Tachometer + water :(

[ddebonis]

I've noticed condensation inside my tach in the past, but is seemed to have escaped.

A few months ago, the back light stopped working sometimes, then completely.

Last week I put in a new bulb, but discovered the old one was fine. I replaced it anyway. When I re-assembled the tach and put the 8-pin plug back in, not only was the back light still not working, but the GEN and NEUTRAL lights were not working. I pulled out the pin, the connection looked good and no rust. Plugged it back in, and if I pressed hard on it, I could get the GEN and NEUTRAL lights to come on. But that was temporary.

I began taking it apart and there's some corrosion on the circuit board, but it doesn't look bad.

I couldn't figure out how to take the whole thing apart. Is there anything I'd need to examine inside the white plastic layer, or is the circuit board the most likely culprit?

Does anyone have experience servicing these?
Anyone have a spare?

[montmil]

Pick up a can of CRC Contact Cleaner at Wally-World, Radio Shack or Ace Hdwr. Spray the electrical contacts, both male and female. The CRC will not harm paint nor plastic. Try the lights again.

The instrument panel bulbs share a common ground. Brown wire is "earth" on the BMW. Start tracing the grounds back to the chassis ground point. Your comment, "pressed hard on it" leads me to think bad ground... somewhere. BTW, those plastic bulb holders are made of a soft German cheese. Check the copper contact strips in the holders and the wire connection to the holder. They are about eight bucks each to replace so be gentle.

Both the speedometer and tachometer can and will develop condensation on the interior face of the glass. You live in a humid area. Sunshine or a low humidity garage will chase off the wet.

While at Radio Shack, pick up a small tube of dielectric grease and start using it on all wire connections. Good stuff.

[georgesgiralt]

If you've air conditioner on, put the opened tacho on it so the air out the A/C will force pass into it. It is the driest air you can have in your area...
+1 on contact cleaner and fragility of bulb supports...

[Luca]

While at Radio Shack, pick up a small tube of dielectric grease and start using it on all wire connections. Good stuff.

and while you're at it get a little tube of thermal compound for the ICU.

The tach is vented and will dry out on it's own, but the speedometer should be sealed and takes longer to dry if it gets wet inside.  After fixing the internals on my speedometer and replacing the rubber reset button boot and collar, it has no condensation, even after sitting out in a thunderstorm.  It's worth replacing the reset button boot and collar if you do get condensation in the speedometer.  Mine was a disaster inside from all the rust.

[Barry]

The instrument panel bulbs share a common ground.

That is the usual arrangement but in our case some of the bulbs share a common +ve feed. Think of the oil pressure and neutral switches. They function by grounding the signal wire.  

[Barry]

Now I'm home from work I can look at the diagram and pin numbers for the tacho multi pin connector. As a starting point you could check for +12V on pin 6  (3 of the lamps won't work without this feed) and earth on pin 8  (2 of the lamps won't work without an earth).

Pin 1 High beam lamp
Pin 2 Instrument back light
Pin 3 Earthy side of Neutral lamp
Pin 4 Earthy side of gen lmp
Pin 5 Tacho impulse input from ignition systerm
Pin 6 Common +ve feed to Neutral lamp, Oil Pressure lamp and Gen lamp.
Pin 7 Earthy side of Oil pressure lamp
Pin 8 common earth to Intrument back light and high beam lamp

[ddebonis]

Good news: A friend had a spare tach from an '81 R65. I plugged it into my '84 and all the lights worked.

It's problem is the needle does not rest at zero. it's moves all over. It took it apart and the needle just needs a dab of glue or loctite to keep it in place.

So I am just going to take the circuit board and put it in mine because that seems easier than rebuilding the tach housing with the crown that I had to pry off. Looking at the two circuit boards, I can see now how bad mine was. The pathway for the backlight is damaged and there's some weird soldering going on with the pathways for the gen and neutral lights.  

Here's a picture- https://www.dropbox.com/s/2c3fat681ynai44/2013-11-12%2020.26.50.jpg

In order to keep vapor out of my original tach, I'm thinking of using a little clear silicone around the bezel since the rubber gasket must be eroding. Maybe pushing a little red & tacky grease in there would do the job, too...

Open to other ideas.

[Luca]

From what I've heard, the tachometer is not fully a sealed unit.  If you do try to seal it, make sure to do so while the humidity is low, or you will trap condensation in the gauge.  I'd use silicone sealant before I use grease.  Just a thin bead.  You could also use something like "indian head gasket shellac" on the rubber seal

The reason not to use grease is that when it gets warm in that gauge (and the light bulb adds some heat) the grease could get runny and start dripping on things.  It would also be difficult to keep the grease exactly where you want it during reassembly.

All the gauges I've ever taken apart had the needle press fit onto the shaft, and plenty tight.  If it's the needle on the new gauge that is worn out, you might be able to just swap it with the one from your defective gauge.  If you do use an adhesive on the needle, I'd use green loctite or another low-strength product.

Also, when removing needles from gauges, make sure you lift the needle over the rest peg and let it fall to its natural resting position.  Mark that position and then reinstall in the same place to maintain accuracy.

[tvrla]

The tach doesn't have pin at zero, so there's no way to lift it over. Sounds like you've got your problem sorted out.

Warning: be careful doing anything with the Tach needle - the shaft is ultra small and bends very easily!

If you'd like to sell your old tach, I'd be interested in getting it for parts.