This is a 1993 model.
Currently it has about 68k miles - automatic trans.

I am the 2nd owner.
Basically this car has almost always run fine since I bought it used (dealer demo) in 1994 with 17k miles on it.

(Background)
Back around 1997 or so I was having problems with the car dying like it would run out of gas and after sitting for a while it would start.
The dealer replaced the ECM at that time with a re-manufactured DELCO unit. But the real problem turned out to be just a bad or clogged fuel filter and after that the car has always been fine.

CURRENT problem:
Around 2005 I think, I started having a problem with random jumping digits in the MPH readout.

I originally only noticed that there was a slight problem only while the cruise control was on and the car seemed to be varying in speed like it was hunting up and down.
The dealer thought it was a bad VSS (vehicle speed sensor) and that didn't fix it so then they thought it was a bad servo but that also wasn't the problem. Then they sent the Digital Display unit out to Delco and it came back as having no problems. So I took the car back home.

It seemed to clear up for a while but eventually it progressively got worse.

Some time later the MPH digits started really wildly jumping randomly around the approximate speed I was driving.

Sometimes the MPH just reads zero, then suddenly starts again jumping around
Otherwise the car seems to run with no problems. Starts fine runs fine.
Just the MPH and of course the odometer will not collect digits either when the MPH is at zero.

==> I recently (this month) took the car to a local mechanic who has a tech that tried to troubleshoot it. That guy told me (after lots of troubleshooting) to to get a new ECM.

They tried to find one but it was taking weeks.

A dealer finally told them he could get a "new" ECM that would solve the problem because he said rebuilt units cannot fix it as he said it is caused by a problem on the ECM circuit board itself but he couldn't get a new unit for maybe 6 weeks!

So Instead I sent my ECM to a shop called "ECU-Exchange" in Cloud Lake, Florida to be rebuilt ($180) since those guys said they were quite sure they could rebuild it and solve the problem and offer a lifetime warranty.

The rebuilt unit came back but it exhibited the EXACT same problem.. but in addition to that ...my mechanic was now also unable to communicate with the ECM via a Snap-on Scanner, diagnostic code-reader unit.
(He previously had been able to get codes and communicate prior to the rebuild.)

The "ECU-Exchange" company told me to send it back.

When they got it back they confirmed a problem communicating for codes but they saw no MPH problem.

So to help me localize the problem they sent me an entirely different ECM from their inventory.

That unit arrived today but it exhibits the same random number digits problem in the MPH readout.


At first for a mile or two the MPH remained at zero. ( Same as before) but then after a few miles it suddenly started to respond..(also same as before)
It's always been either zero or some jumping digits ever since this problem became serious.

The random numbers are not exactly completely "random" but they do jump around. But they seem to jump around at or approximately near whatever actual speed I'm traveling at.
So If I am traveling around 30mph the digits will jump all around in that general area from 22- 35 or so... and likewise if I'm traveling slower about 10-mph or so they jump around from about 5-15 or so.
I don't know if that helps anything to know that.

So anyway I don't know what to say.
Is this bad news or good news?..lol

If this is a "totally different ECM unit" and I am still experiencing the EXACT same problem... does that mean the problem is NOT in the ECM? or is it still possible that this 2nd ECU suffers from the same type of trouble?
Is it maybe a problem with the CCM?
Is it a grounding problem?

My mechanic's garage was closed this afternoon and won't re-open until Monday so I cannot tell anything about being able to read codes or anything until then.

Please let me know what you guys think about all this.

thanks,
Peter Balazsy
pbpix@optonline.net

 

I think your problem is not ECM related.. I'm not anywhere near my FSMs this weekend, so I'm working from memory with this post....

IIRC, the VSS feeds a signal to both the ECM and the CCM. The CCM provides the data to the LCD for speedo/odometer readout. Does your display blank, or is it just erratic?

Also, again from memory, I think there is a splice or connector behind the left head where the VSS signal splits to the ECM/CCM. I would certainly check there. Your problem appears to be either with the VSS signal to the CCM, or between the CCM and LCD.

When I return home tomorrow, I'll check my available wiring prints. Anyone else with a 93 FSM Electrical Supplement, feel free to check and chime in....

 

Thanks soooo much for your help!

Mine does not go blank. The MPH digits just bounce around or stay at zero for a while then jump around then back to zero.

( I have a small, hard to read print here "1993 Chevy Corvette (VIN P) ECM wiring diagram)

The tech that recently worked on this at my local shop said he put his Scope on the VSS signal coming in and it looked clean but it was noisy on the output.
This is why he said I should get a new ECM

Trying to read this print...I DO see the signals coming in from the VSS on page 2 of 3
(VSS Hi) ECM connector "D" pin-12 YEL 400
(VSS LO) ECM connector "D" pin-1 PPL 401

On the same page there is an ECM output signal:
I see one signal labeled on the ECM connector "D" pin (8) that looks like it says (4k mile spd pulse out)
The signal says it goes to:
1.) Radio Control Head
2.) Central Control Head
3.) Cruise Control Module

On another print:
"Instrument Cluster: Gauges Wiring Diagram"

Here I also see a signal on the CCM pin "E2" ( Dk Grn/Wht 817 ) which indicates it comes from the "speed sensor".

