Speedometer interchangability
 

I have a '90 240 that keeps throwing the 3-1-1 code which is no speed signal to the ECU. Also, the cruise control doesn't work. Both the speed signal to the ECU and the cruise control come from the speedometer. The speedometer and odometer both work fine. Earlier this year, my sons were re-installing the instrument cluster and they didn't know where the red/white tach wire goes to (I don't have a tach). Anyway, they hooked it to the same connector as the yellow wire that goes to the cruise control unit.

Assuming that they must have fried something on the circuit board in the speedometer, I pulled a instrument cluster from the junkyard. However, the one I pulled was from a '91. Apparently the circuit boards were changed in '91 as there are visible differences in both the instrument panel circuit board as well as the speedometer circuit board (the one that drives the odometer motor).

The '91 speedometer doesn't work when installed in my '90. Does anyone know if the sending unit in the differential was changed as well? Also, does anyone have any ideas on how I could bench test the speedometer?

The speedo sender did not change. 86 to 89 are pretty much the same, guts wise. Beginning in 90, speedos changed up depending on whether the car had ABS or not. I'd keep an eye out for a 90 on eBay that was from a car set up as is yours. Non-ABS cars have a 12 tooth tone ring in the rear end whereas the ABS cars have 48 teeth. To bench test, you need specialized equipment I've been told. As it receives a pulsed signal from the rear end I believe you need an oscilloscope to correctly diagnose them.

Yeah - to really bench test correctly (like to calibrate), yes a signal generator and oscilloscope are the way to go. I kinda did it cheaply. I just used an old AC adapter I had laying around that put out 9v AC 60Hz as my signal input. Then I used my voltmeter in AC mode to determine if I had a signal out on the taxi meter pin.

Kinda crude but it gives you a go/no-go sort of test. Really that's all I needed since I wasn't calibrating.