Erratic Speedometer (92)
Thread Starter
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Joined: Sep 2010
Posts: 71
Likes: 2
From: Jacksonville FL
Erratic Speedometer (92)
As of July 2010 I was doing burnouts (asr off) and sheared my ring and pinion to pieces. (automatic trans)
Used dealership replaced it with stock 2.59 ring and pinion with the drivetrain warranty.
(previous owner may have had a larger ratio installed but i have no documentation)
got pulled over because i was going 75 (speedo read 51)
compared to GPS readings
1st gear -5 mph to actual
2nd gear -10 mph to acutal
3rd gear -15 mph to actual
4th(OD) -20 mph to actual
didnt make any time to get it looked at
now after I'd say 6 months the speedometer now just shows random numbers
ex:
in park 0-10
driving @25 - 15-35
driving @65 - 55-70
*thinking about trying to find new wheel speed and transmission speed sensors** but something tells me that won't do anything.
any help would be GREATLY appreciated!
Zach
Used dealership replaced it with stock 2.59 ring and pinion with the drivetrain warranty.
(previous owner may have had a larger ratio installed but i have no documentation)
got pulled over because i was going 75 (speedo read 51)
compared to GPS readings
1st gear -5 mph to actual
2nd gear -10 mph to acutal
3rd gear -15 mph to actual
4th(OD) -20 mph to actual
didnt make any time to get it looked at
now after I'd say 6 months the speedometer now just shows random numbers
ex:
in park 0-10
driving @25 - 15-35
driving @65 - 55-70
*thinking about trying to find new wheel speed and transmission speed sensors** but something tells me that won't do anything.
any help would be GREATLY appreciated!
Zach
Drifting
Joined: Jun 2010
Posts: 1,853
Likes: 8
From: Westerly RI
Seems like you have two problems- the linear speed error is due to the wrong speedometer gear in the trans. The erratic display is a different issue- do you have "sys" flashing on the dash? The speedo display is controlled by the CCM, you could check it for codes. This is done by using the trip computer buttons, section 8D in the FSM, or try "search" on the Forum.
Race Director
Joined: Apr 2000
Posts: 15,344
Likes: 4,008
From: Texas
The error is due to the wrong gears in the transmission. There are two...one that goes on the output shaft of the transmission and one on the VSS. You can go to any Chevy dealer and ask them for the gears for your particular setup. The gears used to cost ~$25.
The random speedo readings are typically due to a failing VSS. Replace the existing one with an AC Delco unit and you'll be fine. Many people have had issues with the cheaper aftermarket units thru Autozone, NAPA, etc, hence the reason to stick with AC Delco.
The random speedo readings are typically due to a failing VSS. Replace the existing one with an AC Delco unit and you'll be fine. Many people have had issues with the cheaper aftermarket units thru Autozone, NAPA, etc, hence the reason to stick with AC Delco.
Le Mans Master
Joined: Jul 2009
Posts: 6,381
Likes: 66
From: SF bay area C.A.
C4 of the Year Finalist
This isn't an answer to your post but I had to bring it up. When coming home from our high speed autox fun day I looked at my speedo And it read 156. I know I wasn't going quite that fast so I figured I would have to fix something again.I also noticed the other readouts were very strange. Somehow during the event I hit the metric read out button, it took me a few to figure this out because of being so tired. 
Thread Starter
Advanced
Joined: Sep 2010
Posts: 71
Likes: 2
From: Jacksonville FL
No SYS light flashing. Only codes I got were C12 and *H32 not sure at the moment*
So I do need a new Vehicle Speed Sensor? aka
Transmission Speed Sensor?
*correction
the erractic speed display is only during acceleration. Stays steadily off at the described speeds in my original post.
But whats this about the "gears from the output shaft of the transmission"?
So I do need a new Vehicle Speed Sensor? aka
Transmission Speed Sensor?
*correction
the erractic speed display is only during acceleration. Stays steadily off at the described speeds in my original post.
But whats this about the "gears from the output shaft of the transmission"?
Safety Car
Joined: Jun 2006
Posts: 3,629
Likes: 28
From: FL
You indicate the previous owner likely changed rear axle ratio, when doing that you need to change the speedometer drive gears in the transmission to correct the speedometer reading.
If the 2.59 stock gears were changed to 3.07 it would only have required replacement of the driven gear in the transmission, if lower gears were installed however such as 3.45 or 3.73 you would need to replace both drive and driven gears to correct.
