Speedometer/Odometer issue, old yellow box calibrator.

[Gman094]

Recently bought my buddy's 96 80 series that has 4.88 gears and 35's. He's had intermittent issues with the speedometer the last two years where he'd be driving at like 30mph and the dash is reading 120mph. Bought the car on Saturday and have driven maybe 120-150miles since then but because the speedometer isn't functioning right, it's causing the odometer to roll over at like 5x what it should be. My friend said he had a yellow box installed behind the glove box about 16 years ago and I'm thinking the yellow box may be fried at this point. I ordered the lutzauto calibrator(before I knew he had the yellow box) because it seems like an easy install. Trying to decide what the best course of action is, whether to leave the yellow box in there and install the lutzauto when it arrives or remove it completely and install the lutzauto piece. The yellow box is 16 years old at this point so it wouldn't surprise me if it just died. Any thoughts/recommendations would be much appreciated.

-Will

 

[80t0ylc]

Recently bought my buddy's 96 80 series that has 4.88 gears and 35's. He's had intermittent issues with the speedometer the last two years where he'd be driving at like 30mph and the dash is reading 120mph. Bought the car on Saturday and have driven maybe 120-150miles since then but because the speedometer isn't functioning right, it's causing the odometer to roll over at like 5x what it should be. My friend said he had a yellow box installed behind the glove box about 16 years ago and I'm thinking the yellow box may be fried at this point. I ordered the lutzauto calibrator(before I knew he had the yellow box) because it seems like an easy install. Trying to decide what the best course of action is, whether to leave the yellow box in there and install the lutzauto when it arrives or remove it completely and install the lutzauto piece. The yellow box is 16 years old at this point so it wouldn't surprise me if it just died. Any thoughts/recommendations would be much appreciated.

-Will

Yeah, you need to find the Yellowbox control unit. It's adjustable, but it's possible to be malfunctioning if there is that big of miscalculation. Definitely disco the adapter harness at the transfer case. and reconnect the OEM harness. to see if the Yellowbox is the culprit. Your ECU uses a speedometer input, so the erroneous mph will affect your 80's performance in fuel mileage and engine smoothness.

Edit: Corrected speedometer connection to drive train.

 

[Gman094]

This may be a silly question but I thought the yellow box was installed behind the glove box and not under the car by the transmission. I have to look around tomorrow to see if I see it. Assuming it’s the yellow box, I imagine the OEM harness will work but still be off but by a smaller amount, is that a fair assessment. I need to look at the wiring guide for the yellow box tomorrow and familiarize myself with where it’s located.

 

[80t0ylc]

This may be a silly question but I thought the yellow box was installed behind the glove box and not under the car by the transmission. I have to look around tomorrow to see if I see it. Assuming it’s the yellow box, I imagine the OEM harness will work but still be off but by a smaller amount, is that a fair assessment. I need to look at the wiring guide for the yellow box tomorrow and familiarize myself with where it’s located.

For my Yellowbox, (approx 8 yrs old) there is a control unit mounted ideally somewhere in the cab. A harness (3 or 4 conductor cable) goes from the control unit though the floor to the transfer case where the OEM speedometer connector is. My '94 is on the driver's side (IIRC). The Yellowbox harness end has an adapter that you install between the OEM speedometer connector and the transfer case connector. You can just pull the Yellowbox adapter out and replug the OEM speedometer cable back into the transfer case and see if you get a more reasonable reading on your speedometer.

My Yellowbox is currently disco'd because it got damaged and I just haven't repaired it. I live with the incorrect MPH, because my GPS gives me the correct speed. I'm running 35's with 5-29 gears, so the Yellowbox was nice, but not totally necessary. Although for best performance, you do want to have accurate mph fed to the ECU.

Edit: The speedometer connector is on the transfer case which is directly behind the transmission. I just seem to remember it being on the driver's side near the rear of the case.
2nd Edit: Corrected location in previous edit and text of post.

 

[80t0ylc]

Confirmation - The speedometer connector on my '94 is on the transfer case near the rear output shaft, driver's side. If you ignore the dirt and minor fluid leaks in the following pics, it might be helpful if your '96 is in the same place. Also, the skid plate is custom fabbed by @CreeperSleeper and will be different than the OEM skid plate.

Edit: went back to previous posts and corrected the location reference.