1G 1991 twitching tach gauge mystery

[greekgodchaos]

I have a 91 GSR-4 with a jumping, very weakly moving, tach gauge. At idle, the needle twitches just above ~200rpm and doesn't move when you rev the engine. It sometimes would rise with RPMs while driving, but would just plummet once the RPMs started going down. I so far have swapped 2 (both new) power transistors, 3 instrument clusters (all 91), 4 working CAS, a new coil pack, and new Denso plug wires (cheap ones were arcing and causing a misfire). I ran new wires that go from the transistor to the ECU and the ECU to the tach gauge, so as to assure no possible with damage. The connector pins for the instrument cluster have been cleaned and bent the pins to make solid contact, same with the ECU plugs. The ECU board is clean (WD40 circuit board cleaner ftw) and has had new caps installed a few years back. I have toured around the engine bay and jiggled all the possible connectors that could have any relation to the issue, without anything odd happening.

This problem is also causing my RPM switch to trigger randomly as the switch is seeing an erratic signal that bounces around anywhere from 0 to 9900; causing my actuator on the cyclone manifold to open and shut the runners sporadically.

However, if you log it in ECMLink, guess what? You get a smooth signal.

I have already ordered a new RPM switch (Jegs not that good I guess) by Summit Racing so I'll install that on Thursday and see what happens. I also did attempt disconnecting the tach signal wire to the RPM switch, so as to be sure it wasn't somehow affecting the tach gauge circuit: no difference.

Has anyone installed tach noise filters for 91+?

At the moment I have no idea where to go from here. Any ideas?

 

[greekgodchaos]

It's always funny how whenever I post up a thread, less than 10 minutes later I fix the problem.

The problem was the tach and RPM switch were getting their signals from pin 109 when they should have been getting them from pin 21.

 

[steve]

The factory tachometer is normally fed from 109 but I can't say the RPM switch should be. As I'm sure you know, pin 21 is the crank signal from the CAS and is what the ECU uses for RPM. I hope whatever you connect doesn't load down the signal.