During normal driving my tach is easily bottoming out at 7000rpm. Could this be caused by a faulty wire somewhere? I don't know where the tach gets it signal from, the repair manual suggests it gets it from the coil or the distributor but I can't be sure as it's a pretty vague manual and doesn't go into much detail.

I have an automatic transmission, so the tach not working right isn't neccecarily that dangerous, I guess it's more annoying than anything but I'd still like to get it fixed. It stopped working right after being parked outdoors for quite some time.

Thanks
Justin

 

There is a white wire from the wiring harness going to your coil (whether it's an earlier style with the coil built into the top of the distributor or a later style with a separate coil mounted to the intake). This wire drives the factory tach just like if you had an aftermarket one. Same deal.

If the motor runs good (no missing/popping/breaking up) but the tach reads wrong then the problem is almost certainly the tach itself.

 

These tachs are almost all already failed. I have 3 or 4 of them, only one of which is anywhere near accurate. All the rest read high. It is very common for them to read twice the real RPM, and not at all uncommon for them to read many times reality. So your situation is somewhat typical. Probably not the wiring, since it's so simple; but you should always check what you CAN repair, before jumping to the conclusion that something you CAN'T is bad.

The actual part that fails is a thick-film resistor network that's factory-trimmed to calibrate it. I suppose it would be possible to cut it all the way through and solder an appropriate resistor in its place, but I've never tried that.

 

Well my engine hasn't been running all too smooth lately, this afternoon I pulled all my plugs to check to see how they were burning, I found carbon deposits on all of them and my car has always run noticeably rich. Anyway, when I was pulling my plugs, one of the boots was serverely hardened due to heat, being right by one of the header downpipes I guess I should call it, so the plug wire snapped out of the boot when I pulled it off, so that may be why it's been running rich and missing, I'll find out when I get a new boot for the wire.

This couldn't have anything to do with the tach reading wrong, right?

 

Not likely.