are. I have a 91 that has been changed over to carbed. I removed the ECM and rewired the engine compartment. All the gages on the dash work except the speedo. Here is what I have found out. The VSS (vehicle speed sensor buffer) is located under the passenger side dash right next to the ECM. The wires from the speed sensor off the trans end up there colors are yellow and purple. The speed sensor in the trans sends its signal to the VSS which splits the signal to the ECM, Instrument cluster, and cruse control. Thats where I am with it. I have also been told that the with the ECM removed the speedo should still work. Since yours didn't work and your speed sensor works with an aftermarket speedo leads me to the VSS or speedo itself not working. I'm going to try changing my VSS then maybe my speedo. Where did you get your aftermarket speedo since that maybe the way I will half to go.....

 

I know this is a lot to ask, but is there any way you can post or email me a photo of the VSS, because I'm just not seeing it. Is it in the silver box with the ECM?

My aftermarket speedometer is an AutoMeter, but they are damn expensive and hard to mount... I couldn't find one that mounted on the dash so I had to buy an in-dash one and buy a mounting cup. The mount and speedo weren't compatible so I had to drill holes in the cup; it's really ugly.

 

The VSS is under the dash mounted next to the ECM it is a light yellow plastic box that is held in by two screws from the bottom. You need to stick your head under the passenger side with your head laying on the floor mat. Its hard to see but its there. It has two plugs comming in at the top one with two wires and one with five. The two wires yellow and purple are from the speed sensor. Here is a link to the wiring dia . http://www.austinthirdgen.org/mkport...SS_V8_vinE.jpg

 

I may have stumbled across an important fact. This link: https://www.thirdgen.org/forums/post...777-post7.html explains that since the car previously had a V6 that I don't have a VSS buffer box!

If in fact the ECM is somehow disabled that probably explains why the speedometer isn't working. What it sounds like I need to do is find out how to bypass the ECM, but I don't know how to do that without knowing which wires go to the speedometer.

Unfortunately, I don't know how the ECM was disabled either or I'd just re-enable it. Anybody know how someone would go about disabling an ECM without disconnecting it?

 

How about a junk yard or Ebay speedometer from an earlier model? Are the clusters the same up to the 91 year? Cable drive from the tranny? Might be the simplest way to go.

 

It's not a cable drive... the VSS is in the tail of the tranny and has two wires running from it. I don't think the speedometer itself is bad, just that the ECM is getting in the way of the signal.

 

The VSS signal goes to the ECM for engine management and to the VSS buffer box which will be either on the passenger side, or mounted to the back of the speedometer (a little yellow box) from there you will need to find the input from the tranny and make sure you are getting a signal to this box, if not the speedo won't work (I believe that it gets the signal from the ECM, so if it's not present, there's your problem).

 

No, 82-89 are mechanical, 90-92 electronic.

JamesC

 

I just did the carb swap in my 89. My speedo isn't work either. Hopefully we can figure this out.

 

This is strictly for 90-92 cars for it to be 100% accurate...

V6 and TPI, the VSS (speed sensor, this is mounted on your transmission, not a box in the dash as someone else tried to state), sends its signal to the ECM, it is a 4000 pulse/mile signal. The ecm then outputs a 4000 pulse/mi signal to the speedometer and a 2000 pulse/mi signal to the cruise control box.

In the factory wiring harness, the VSS pair of wires (purple and yellow) go from the transmission through the passenger side firewall grommet and into the ecu connector.

V8 TBI the VSS sends its signal directly to a buffer box, this buffer box receives the signal and outputs a 2000 pulse/mi signal to the ecu and cruise control box, and a 4000 pulse/mi to the speedometer.

Wiring in the TBI harness, and chassis harness has the wires from the vss go to the C100 (the big block connector on the drivers side by the brake booster) the interior "chassis" harness then has the wiring for the buffer box and takes the signal wires over to the buffer box which is mounted on the a/c box on the passenger side.

V6 and TPI 90-92 cars have no buffer box wiring at all, and are identical to each other as far as the chassis harness.

You will need to get a buffer box from a TBI car, and wire it up to take care of your issues.

Earlier cars used some oddball configurations, as i know the camaro's used a vss from the instrument cluster since the speedometer was cable driven, im not sure about the pulses to the ecu, or if they use a buffer box to split it after this.

 

Z28*****,

If I'm understanding you correctly then the speedometer accepts a 4000 pulse/mi signal either way, so why would a buffer box be required to get the speedometer working? Sounds like the buffer is only for splitting and modulating the signal to the ecu and c.c.

If I need to get a buffer box and wire the VSS signal to it, where/what other wires would need to come out of it and where would they go?

