I've been having an intermittent issue (seems to occur more on rainy days but not definitive on that yet) with tachometer.

Upon starting the truck, for the first ~10mins or so of driving the tachometer is dead (reads 0) but will come rise up to ~2k rpm when I depress the brake...weird

After some time it comes back as normal.

Here is what I know about how the system works. Please check my work.
- RPM counts are sensed by the ECU via the negative contact on the distributor coil. When I hookup Rovergauge I can see RPM just fine.
- Separately, the tachometer is driven by interpreting rotations of the generator/alternator and converted to voltage turning the dial somehow within the cluster
- I believe there is no connection between ECU RPM and tachometer RPM

Rovergauge is also telling me I have 14V at the ECU so I believe the alternator is working fine.

Here is what I have checked so far:
- Wires on the back of the alternator, which I have re-tightened and applied di-electric grease to.

I was about to go hunt down the ground for the system and tighten it, but after reading the ETM, the ground is shared by lots and lots of other stuff including other parts of the cluster and fusebox and the radio etc which are all working fine, so feels like that is probably not it...

Next step I guess is to chase the wire from the alternator over to the tachometer and make sure there isn't anything funny going on? I have a sneaking suspicion the issue might be in the alternator itself and the rainy weather and condensation is causing issues with the signal pickup which then burn off after 10mins of use.

Anyone else been down this road?

Note - title should have been "alternator driven tachometer puzzle"

 

Prior to my 94 alternator dying, when the tach signal failed, I noticed the battery symbol glowed very faintly in and out, it could only be seen in the dark.

I bought a rebuild, but looking back I should have tried replacing the brushes and kept the original alternator.