I'm having a strange electrical issue with my 90 Civic. Yesterday while driving, I noticed that my tachometer needle was starting to act crazy and bounce way down, sometimes all the way down. It was raining, so I also had my windshield wipers on and set to the slow setting (not intermittent). When the tachometer needle started acting up, my windshield wipers would speed up noticeably faster. This happened for 5-10 seconds, then stopped, and repeated a few times. Over the last few weeks, I thought I had noticed in my peripheral vision that the tachometer needle had flicked down, but by the time I looked, it was back to normal every time. Also, when the tachometer needle started going crazy, the performance of the vehicle was unaffected (no bucking) - making me think it isn't something directly related to the distributor. There were even a few moments lasting a second or two when the needle dropped all the way to zero RPM, still with no hesitation / bucking.

Here's the status of the car/parts:

1990 Civic Sedan DX
DPFI to MPFI conversion 1 month / 2K miles ago - I highly doubt it is anything to do with the wiring on this...it was meticulously executed and has been running good, but I'd be happy to discuss.
OBD0 / PM6 ECU
Alternator was replaced almost 7 years / 105K miles ago with a new unit
Distributor was replaced 1 month / 2K miles ago with a new unit

I'm wondering if this might be something wrong with the voltage regulator in the alternator, since two unrelated systems are affected.

I'm also wondering if it might be something wrong with the C102 / C211 connector - this is the one on the passenger shock tower that has the reddish-brown soft plastic cover over it, making it hard to get apart. The connectors were loaded with what I assume is some kind of dielectric grease - with the car being 28 years old, could it be time to clean out the old grease and re-pack it? The reason I suspect this connector set is because both the alternator and distributor have power wires running through this connector.

I'd appreciate some input from folks that may have seen this issue before.

 

It could very well be something with the alt. Assuming you have some basic garage tools, I would backprobe the alternator and connect to a multimeter, and see if you could reproduce the issues and see what the alt is doing. If you suspect you have some oxidation on the connectors, you can always try a spray called deoxit. You can find it on amazon, or maybe even locally. I have used it before, and it works very well.

 

There is a test procedure for the voltage regulator in the workshop manual.

Your problem also made me think grounding issue.

 

Yeah, I plan on going though that, but with it being a VERY intermittent problem, I'm worried it may not present an issue when I test it. We'll see. I'll also check grounds again. Thanks for the input.