I have a 91 Z28 305 TPI.

This all started when I noticed my speedometer would stop working, not freezing up but it would either bounce around or just stay on the peg and not move at all. Eventually the car started to have very low idle and sometimes die when I came to a complete stop or even when going over speed bumps. If the car died it would start right back up but usually run very poor.

After searching the boards here I found that the VSS sends the signal to the ECM then from the ECM its sends the signal to the speedo and cruise control (which also doesnt work when the speedo is inop). I had the VSS replaced and the car ran fine for a couple days but began to act up again.

I took the car to the dealership and they were unable to duplicate the problem due to the computer not setting a code and the car not acting up for them. The tech's notes say possible ECM failure or power feed problem and wanted to know if the car had a remote start on it (it doesnt). Im uncertain what he means by power feed problem and I am now unsure what to do next. I dont want to throw money towards replacing the ECM if that wont fix the problem. Has anyone had similar problems or know of a possible solution?

 

if ecm is your problem you can usually induce fault by tappping on ecm w/ screwdriver handle after running vehicle for a while.worth a shot...ran into this often w/ late 80s units.

 

Any other suggestions? I have zero experience with ECM's/electronic repairs.

Is there a way to test the signal with a volt meter?

Can I open up the ECM to look for signs of corrosion without damaging it?

Is there anything else that controls the speedo besides the VSS and ECM?
(Im pretty sure the cluster is fine all other guages work and there is no corrosion on the back of the IC or harnesses)

If I do decide to just buy a new ECM should I also get a new PROM?

The repair guides I have are worthless and repair shops can't/wont fix anything until something is completely broken. Im trying to be proactive instead of reactive since this is a daily driver any other ideas/help would be great, thanks.

 

did the speedo stop woking again?did you try "tap test"?

 

also recall yellow vss signal buffer module?changes ac signal generated by vss to digital signal that ecm can"read"?

 

The speedo is always intermittent while driving, it either works like normal, bounces around or doesnt move at all and stays on the peg like the key is off.

Not sure what the tap test is.

 

I thought that could be the problem but after doing some research I found that the 91 - 92 speed density cars don't have a seperate buffer box because it is incorporated into the ECM. I went to check under pass. side dash/kick panel area and I didn't have one.

 

"tap test" is tapping lightly on ecm w/ screw driver handle and see if this induces stall/poor running.

 

I pulled out the ECM and tried this, no change. While the ECM was out I checked plugs for any signs of bad or corroded wires, nothin. I tried to remove the cover to see inside but it wouldn't easily come off and I didnt want to force it. From what I could see inside it looked fine.

 

Have you tried a new vss sensor in the tranny?

 

Yes, that was the first thing I did even though the ECM wasn't giving a code. I drove it 40 miles today and its still acting up. Can anyone tell me if there is something else that plays a role in the signal from the VSS to the ECM and then from the ECM to the Speedometer? My car doesnt have a buffer box so im out of ideas. Id hate to replace the ECM and PROM to not have it fix the problem. Thanks.

 

Well, you do not have a buffer box because it is a sign wave producing vss not mechanical like earlier trannys. I learned this yesterday. How does the engine run when you are in park? I had the same problems but it was because of a tranny swap from 700R4 to T56. You could check the signal wire from the vss to the computer by doing a resistance test on the wires. There is a purple and yellow wire on the black plug going into the computer at location B9 and B10. Only Y and P wire color on that plug. Test that plug point on each wire to the two wire ends at the vss. Make sure there is no large resistance which would identify a bad wire. If that is OK I would go for a computer first as they are much cheaper than a prom, about $80.

 

Ok, though diagnosis of electronics is a foreign language to me. Do you know which happens to be the output from the ECM to the speedo so I can test that as well? (or link to good diagram)

I do have a decent voltmeter, what setting would I use to test this (20VDC)?

Are the pins B9 and B10 for the VSS input located on the smaller or bigger black plug?, (theres 2 black plugs, cant remember if they both have Yello & Purp).

Does the car need to be running or just key turned on? (dont want to short something)

What kind of numbers should I be looking for? (do higher numbers mean higher resistance?)

Sorry for all the questions, I like doing things right the first time and dont want to ruin something in the process due to lack of experience.
Thanks for your help.

 

Smaller black plug. I will check which wire is the output to the speedo. Place your meter on ohms. 0 is a good wire. When you start to see whole numbers then you have a wire issue. You may see .4. Ignition is off.

 

http://tpiparts.net/1227730_pinout_diagram

Looks like B11 is the signal output from the ecm, brown wire

 

Awesome diagram. Ill have to do some testing to see what I come up with. Thanks for your help with this.

 

Ok so I went and tested the Grey B11 for resistance with 20k ohm meter. I checked continunity from ECM plug to IC plug and it was good. Ohm's checked out good as well at 0.00. So it apperars the speedo wiring from ECM to cluster is good.

 

B9 and B10 are OK. B11 you traced that to it end point at the speedo? What you a trying to do is see if the wire has been grounded out or plastic covering fraid. So you test the ohm reading of that length of wire from the ecm plug to the speedo connection. If that is what you did then the resistance is high and wire is bad.

 

After doing some reading and a bit of research I realized I was testing these wires improperly and need to check from each end of the wire to check for continunity. Because of that my results from B9 and B10 are inaccutrate as I have to test from VSS to ECM for both continunity. I edited my above post to reflect the changes.


Slightly off topic but the engine also has a issues with idling while coming to a stop and wants to die when the speedo is inop, they kind of go hand in hand. Is the VSS signal and IAC linked through the ECM?