quick make a youtube video, people will go nuts
Speedo issue
Thread Starter
Race Director
Joined: Apr 2000
Posts: 15,344
Likes: 4,008
From: Texas
Speedo issue
Finishing up the part throttle tuning and have been blessed with a new issue to resolve 
After the car warms up, the speedo display will randomly display various speeds or even "0" while moving. Highest I've seen is 162 (in a 25 zone
).
Some observations...
- The engine runs fine when this is happening
- Can happen at any speed...dead stopped or cruising along at 70mph
- No error codes in either the CCM or the PCM
- No Service Engine Soon light
- If the cruise control is on when this happens, it of course goes nuts
- The Speed Compensated Volume reacts to whatever speed is being shown (ie at 162, the volume significantly increased)
- During one of the episodes, I happened to be logging data. The speed output in the log file bounces around
- I had a spare VSS, so I swapped it out and there was no change
Given the above, it looks to me that I have a connection in the PCM that is calling it a day...ie a cold solder joint.
Before I disassemble the PCM, I'm open to other suggestions regarding what it could be
:
After the car warms up, the speedo display will randomly display various speeds or even "0" while moving. Highest I've seen is 162 (in a 25 zone
).Some observations...
- The engine runs fine when this is happening
- Can happen at any speed...dead stopped or cruising along at 70mph
- No error codes in either the CCM or the PCM
- No Service Engine Soon light
- If the cruise control is on when this happens, it of course goes nuts
- The Speed Compensated Volume reacts to whatever speed is being shown (ie at 162, the volume significantly increased)
- During one of the episodes, I happened to be logging data. The speed output in the log file bounces around
- I had a spare VSS, so I swapped it out and there was no change
Given the above, it looks to me that I have a connection in the PCM that is calling it a day...ie a cold solder joint.
Before I disassemble the PCM, I'm open to other suggestions regarding what it could be
:
Team Owner
Joined: Sep 2003
Posts: 25,331
Likes: 2,722
Do you have access to a 2nd PCM to confirm your thoughts before ripping into the PCM?
I don't have a '96 FSM and I've never investigated the "pin-outs" for a '96 but does the CCM, the cluster and the ALDL rely on the same source from the PCM?
I think I'd be inclined to check the VSS signal to the PCM first! There's only I'd think two pins to check lo in/hi in and then I'd guess it would be isolated to an internal PCM. If the PCM has been in/out frequently I think that would be more likely than an internal problem.
I don't have a '96 FSM and I've never investigated the "pin-outs" for a '96 but does the CCM, the cluster and the ALDL rely on the same source from the PCM?
I think I'd be inclined to check the VSS signal to the PCM first! There's only I'd think two pins to check lo in/hi in and then I'd guess it would be isolated to an internal PCM. If the PCM has been in/out frequently I think that would be more likely than an internal problem.
Last edited by WVZR-1; Aug 1, 2009 at 05:23 AM.
Thread Starter
Race Director
Joined: Apr 2000
Posts: 15,344
Likes: 4,008
From: Texas
Do you have access to a 2nd PCM to confirm your thoughts before ripping into the PCM?
I don't have a '96 FSM and I've never investigated the "pin-outs" for a '96 but does the CCM, the cluster and the ALDL rely on the same source from the PCM?
I think I'd be inclined to check the VSS signal to the PCM first! There's only I'd think two pins to check lo in/hi in and then I'd guess it would be isolated to an internal PCM. If the PCM has been in/out frequently I think that would be more likely than an internal problem.
I don't have a '96 FSM and I've never investigated the "pin-outs" for a '96 but does the CCM, the cluster and the ALDL rely on the same source from the PCM?
I think I'd be inclined to check the VSS signal to the PCM first! There's only I'd think two pins to check lo in/hi in and then I'd guess it would be isolated to an internal PCM. If the PCM has been in/out frequently I think that would be more likely than an internal problem.
) are from two different PCM terminals. The magic that distributes the VSS signal happens inside the PCM.The PCM has never been off the car, only moved around on occasion.
Going to figure out a way to test the VSS wiring to the PCM as well as check for corrosion. I also posted to see if someone local has a '96 PCM they can loan me.