I'm a little confused how the buffer box would come into play, and it sounds like it's not needed. Thanks for replying and helping us work through this!

 

I dont know if the stock signal out of the box is switched to ground or +, i know that you've got 2 wires from the vss to connect to the buffer box, and one wire output to the speedometer.

You *may* be able to ground the one vss wire and the other straight to the speedometer, but i havent tested this, the other issue is if you'd like to retain cruise control the buffer box would take care of this too.

Buffer box just has hookups for the vss, outputs, and power /ground iirc.

 

No cruise control on mine, so that's not a concern.

My VSS currently has one wire going to the aftermarket speedometer and the other to ground, so essentially all I need to know is which wires go specifically to the speedometer from the harness and I should be able to splice into them and be good to go.

Do you know which wires they are or where I can find a diagram showing which wires to use?

 

The clear connector near the ecu, pin "P" its a red wire.

 

I'm assuming this is the 4000 pulse/mi signal wire to the speedometer. Will I need to supply 12+ and ground to the speedometer as well? The rest of the guages work, so I would hope not, but I wouldn't be surprised if I had to anyway.

 

I think you have to power the buffer box though.

 

Don't have a buffer box, and shouldn't need one it sounds like. Assuming that's the case, how do I power the speedometer?

 

Instrument panel has its power already, if its going to work it will work with just the one wire of the vss hooked to the pin P of C207 under the dash and the other hooked to ground.

 

Hay BigRuss
You need to have a VSS buffer box to make it work eventhough Z28***** thinks I don't know what I'm talking about my speedo now works..............

 

I simply corrected your screwup, dont start crying because you posted incorrect info, you called the buffer box the "VSS" in your post, incorrect info leads to people not knowing whats going on, which is what you posted. Its a buffer box.

Really your speedometer works after installing the buffer box ? Go figure, nobody said it wouldnt, if you'd read the posts, thats been determined long ago, what we were discussing is the fact that BigRuss is right in theory, the buffer box technically isnt needed to make the speedometer only work, in actual application, it may prove needed, however in how everything works, his speculation that unless you are using cruise control, or the tbi ecm, the buffer ***shouldnt*** be needed.

 

I beg to differ on the buffer box not being needed. The box supplies 10.2V to the gage cluster. I have the electronic diagram and specs. As for the name, buffer box, if you want to get technical it is listed on the electronic diagram as VSS-Buffer not buffer box or just VSS. Since you think you’re the raging expert then you need also to get your information correct. Its people like you that stop others from sharing there experiences, that maybe helpful to someone else, for fear of ridicule. That’s a shame.

 

If you feel the need to specifically call it a "VSS Buffer" go you, just dont call it a VSS, that leads people in the wrong direction, people know what I mean when i say "buffer box."

If the signal to the speedometer is a switched 12v signal, thats great, then he may need to put 12v to the other wire of the vss instead of ground, at any rate you havent posted anything set in stone that the buffer is required for speedometer operation, the signal from the vss to the buffer itself is 4000 pulse per mile, the signal output from the buffer box to the speedometer itself is 4000 pulse per mile...

 

That's odd, when I put my LT1 in, I took that box out, hooked my output from my LT1 PCM directly to the 4000ppm speedo input wire on the plug that went into the bufferbox and I have a very accurate speedo working for over 10k miles now.

 

Thats not exactly the same situation. Take a look at the breakdown I posted previously, TPI and V6 cars didnt use a buffer box, the signal was fed straight into the ecm, and then the ecm took care of splitting the signal, the LT1 and LS1, they used a bit different setup on the cruise control, but nonetheless the vss signal is input to the pcm, then it outputs the same 4000 PPM signal that the tpi and v6 ecm outputs, the tbi cars, the vss signal is fed into the buffer, and it outputs a 2000 pulse / mi signal for the tbi's ecu, the cruise, and a 4000 ppm signal to the speedometer head.

He got the speedometer working by hooking up the buffer box, however it may very well work without the buffer, because without the tbi's ecm and the cruise control there really is no need to break the 4000 ppm vss signal into two 2000 ppm and one 4000 ppm output.

 

I know this thread is old but I got my speedo working today. The buffer box wasn't getting any power or a ground. The pink/black wire needs 12v ignition and the black/white wire needs grounded out.

I am guessing the buffer box gets it's ground/power from the ECM which I took out.

Hope this helps someone in the future.

 

Z28***** can you Be a little more descriptive of the schematic I have the purple and yellow wire coming from the vss They are cut right now do they go to the yellow and purple below the brake booster or to the pass side the car is a 91 TBI car, Changed to carburator Thanks Ron