orangecurry

I've done a search but.... anyway bear with me as the detail may be important!

I've just replaced the odometer gear at 77k miles. All went well. Thanks to the guides on here especially about not needing to remove the face.... anyway....

Coincidentally one of the bulbs had gone on the tach, so I pulled that as well, removed the dud bulb (which I can see has lost a filament) and put it back with a bulb from the clock.

Now at first I couldn't get either of the tach bulbs to work.... no idea why, and then suddenly they both did, so I've replaced the tach and both bulbs are still ok.

As I was working on the odometer, I put a previously 'good' odo bulb into the clock, and put the clock back..... now neither of the clock backlight-bulbs work. BUT the clock works as do all of the ignition-on warning bulb lights.

The odometer is finished/repaired, so I've put the 'good' bulbs into that, and none of them work!

Back and forth between clock and odo - can't get any of the four bulbs to light!

Questions:

1 - can I test the bulbs without blowing them?
2 - are there any fuses/relays that are for each individual instrument?

Puzzled - thanks for any ideas!


PS I have rotated the bulb-holders, once in the housings, to attain a good contact - no difference.

Heuerman

Sounds like an earthing problem.

You can test the bulbs with an Ohm meter or continuity tester. Alternatively connect a 12V battery (or even a 9V will do) and they should light, not polarity sensitive so no damage can be done. All instrument lights are on a common fuse (see inside fuse box cover for assignments - fuse #18 IIRC). If the fuse blows none of your instruments inc. bulbs would work.

jay@EZimmoblock.com

OC - All backlight lamps run off the dimmer relay R24 which outputs a variable voltage (controlled by the dimmer pot). This circuit is active whenever the headlight switch is on. In addition to all backlight lamps in the gauges, that same circuit drives other dash lamps like the switches, ashtray, etc. This backlit circuit is protected by fuse 36. This circuit is separate from the one that drives the instrument warning lamps, which is protected by fuse 18 (on whenever the ignition is on). It's unclear which of your backlight lamps are currently not working but if R24, or the pot, or fuse 36 were bad, ALL backlight lamps would be off, not just some (assuming the lamps are good). If backlighting is off in just one instrument, look there, it could be a ground as was suggested. Note that the backlight bulbs share a common ground with the instrument so if that were the case, you would notice the instrument not working.

orangecurry

Thanks for confirming that - which is why it is so odd. Speedo and odo, indicators and full-beam all working - clock and it's warning lights all working.

Either the backlighting-bulbs have blown with me moving them after 27 years, or there's a connection issue in the housing for each bulb I've moved.... not likely.

I'll test the bubs now I think I can't fry them

jay@EZimmoblock.com

So to recap...
1) If some backlight lamps are working and some are not, your can rule out R24, the pot and fuse 36.
2) If the instrument itself works, but the backlighting in the same gauge doesn't, you can rule out the ground return for that gauge

That pretty much leaves the bulbs (or their connection to the circuit board).

pp000830

Hi OrangeCurry,
Things to check first:
-Clean. use some spray electronic cleaner such as CRC QD Electronic cleaner spray and hose out the gauge connector and wiring harness plug for both gauges.
-Check the lamps and lamp socket connections. They are very delicate and sometimes bending them a little make for a more positive connection.
-Clean, spray some of the cleaner saturate a swab and rub the bulb and socket contact points if you can get to them.
-Replace all the bulbs, some may be original to the car and have filaments that have been burned off to the point that they are intermittent and it cannot be observed by visual inspection or meter testing.
When I had a similar problem I also reheated all the solder connections on the clock's internal circuit board, failed solder joints being a known issue in the clock. It was a while ago but I seem to recall I had to heat the solder connections on the board going to the clock motor to release the board for inspection and reheating. Once out it can be spray cleaned as well.
Some of the above are discussed in more detail at the bottom of this page:
https://993servicerepair.blogspot.co...ing-light.html
All easy DIY efforts.
Andy