Le Mans Master
Joined: Aug 1999
Posts: 7,337
Likes: 137
From: WilkesBarre PA
Mine does this sometimes also. If I start the car the stereo goes full blast if the SVC is on on the radio. Sometimes I see a high speed. Mine only seems to happen on a startup or comming to a stop. It did this with the stick and also the auto, with different sensors.
Someone awile ago thought it was interferance from the MSD box. Don't know if that is the case or not, I never really looked into it since it happens very rarely. I'll be curious to see if you find what it is.
Someone awile ago thought it was interferance from the MSD box. Don't know if that is the case or not, I never really looked into it since it happens very rarely. I'll be curious to see if you find what it is.
Le Mans Master
Joined: Aug 2002
Posts: 6,079
Likes: 3
From: Clearwater FL
St. Jude Donor '06
My 96 LT4 does the same thing. Replaced the VSS; no change. Ususlly happens when inching forward or coming to a stop at less then 10 mph. Speedo will jump up to 220+. Does wonders for the VSS radio which I leave off now. Wonder if this is a LT4 only issue which would have something to do with OBD2 possibly?
Corvette Stories
The Best of Corvette for Corvette Enthusiasts
2027 Corvette vs The World: Every Model vs Closest Competitor
Joe Kucinski
10 Most Common Corvette Problems of the Last 20 Years!
Joe Kucinski
5 MOST and 5 LEAST Popular Corvette Model Years in History!
Joe Kucinski
2027 Corvette Buyer's Guide: Everything You Need to Know!
Joe Kucinski
10 Things C8 Corvette Owners Hate (But Won't Tell You)
Joe Kucinski
10 Best Corvettes Coming to Barrett-Jackson Palm Beach 2026!
Brett Foote
Every Corvette Grand Sport Explained! (C2, C4, C6, C7, & C8)
Joe Kucinski
Grand Sport & Grand Sport X Launch Alongside All-New 535hp LS6 V8!
Michael S. Palmer
5 Reasons Bad Drivers Crash & 5 Ways to Avoid a Costly Mistake!
Joe Kucinski
Thread Starter
Race Director
Joined: Apr 2000
Posts: 15,344
Likes: 4,008
From: Texas
Hope to know something by next week. A loaner PCM is on the way which will prove/disprove my theory.
I noticed this happening a couple times last year but it was so quick that it was one of those "did I just see that?" sort of moments. Now, after 20 minutes or so of driving, the speedo will start acting up.
Team Owner
Joined: Sep 2003
Posts: 25,331
Likes: 2,722
Since there's a few of you that have related "similar" experiences I tossed a couple/few random GS VIN's here and there looks to be maybe a calibration change available or maybe a "rolling change" in production that the LT4's have seen.
Go here: http://tis2web.xw.gm.com/tis2web
and enter your VIN to check the available calibrations. Cars #007, 857 & 016 all show the same two. Car # 001 shows a different calibration. I didn't check the cars for matching options and I meant to inquire about the #'s for 007, 857 & 016 today but the workplace was fairly busy.
It would be interesting to have the calibration for a couple base LT4 cars for comparison. Perhaps "C4DC" will post his.
If you've access to a Tech2 or other OBD2 scanner you could maybe check the calibrations recorded in the PCM's! Maybe the PCM just needs a "flash"!!!!
I believe I've read where it is "suggested" that you don't attempt to "re-flash" a PCM with the same calibration. The likely hood of it being corrupted are reported an "slim"! I don't really know! A good question for some of the "tuners" out there maybe!!
Go here: http://tis2web.xw.gm.com/tis2web
and enter your VIN to check the available calibrations. Cars #007, 857 & 016 all show the same two. Car # 001 shows a different calibration. I didn't check the cars for matching options and I meant to inquire about the #'s for 007, 857 & 016 today but the workplace was fairly busy.
It would be interesting to have the calibration for a couple base LT4 cars for comparison. Perhaps "C4DC" will post his.
If you've access to a Tech2 or other OBD2 scanner you could maybe check the calibrations recorded in the PCM's! Maybe the PCM just needs a "flash"!!!!
I believe I've read where it is "suggested" that you don't attempt to "re-flash" a PCM with the same calibration. The likely hood of it being corrupted are reported an "slim"! I don't really know! A good question for some of the "tuners" out there maybe!!
Last edited by WVZR-1; Aug 3, 2009 at 09:15 PM.
