digital speedo problem (only shows 0 mph)

[SHIFTrl240]

i bought a new digital cluster for my 91 240 coupe. i bought it off of someone that must have had a sohc motor in it because after i hooked it up the first time the tach would not work, also the digital display will only read 0 no matter how fast you go. i figured it had something to do with the tach not working so i swapped the sohc tach with the dohc tach out of my old cluster. after that the tach would work but the speedo still only shows 0 mph. anyone have any clue on what i should check? do the speedometers from a sohc work on a dohc or is it the same as the tach, it dont work? or is it something else? thanks for the help!

I also tried putting the new digital speedo unit and the old tach into the old cluster, but still does not register any speed. im thinking maybe the speedometer sensor, but i dont have a clue where it is or how to go about checking it.

btw, i did search.

thanks.

[NISTECH]

get a buddy to help you out first. have him sit in the drivers seat with the key on. you go under vehicle remove speed sensor off of tail shaft of trans. have a pan under the trans cause you will loose fluid. if you have to lift one end or the other of the car. recommend you lift the rear that way the fluid will move to the front of the trans and not spill as much. whith the sensor in hand spin it as fast as possible and have your buddy watch the display. this will assure the drive gear in the trans is not your problem. if that tests ok unplug your sensor and plug in a good one (that is if you know someone with a similar 240) and repeat test if it now works you know it was the sensor. if not now your problem lies in your instrument cluster. or harness.

[SHIFTrl240]

ok i think i found the sensor, but how do you get it out? i took out the single bolt but it still is tight in there.

[NISTECH]

grab it with a pair of channel locks and rotate it back and forth while pulling on it. it has a tight rubber oring helping keep it in

[SHIFTrl240]

ok i removed the sensor and spun it and it still showed no speed. unfortunately i dont know anyone else with a 240 in my area, so how much does that part run for? also, how do you take the sensor off the car? do you have to cut the wires? is there a fuse i can check? when i bought it the guy said it worked on his car, but it was an sohc. would that be a problem, like the tachs are different for sohc and dohc, is it the same with the digital display?

also, i dug up the old paperwork from my car and the guy who owned it before me took it to a dealer, they diagnosed the problem as the 'speedometer head' what exactly is this? i was thinking it was just the digital display, am i wrong?

thank you sooo much for the help!

[NISTECH]

ok i removed the sensor and spun it and it still showed no speed. unfortunately i dont know anyone else with a 240 in my area, so how much does that part run for?

not sure on how much it would run but i wouldnt suggest running out and buying it. that was only a test step to determine if it was bad or not.

also, how do you take the sensor off the car?

you follow the wires from it to a connector and disconnect it.

do you have to cut the wires?

no

is there a fuse i can check?

if it were a fuse it would most likely not light up at all

when i bought it the guy said it worked on his car, but it was an sohc. would that be a problem, like the tachs are different for sohc and dohc, is it the same with the digital display?

it may be they are different but i would only think that would be related to the tach exclusivly. if the tach works i think your ok

also, i dug up the old paperwork from my car and the guy who owned it before me took it to a dealer, they diagnosed the problem as the 'speedometer head' what exactly is this? i was thinking it was just the digital display, am i wrong?

correct the "speedo head" is the speedometer portion of the cluster. which you have already replaced.

thank you sooo much for the help!

note the color of the wires to your speed sensor at the connector i mentioned above. look at the harness side of the connector . remove the cluster and find the 2 wires feeding into the cluster that match those two wires. use a multi meter across the two wires on volts scale and rotate the sensor again and see if you get a response on your meter. specifics are not important just as long as you get some sort of response indicates it is sending a signal. if you get nothing with your meter on the volts scale try ohms.

[SHIFTrl240]

ok i went and bought a multimeter and checked the voltage and ohms at two spots. the wires coming out of the speed sensor are solid black and solid green. i first found those two colors that were going into the speedometer and checked those. there was about 12v there, and no ohm reading. there was no change in the readings when i turned the sensor. i then checked the wires coming directly out of the sensor. down there were no voltage, but the ohm reading would go up when i turned the sensor. does this give you any ideas on what i should check for?

[NISTECH]

ok let me see if i understand what you said. you had a change at the sensor when you spun it? if so this means the sensor at the trans is sending a signal. now the 2 wires from that sensor should end up going to the instrument cluster find those same 2 wires and do the measuement again when spinning the sensor. you should see the same results if the wiring from the sensor to the cluster is good. it would really help to if you had a schematic of the circuit. I dont have one readily available to me or I could tell ya what 2 wires to check. but the wires in the connector at the speed pickup in the trans, on the main harness side of the connector, should be the same color as whats at your cluster.

[NISTECH]

do not use the wire colors on the sensor side of the connector cause those will be different then the colors on the main harness side.

[SHIFTrl240]

ok i figured it out and its working now what happened was that i didnt notice since the back wheels were up that the wire coming out of the sensor was so long before it clipped onto the harness.... the plug was just loose :bash i was kicking myself the rest of the day for that....

but at least now i have a cool working speedo with HUD! :ylsuper