I don't know but it looks to me like the VSS HI signal goes into the ECM on pin 12 (yel) first and then it comes out on pin 8 (no wire color specified) and then does it (maybe?) go from there straight onto E2 at the CCM? ..Is that right? ... or does that CCM (E2) input signal (speed sensor) signal come in .. right from the VSS too?

In any case... if the tech was right when he thought that the ECM signal out looked noisy coming out... then a different rebuilt ECM ought to have helped... no?

Another thought here tells me that MAYBE the signal coming out of the ECM is STILL the problem because it not only feeds the CCM BUT it also feeds both the Radio Control Head as well as the Cruise Control Module.

And when this problem First started I was noticing the Cruise Control was creeping high and low!
Also the radio volume would randomly go low and high.
And now I don't dare turn on the Cruise control because it goes crazy like the MPH digits leaping forward and then not. Also if I turn on the radio/speed sensor volume control... the radio vol will BLAST when the MPH digits go randomly high too.

Those two are BOTH fed directly from the ECM... right?
So if they are BOTH also being effected (and they are) doesn't that indicate the bad signal is from the ECM output?

So ALL three of the items that are fed directly from the ECM output are going erratic.

Maybe all this relates to a BAD ground?

 

OK. As the digits are constantly present (no blanking), reading zero or jumping tells me the link between the CCM and LCD is ok, but the CCM is not receiving a solid VSS input signal.

So, again, that leads me to the wiring between the VSS and CCM. After reading your post again, and thinking about your original issues that led to an ECM change, it sounds more and more like a wiring issue or at the connector from the VSS back to the main harness or the splice......

 

Okie sounds like hes onto something.

 

I had a similar problem years ago. My readout was erratic. I finally found that one of the connectors plugged into the CCM had a bad connection. I found it by moving the connectors with the car running. I pulled it off, cleaned with contact cleaner spray and it's been fine since.

 

Going back to the FSM Circuit Operation description, the ECM receives the VSS sine wave signal, amplifies it and converts it to a square wave signal which is then supplied to the CCM, Cruise Module and the radio control head.

That output signal is what you describe as the 4K/Mile Speed Pulse Output ECM D Pin 8 and is the signal used for speedo/odometer readout by the CCM.

Looking at the print you have, it should go through a connector and a splice between the ECM and the 3 components (CCM,Cruise,Radio).

I'm using a 91 FSM Electrical Supplement and it lists it as C238 and S224, but if anyone has a 93 specific FSM Electrical, could you please look up the VSS wiring prints around 8A-33-0 and check the connector/splice numbers and the location for the connector?

As you describe problems with all 3 systems, then the first place I would check is the connector (equivalent to the 91 C238) for being dirty or corroded causing erratic ECM output signal..

 

Partial UP-Date:
Well this is getting more and more curious.

I no-longer believe that the ECM is bad.
After the ECM repair place sent me a 2nd different unit and I still had the same problem.

So I decided to work with my mechanic and his tech to try to solve the jumping digital MPH digits problem together so that I could see 1st hand what's actually going on with signals etc.

I had a spare VSS (vehicle speed sensor) here from back when it was changed after the 3rd time ( I think) and changing it never made a difference. It was changed 1st by the dealer when I first had initial symptoms of only a slight variation in speed while in Cruise control.
Eventually that problem worsened to this wild random digits in the speedometer.

The ECM uses a sine-wave input from the VSS .... and then the ECM processes it ....and puts out a square-wave to 3 places...
1.) the CCM that feeds the dash digits
2.) The radio volume control for vol increase with speed
3.) The cruise control

ALL three places are affected by the varying signal. They (all 3) each go wild when the digits go erratic.

So I brought my spare VSS to the shop and we put a scope on the input (sine-wave) to the ECM and the other scope trace on the output Square waves.

Then we spun the shaft on the VSS by hand and everything was good.

We observed a nice clean sine-wave signal from the VSS going into the ECM... and good square-waves coming out of the ECM ....with a nice steady, solid display on the dash.

So then we put a variable speed drill on the VSS shaft and raised the speed.... and still everything looked good.

So at 1st we thought the existing VSS itself might be bad.

We removed that existing VSS and tested it by hand, spinning the shaft... and with the drill and everything STILL worked well too!!

BTW: (I have a 4spd automatic trans with OD)

The VSS has an orange-colored "driven" gear on the shaft that fits into the transmisssion tail shaft and meshes with the RED "drive" gear.

So the RED "drive" gear inside the trans ... drives the orange "driven" gear on the VSS shaft.


The colors of those gears relate to the gear-ratio of the rear end.
We spoke to GM and confirmed that the VIN number is correct for those color gears.

Okay.... onward... lol

Now Everything was good when we turned the VSS by hand ....but as soon as we mounted it back inside the transmission the digits would become erratic again.

We STILL are not 100% sure what's really going on here but here's where we are so far.