Thread Starter
Race Director
Joined: Apr 2000
Posts: 15,344
Likes: 4,008
From: Texas
WVZR-1, appreciate the homework! FWIW, my calibration is customized using LT1 Edit Granted, could be something in the base file that LT1 Edit doesn't see/touch. Having said that, it's worked fine for 13 years and it has the original PCM
Today I hit a new top speed....in 1st gear
Granted, could be something in the base file that LT1 Edit doesn't see/touch. Having said that, it's worked fine for 13 years and it has the original PCM Today I hit a new top speed....in 1st gear
Le Mans Master
Joined: Aug 2002
Posts: 6,079
Likes: 3
From: Clearwater FL
St. Jude Donor '06
I have used LT1 edit as well. Did the fan temp mod, 3.91 gear correction, ASR desensitization, and tried some timing changes, but left it stock as it was as good as it could get with the stock H/C.
16230041 PN
Cal ID #s 16229910 & 16229909
16230041 PN
Cal ID #s 16229910 & 16229909
Thread Starter
Race Director
Joined: Apr 2000
Posts: 15,344
Likes: 4,008
From: Texas
Tried the loaner PCM today and after 25 minutes or so of driving....I got the same symptoms 
The last two days I've been getting a P0500 code with my original PCM. P0500 is the VSS.
It'll be a bit due to work, but the next step will be to recheck the wires and then see if the (new) sensor is bad. I will admit that since I wasn't 100% sold on it being the VSS the first time around, I bought the cheapest replacement I could find. I really hope it's not the CCM.
It does seem to be heat related since I have to drive it for awhile before it starts acting up. Of course that really makes it a pain to troubleshoot.
The last two days I've been getting a P0500 code with my original PCM. P0500 is the VSS.
It'll be a bit due to work, but the next step will be to recheck the wires and then see if the (new) sensor is bad. I will admit that since I wasn't 100% sold on it being the VSS the first time around, I bought the cheapest replacement I could find. I really hope it's not the CCM.
It does seem to be heat related since I have to drive it for awhile before it starts acting up. Of course that really makes it a pain to troubleshoot.
Thread Starter
Race Director
Joined: Apr 2000
Posts: 15,344
Likes: 4,008
From: Texas
So just a quick update. As mentioned the symtoms remained with the loaner PCM. I tracked through the wiring and it all passes....5v on the VSS high circuit, VSS low is a ground. Insured all the connectors were clean, flexed the wires, etc.
Soooo, my conclusion is I have a bad VSS. I called Chris May at Superior Chevrolet today and have a new AC Delco unit on the way vs buying another AutoZone special. We'll see how it works out. Probably be next week before I get around to installing it.
Soooo, my conclusion is I have a bad VSS. I called Chris May at Superior Chevrolet today and have a new AC Delco unit on the way vs buying another AutoZone special. We'll see how it works out. Probably be next week before I get around to installing it.
Melting Slicks
Joined: Oct 2006
Posts: 2,238
Likes: 10
From: College Station TX
I think a good test would be to log data from the PCM with something like Datamaster and see if the PCM thinks the speed is going crazy, or the data to the cluster from the CCM is just wrong. The PCM will still output the speed to Datamaster even if it has no connection to the CCM (don't ask how I know that).
Thread Starter
Race Director
Joined: Apr 2000
Posts: 15,344
Likes: 4,008
From: Texas
I think a good test would be to log data from the PCM with something like Datamaster and see if the PCM thinks the speed is going crazy, or the data to the cluster from the CCM is just wrong. The PCM will still output the speed to Datamaster even if it has no connection to the CCM (don't ask how I know that).
"During one of the episodes, I happened to be logging data. The speed output in the log file bounces around"
Had to give you a hard time
In all seriousness, the fact that the PCM data is messed up points back to the sensor vs the CCM as you pointed out
Melting Slicks
Joined: Oct 2006
Posts: 2,238
Likes: 10
From: College Station TX
From my original post...
"During one of the episodes, I happened to be logging data. The speed output in the log file bounces around"
Had to give you a hard time
In all seriousness, the fact that the PCM data is messed up points back to the sensor vs the CCM as you pointed out
"During one of the episodes, I happened to be logging data. The speed output in the log file bounces around"
Had to give you a hard time
In all seriousness, the fact that the PCM data is messed up points back to the sensor vs the CCM as you pointed out