Original speedometer gears should have been a 17 tooth red drive gear and a 35 tooth orange driven gear, you can examine them when replacing the speed sensor on the transmission tailshaft housing.
If the 2.59 stock gears were changed to 3.07 it would only have required replacement of the driven gear in the transmission, if lower gears were installed however such as 3.45 or 3.73 you would need to replace both drive and driven gears to correct.
Original speedometer gears should have been a 17 tooth red drive gear and a 35 tooth orange driven gear, you can examine them when replacing the speed sensor on the transmission tailshaft housing.
Thread Starter
Advanced
Joined: Sep 2010
Posts: 71
Likes: 2
From: Jacksonville FL
Race Director
Joined: Apr 2000
Posts: 15,344
Likes: 4,008
From: Texas
No SYS light flashing. Only codes I got were C12 and *H32 not sure at the moment*
So I do need a new Vehicle Speed Sensor? aka
Transmission Speed Sensor?
*correction
the erractic speed display is only during acceleration. Stays steadily off at the described speeds in my original post.
But whats this about the "gears from the output shaft of the transmission"?
So I do need a new Vehicle Speed Sensor? aka
Transmission Speed Sensor?
*correction
the erractic speed display is only during acceleration. Stays steadily off at the described speeds in my original post.
But whats this about the "gears from the output shaft of the transmission"?
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Drifting
Joined: Jun 2010
Posts: 1,853
Likes: 8
From: Westerly RI
Then, if the VSS is bad, the CCM should set a code 24. The CCM is read thru the trip buttons, not from the ALDL connector under the dash. I would not change parts at random, hoping for a win.
Race Director
Joined: Apr 2000
Posts: 15,344
Likes: 4,008
From: Texas
There was nothing wrong with the accuracy of mine once the VSS was replaced. The point of the post and the picture was to illustrate that a bad VSS will cause odd readings even though there was no trouble code(s) set. Contrary to popular belief, not all issues with a component will set a code.
You have two things to solve:
1) You have an accuracy issue due to wrong gears on both the transmission output shaft and the and VSS *or* the tune was changed to reflect the gearing used by the prior owner.
2) Your VSS is bad and needs to be replaced.
The fix is as simple as everyone has been telling you:
- Verify the need to go buy the right gears by pulling the old gears and counting the teeth. Install new gears.
- If you pull the gears and find they were correct for your current rear end gears, then you have a tuning issue and you'll need to do some math to figure out which gears will work with the existing tune.
- Go buy a new VSS.
If the above is not clear, then I'm sorry....take it to a mechanic.
Thread Starter
Advanced
Joined: Sep 2010
Posts: 71
Likes: 2
From: Jacksonville FL
There was nothing wrong with the accuracy of mine once the VSS was replaced. The point of the post and the picture was to illustrate that a bad VSS will cause odd readings even though there was no trouble code(s) set. Contrary to popular belief, not all issues with a component will set a code.
You have two things to solve:
1) You have an accuracy issue due to wrong gears on both the transmission output shaft and the and VSS *or* the tune was changed to reflect the gearing used by the prior owner.
2) Your VSS is bad and needs to be replaced.
The fix is as simple as everyone has been telling you:
- Verify the need to go buy the right gears by pulling the old gears and counting the teeth. Install new gears.
- If you pull the gears and find they were correct for your current rear end gears, then you have a tuning issue and you'll need to do some math to figure out which gears will work with the existing tune.
- Go buy a new VSS.
If the above is not clear, then I'm sorry....take it to a mechanic.
Im sorry for the "lack of understanding"
whenever I get the time and resources to fix this I will.
I guess it would be safe to say that this thread is closed.
Thread Starter
Advanced
Joined: Sep 2010
Posts: 71
Likes: 2
From: Jacksonville FL
Seems like you have two problems- the linear speed error is due to the wrong speedometer gear in the trans. The erratic display is a different issue- do you have "sys" flashing on the dash? The speedo display is controlled by the CCM, you could check it for codes. This is done by using the trip computer buttons, section 8D in the FSM, or try "search" on the Forum.
Last night I did notice that the dash started displaying SYS.
What does this mean?
Drifting
Joined: Jun 2010
Posts: 1,853
Likes: 8
From: Westerly RI
This means that the CCM (Central Control Module), not the ECM, has a stored fault code. You can read it yourself without tools, through the trip computer buttons. If you can't find the instructions on "search", I will post them for you.