I suspected that maybe the VSS isn't mounted quite right such that maybe the 2 gears inside the tail-shaft don't mesh properly ....or only mesh partially ....and so they don't generate a good steady signal. Maybe sometimes the gears are slipping or not properly meshed.

We watched the scope traces when the VSS is inside the trans and whenever the sine-wave signal INPUT to the ECM goes wild .....then of course so does the output square wave signal that feeds the dash ... hence the erratic digits.

I wondered if maybe the VSS was not the proper one since it had been changed several times .......and if it is the wrong one, .....then maybe that's why the 2 inside gears don't properly mesh.

Well.. sure enough the existing VSS has a WHITE plastic body ... and that is the one for the manual transmission!!.

The proper VSS for the auto trans with overdrive has a BLACK plastic body.

option "D8L" = sc51 - Speedometer sensor 10456092

So we've now ordered the new black one.
(very hard to find ....BTW... a new one was finally found in NY somewhere ($50) and is on the way .....and I also found a good USED one ($100) that was still mounted in an automatic overdrive transmission... so I'm sure it's ALSO the right one)
Both are being shipped...lol

So this afternoon my mechanic put the whole thing back together ... just for tonight, so we can drive it until the parts get here.

WELL ...lo and behold ....he said that when he put the same old existing VSS back in the trans ... it seemed to be working for a few moments.
.... but then it went wild again while driving for a few moments.. AND then suddenly it cleared up again and worked solidly. lol

He was astonished and called me to come test drive it.

So then I decided to see if it was REALLY working properly. So I started it up ....and as soon as I started moving the digits went wild again for a moment .....then stopped jumping around .....and then worked properly again!!
OMG

So I road tested it for five miles or so using the cruise control and testing the speedometer at different driving speeds and I observed that it went into OD properly ....right at 40mph... and EVERYTHING seems very solid and beautiful again...lol

How screwy??

Well anyway ...The upshot here is that we have no-clue why it's working... except or unless that ... maybe...when he installed the VSS again ...perhaps the gears may be JUST catching a bit properly.

I don't really know what else to think. lol

When the new "proper" black VSS gets here tomorrow, we can then try to compare them to the existing WHITE VSS... just to see if there is any slight physical difference in the part such that might help the gear fit more properly into the shaft.

If so, perhaps the new black VSS truly allows the "driven" gear to go far enough (properly) into the transmission just a little deeper allowing for correct gear meshing. Who knows?

There might also be some difference in the electrical sine-wave it puts out (from the White unit compared to the Black unit).. but I doubt that.

I think that "ratio stuff" seems to be determined solely by the proper colored gears where each have a different tooth count for the right circumstance.

As for the three individual VSS units ... the 3 different types might have to be physically different due to the physical differences in the 3 transmission ( 1-manual, 2-auto & 3-auto/OD)

Tomorrow will tell the story.. I hope.
Wish me luck...lol

 

Partial UPDATE #2 (I think) ...lol

Well it seems you can't trust anything people "tell" you.

The part number for the VSS that I actually HAVE installed in the car is 10456091 White plastic.

According to GM - Dealer when we called them we gave them MY VIN- number and they said that translates to a build code D8L
And that means I should use part number 10456092 they also said it should be black plastic.

( I only assumed that meant it was for the Auto Trans with OD because I do have the OD auto trans in my car)

So I went out searching for the xxxx092 part.

There are 3 VSS part numbers.
Manual Trans= 10456089 build code D8I -- (white plastic)
Auto - Trans = 10456091 code D8K --- maybe black?
Auto - Trans = 10456092 code D8L --- maybe black?

Since the GM dealer told me I needed 10456092 for a D8L ... this is what I was looking for.

So I found a guy who sells used parts in Mississippi. He said he had the auto-trans w/OD right there ...and he would pull off the VSS and send it to me.

But today he called and said it's WHITE part-number 10456091

He said everything else matches... the RED "drive" gear and the orange "driven" gear

So I was puzzled and told him NOT to send it... because I thought I needed a black one part# xxxx092


So I went to the shop and looked in my car's console where the build codes are printed and I found out that I have code D8K

Then I checked the number stamped on the WHITE VSS that is in my car and it is part# 10456091.

How crazy... I should have done that FIRST.

So according to the build code chart.. I DO already have the correct part in the car.
D8K = 10456091

So it seems the mis-information came directly from the GM dealer who said my VIN number means code D8L and xxxx092 part number


And... for whatever reason... NOW the speedometer is working perfectly.
I test drove it again today. It performed perfectly.. including the Radio vol control and the cruise control.

This makes no sense since I've had the erratic digits for years now with these SAME parts in the car!! ...lol

So I have no clue now why it started to work so solidly.

Now what? ...lol

 

I suggest you go back above and read Okie and Shuman's posts about loose connections. I'm betting they've already identified the problem for you. If all the various components you test are OK and the problem persists, it seems likely the issue is somewhere in the communication BETWEEN the various parts.

 

I had the same problem. Look at this link, post#7 and #10. Maybe you have the same problem.
http://forums.corvetteforum.com/c4-t...0-my-dash.html