Tacho Now Acting Funny

Well, as part of the saga of replacing my two oil pressure senders/sensors/transducers, after finding out that the “upgraded” version for the pressure gauge is not doing well (i.e. needle reads near the bottom instead of at mid-range all the time) my tech got the billiant idea maybe there is nothing wrong with the new sender (or the wire going to it) and possibly the pressure gauge itself was at fault. :angry:

After removing my instrument cluster and tearing into things in there, he did indeed find that there were what appeared to be brown “scorch” marks at two of the connections on the back of the pressure gauge (IIRC). Very fortunately, he was able to locate and obtain a used replacement gauge and put it in. However, same old story with the replacement gauge, so apparently the problem does lie with the new sensor/sender and/or the lead wire to it.

After putting everything back together and back in place (and after shooting most of the electrical connectors and ribbons in the back of the panel w. electrical contact cleaner spray), we went to turn the ignition on and the tach is totally out of whack (hey, that rhymed :laughing: ). Bottom line is that, with the engine idling (cold or hot), it wants to stay right @ 1,000 RPMs even (vs. 750 or so for hot idle). It does go up higher than that somewhat at higher rpms, but not like it should. When you go to shut the engine off, the needle will suddenly swing clockwise up to about 8K RPMs or so and then drop back down to right at 4K, where it remains until the engine is started again. So, despite the engine not even running, it’s @ 4K RPMs ? :crazy_face:

The tech tells me he has heard that tachs sometimes have to “relearn” their settings over time. I have never heard of such a thing. I have since then driven the car several times, and the needle still behaves as described above. He supposedly has also heard about some way to “reset” tachs. Again, something I have never heard of. I’m suspecting somehow he damaged it during the r/r of the oil pressure gauge (despite the neg. battery cable being disconnected as a precaution). I would think if the damage were electrical, the needled would still drop down to “0” RPMs when shut off (i.e. no power going to the tach). If mechanical, then that would explain it staying up like it does (@ 4K) w. no power applied. :thinking:

Thoughts on what is wrong with it, anyone?

Mine has been stuck on 7-800 rpm when off but moves OK when the engine is running, I think your tach meter is faulty, I need to get a new one too.

Being as it is the temperature gauge and the tach, and both after the R&R … could be a bad ground issue as everything depends on a good ground connection.

I have a bad instrument cluster ground on another project I have (whuch is waiting on me to finish with the Jag before I can get back on it), but I’ve got lots of other work to do on the project before I mount the body of one manufacturer onto the chassis from another manufacturer,and then get things sorted out and working properly.

It is actually the oil pressure gauge + tachometer. The full story is that when it appeared the upgraded sensor for the gauge was not acting like it should (i.e. reading mid-range as long as engine running and there is pressure) after it was installed then my tech suspected a bad gauge, and r/red it. When he first put the cluster back in after that, it was a major cluster - (well, yanno :poop: ) … no pun intended: the speedometer, tachometer, odometer, trip odometer and fuel gauge (along with the oil pressure gauge) were all inoperative. Even the left turn signal indicator light was not blinking (although I could hear the relay clicking and the light itself did actually flash) … I also noticed the indicator lights for the two “special” modes of transmission were not coming on whenever the rocker switch was placed in their respective positions. :open_mouth: The tech pulled the cluster again, and this time went to town with the electrical contract cleaner spray and making sure all the connections were firm and tight. That took care of everything except the tachometer (and the oil pressure gauge, of course :roll_eyes: )

This is what a ground issue can cause - see photos.

Instrument cluster

Ignition off (there is mo “key”, just the switching device itself)

Ignition on

Engine running

A lot of us put in a redundant ground for cluster. There are plenty of places around steering column frame for a ground.

A bit difficult to do right now, with everything in a bucket. LOL