Jumping tachometer

I have a 96 taurus and my tach recently went from "Jumping erratically" to totally inop. Does anyone have any clues if one could troubleshoot what feeds the tach or anything that sends a signal to the tach? I don't want to just throw one in it and hope it cures the problem...just wondering if anyone else had had this happen...thanks Dave

The tachometer gets its feed (through a grounding circuit) from the Powertrain Control Module (PCM).

The first thing I'd suggest you do is have the PCM scanned for diagnostic trouble codes. If your car has some stored, post the exact codes here and that might help indicate a direction to begin.

Without codes, probably the quickest way to start tracking the problem down would be through the use of an oscilloscope/lab scope. If you have access to one, apply battery power to one lead and the other lead on the tan/yellow wire for the tachometer (pin 8 at connector 251 - instrument cluster; pin 48 at C191 of the PCM). With the engine running, you should see a signal that changes with engine speed.

If you see a changing signal at the tach, then you most likely have a bad tachometer, assuming you have verified you have battery voltage to the other end of the guage coil.

If you don't see a changing signal, and you are not already measuring the signal by backprobing the wire at the PCM, see if any signal is coming from the PCM. If there is signal at the PCM, but not at the tachometer itself, then you need to carefully inspect the wiring between the PCM and the instrument cluster.

If there is not signal at the PCM connector either, then you need to determine what sensor or sensors is/are not providing a proper signal to the PCM. Most likely, this would be either the Camshaft Position Sensor (CPS) or the Crankshaft Position Sensor (CKP). Of the two, the CKP would probably prevent the car from starting, so start first with the CPS. The scope will help you test these as well.

-Rod

Thanks for the suggestion. I was hopeing for maybe a "pull this plug or pull that plug" and see if the connection might be loose or not making connection...I was not sure where anything that fed into the tach are even located.........I will keep looking but I appreciate the possible causes...Dave

The first thing I would do is use an ohmmeter to test the continuity of the tachometer actuator (this moves the needle), as well as the wiring to the PCM. I would bet that it jumping, and then dead would be an intermittent connection. Fortunately, taking off the instrument panel isn't that hard. I would bet that if a sensor wasn't working right, the car wouldn't run worth a crap, if it did run at all. I would be very supprised to even find it would run with a bad sensor, and doubly supprised if you don't have a check-engine light if there is a bad sensor. "Pull this plug or pull that plug" would be where my money would be on this one.

I'm betting that the problem is a bad connection. Let's chase down the PCM. It is on the passenger's side and about 3" under the cowl. It looks like a sideways-mounted "vial" and has a large blocky connector on it, and a ground ribbon (flat wire) right by it. Disconnect that connector and ohm out between the red/yellow wire and the tan/yellow wire on the side going into the vehicle. You want a resistance, but a dead-short and a wide-open will tell you about the problem. If you get a resistance, take off the instrument panel and use a battery (a 6V, a 9V, a AA, or something like that) to see if you can get the tach to move. Once you're done, you'll have to "retune" the PCM to the engine, which is easy. Once you hook it up again, start the car up and let it idle in park for about 60 seconds. Then, shift it in gear for about 60 seconds and you're done. It will take this time to try different settings and determine how the engine responds. The PCM has to literally learn what the engine does.

Well the car still runs like a champ...despite the 190.000 on the odometer. I have a couple of light bulbs burned out on the instrument cluster anyway so I might as well pull the cluster, and reseat the plug on the back. If the tach comes out of the cluster relatively easy, I may look on flee bay for another tach and throw one its way. I have heard "yes and no" as to the ease of removing and replacing the tach so I will have to look for myself. As far as the PCM an other electronic troubleshooting....that may have to wait for the confidence to do it, or the money to pay for someone who can guarentee thier work...Thanks for the help agian...Dave

Well I pulled the instrument cluster and attempted changing out the tach alone. Upon inspection of the old tach, when you manually move the needle, it rises and falls at a slow steady pace. The "new" tach, which is out of a 97 acts the same way. When I start the engine and manually move the needle up it "speeds" down and slams into the stop. I removed the old and installed the new one, and it is dead upon starting the engine and with the engine off, it moves down slowly and smoothly when manually moved, but with the engine on, it goes up and slams down against the stop...any other suggestions? I am not sure where the cam position sensor or the crankshaft position sensors are, but the cam position sensor was changed recently and just before the problems started....maybe if I could unplug and replug the connector I would have some luck...Thanks again...Dave:banghead: