Tach works intermittently after driving in rain

[oneblueyedog]

78 911SC

What am I missing?

My tachometer is malfunctioning. It happened after I was caught in a heavy downpour. The tach doesn't function during start up. It then springs to life after driving a while. The readings are accurate sometimes, or they are a few hundred RPMs less at idle and through the range. When the readings are off, it seems as if the tach wont go over 4000 but I did not want to risk over rev.
Then with no warning, the needle will drop dead to zero while driving.

The needle is dead usually when starting up. Then comes to life after a few miles, but not always.

I've cleaned the grounds in the trunk, engine compartment, transmission and in the cockpit behind the fresh air levers.

I've cleaned every terminal on the Tach.

I've cleaned the 6 pin connector terminals on the cdi box.

My voltage across the battery terminals with engine on is 13.9V

I tried to measure the BLK VIOLET wire to TD on the Tach in AC voltage mode and it's .5xx and this voltage varies when revving the car. It counts down in voltage.

I checked, cleaned and spread the 14 pin connector in the engine compartment.

I plugged and unplugged the 3 connectors in the cockpit that go through the firewall.

This has been going on for a week. The climate is dry now. The car has been kept dry.

The tach illuminates normally except when the revs are registering low on the meter when it comes back to life. The turn signal arrows seem to both glow until you use the blinker.

The car runs well regardless.

The CDI box has been tested and verified as of last year.

It's been a few years since I replaced the VR (6yr?). No warning lights.

Thanks for any help.

[wwest]

Poor ground return somewhere in the intrument power supply or illumination circuit.

[ischmitz]

To measure the tach signal from the CDI box you need a termination resistor. Use a 1 - 5 kOhm resistor between the tach signal wire and +12V and then observe a square wave. Without that resistor you won't see munch.

[wwest]

Wiper motor arm movement wearing through wiring insulation?

[oneblueyedog]

Baffling. I'll try these suggestions out.

"The tach illuminates normally except when the revs are registering low on the meter when it comes back to life."

I support wwest's opinion. To me this points to a bad ground connection of the instruments. Check the brown wires and connectors and clean its contact to ground.

However, that the instrument starts reading when driving for some time, lets me think of water/humid, dirt or even some corrosion - or a combination of these - in the instrument itself.

Otto

[oneblueyedog]

To all,

Thanks for the suggestions.

I'm out of town but tracing the wiring diagram, the wire vi/blk, r/wht going to TD on the tach relates to the ground coming off the "speed switch". I'm not sure where it is. A thread mentioned it in the trunk. It is connected to the fuel pump relay. It does say that the speed switch is grounded to G2. I don't have my Manual but I think it is still the ground terminal for under the heater controls. I will trace that ground, but I think that it may be the instrument. The fuel and oil pressure gauges fogged up nicely that day. The tach did not but that doesn't mean anything.

All of the other grounds coming off of the Tach seem to relate to illumination.

[oneblueyedog]

This weekend I traced the ground from the limiter switch to the ground terminal inside the trunk behind the fresh air vent valve. You have to remove the upper duct and the side AC tube.

There were many wire terminals under the nut. I glued some 400 grit to a pencil eraser pencil and polished the brass base terminal and Scotch brite on each terminal. I also found the washer tube on the driver side was leaking (blue residue). I fixed that and lubed the vent wires to the vent valves.

And I replaced and reconnected the bulbs to the vent levers inside the cockpit. I had to remove the radio for this so I traced every ground and cleaned the terminals and ground again inside the cabin.

The tach still had the same characteristics as before.

I found this thread on Rennlist and the 930 a few posts down had the same prob as I
another bouncing tach thread - Page 2 - Rennlist Discussion Forums

Too bad he did not share the remedy.

I removed the Tach and CDI and they are being tested this week at Zims.

At least I found some more things important to fix.

I doubt it's the CDI.

Maybe the VR, but my voltage read seemed good.

[1983Cabrio]

I had an intermittent Voltage Regulator problem in my 83SC. The tach was very unpredictable... it wasn't until my battery cooked that I replaced it. Since then, my Tach is perfect.

[oneblueyedog]

I removed the alternator and found that I did have bad bearings in it. I replaced the bearings and sleeve that had disintegrated, the VR and rectifier set. I bead blasted the the housing and fan. I reinstalled it. I still have the same problem. I took it by a reputable Porsche mechanic at noon today and just from him listening to the engine run found numerous things, that I knew it needed, like valve adjustment. But I did not notice it missing. I told him it was a rebuilt CD box and he felt that may be a good place to look from. But that there was something wrong with the ignition. I'm going to get the car to him next week. It was great to let him witness the tach go live at about 500rpm lower at idle then go dead. His shop is a wonderland and I was in awe.