Erratic Speedometer reading..

[Pointman0853]

I'm finally getting around to addressing this issue. Once started in Neutral, the speedo needle takes a big jump when 'drive' is chosen. Matter of fact, anytime a different drive mode is selected, the needle jumps...

Additionally, the speed shown is around 50% of what the GPS indicates. Am I looking at replacing the sensor on the torque converter housing?

Thanks in advance!

Pointman

 

[tennmogger]

Mine jumps around too. Bet your speedometer is showing the speed in km, reading the black scale, ratio 5/8. Look closely for an inner scale in red-faded-to-pink on the speedometer and compare that scale to the GPS. That red scale read in MPH on mine, at first, then changed.

Before you all think I am looney, my speedometer and odometer both jump around. When I first got the truck the speedometer read kph on the black scale and mph on the red scale. The odometer read miles accumulated. Verified many times by GPS.

Then the speedo needle bounced around a little and started reading MPH on the black scale, like it should. The odometer started recording kilometers accumulated. All this was verified by GPS and this condition continued for the entirety of a 4600 mile trip I just got back from Aug 3.

Yesterday morning the speedometer is reading KPH again, and the odometer records kilometers.

I changed the speed sensor on the transmission about 6 months ago with no improvement.

That sensor on the bell housing, left side, the one you can see when you check the engine oil dipstick, has not been changed on my truck. I thought that was an engine speed indicator for the transmission's use. Logic says this signal would not be related to road speed, doesn't it?

The speed sensor on the transmission would logically be the sensor at fault, it this is a sensor issue.

I mention all this because we may be fighting the same failure.

Bob

 

[Suprman]

Does the trans shift funny? Check the error codes on the trans keypad its pretty good about telling you whats wrong.

 

[tennmogger]

It's Pointman's thread but my transmission shifts just fine. When I first got the truck there was a shifting delay issue but idle was too high and wide open throttle rpm was too low. Adjustments were made to linkage, Allison read the transmission again, and no problems found. Speed, shift points, all look fine. To this day there has been no indication of trans issues, including when the speedometer decides to shift from metric to normal.

I had not thought to dump codes on the pad recently. Will do that next time I have it going.

BTW, I read that there is one model of speedometer that has programming switches. This speedo is not one of those.

Bob

 

[Shark Bait]

Mine does the jumping around sometimes too. Sometimes tapping gets it to stop, but I haven't pulled dash loose to check for a loose connection. Occasionally when it idles it'll show a few miles per hour causing it to rack up mikes. Lol. I know I. Not breaking any speed limits but would rather it not rack up un driven mikes.

Dave

 

[Young Jedi]

Pointman, have you resolved the speedometer issue? I'm having the same exact problem and I've tried just about everything imaginable. I noticed the speedometer doesn't jump around with a different TCU but still reads extremely low. I've tried swapping speed sensors and checked the harness and pdp a dozen times. No error codes shown on tcu. My vehicle is an early 94 with the white faced gauges.

 

[Suprman]

Unplug the ctis controller see if any difference. Any error codes on the keypad? Sometimes the dash components need a better ground. You can try running a ground wire to the trans computer to a good solid part of the dash underneath.

 

[Young Jedi]

I swapped out the gauge today and it worked. So hopefully the new gauge will solve this. I didn't have any error codes and grounded everything. I had tried everything ever mentioned on SS aside from trying another gauge.