Tachometer Light Strange Problem

[arbita1]

I'm having a strange issue with the gauge lights on the tachometer.

On occasion (more times than not) when I turn on the lights, the tachometer gauge lights that illuminate the gauge are not on. All other interior/gauge lights work.

Here's the strange part. The high beam indicator is illuminated (but the high beams are not on).

Next, I will tap on the gauge window a few times and the gauge lights will illuminate and the high beam indicator will extinguish. Sometimes the high beam indicator will just dim...followed by a few more taps and it will go out.

It's fine after that, until the next time I turn on the lights which starts the process over again.

I'm thinking this is something that is a problem on the circuit board of the gauge. Any ideas?

[woland]

Remove the tach to verify if a previous owner made some confusion with the wires.
All the gauge lights are gronded with the lamp body and the positive is a black-blue wire without fuse protection (better you disconnect the battery)



If the tach it's ok go and look for a burned black and blue wire in the light switch



There's a connector under the dash. Keep note of every wire you remove from the switch.



Verify that the turn signal switch works correctly (high-low flashing). If something is wrong you can already repair it with this instructions:

turn signal switch quick fix pictures

Let me know

[arbita1]

Thanks for those pictures Paolo.

I have personally replaced the turn signal stalk and the headlight swtich myself (due to other issues). And I have removed and reinstalled the tachometer previously as well. So my hand has been on all three of those items and I cannot blame the P.O. for anything here.

I did some troubleshooting last night.

When I pull out the high-beam indicator lamp from the tachometer and ground it, it illuminates dimly. This shouldn't happen as the turn signal lever is not being pull/pushed. According to the wiring diagram, when you pull/push the lever, it sends 12V from the headlight switch to the indicator bulb, which is then grounded to the gauge itself.

When I put a multi-meter from the bulb socket to the tachometer I got a negative voltage (about - 2.5V if I remember right). When I put the multi-meter from the bulb socket to another ground it showed about .3V. I don't think I should be getting any reading from this unless the turn signal lever is pulled. Correct? And why would I get a negative reading when I touch the gauge body as the ground?

After disconnecting/connection most of the wiring to the tach last night, the lights and high beam indicator seemed to be working properly. But I'm not sure the problem is 100% solved.

[schumicat]

I had to run a separate ground wire to get the tach illumination to work (spade connector under screw on the back with wire run to screw on chassis in trunk).

[wwest]

Ground return connection to/on the tach CASE is poor or intermittent resulting in the guage illumination bulb ground return flowing through the high beam bulb to "its" ground return.

On the tach the case ground is through the multi-pin connector, grounded internal to the case. On the back of the tach you should see a small round copper washer spot-welded to the case. Inside that washer is a "stem" that should be solidly connected to the copper washer. If it isn't a quick solder job is in the works.

[arbita1]

Originally Posted by wwest

Ground return connection to/on the tach CASE is poor or intermittent resulting in the guage illumination bulb ground return flowing through the high beam bulb to "its" ground return.

On the tach the case ground is through the multi-pin connector, grounded internal to the case. On the back of the tach you should see a small round copper washer spot-welded to the case. Inside that washer is a "stem" that should be solidly connected to the copper washer. If it isn't a quick solder job is in the works.

That sounds like it. Is there a fix for this?

I thought of running a seperate ground to the high beam bulb similar to what schumicat did.

Also, why would tapping the gauge fix this?

[wwest]

Originally Posted by arbita1

That sounds like it. Is there a fix for this?

I thought of running a seperate ground to the high beam bulb similar to what schumicat did.

Also, why would tapping the gauge fix this?

The copper washer connection is probably broken but still "touching" with vibration.

[arbita1]

Originally Posted by wwest

The copper washer connection is probably broken but still "touching" with vibration.

I'm sorry. Are you referring to the metal ring that is attached to the back of the gauge body, where the bulb is inserted into?

[arbita1]

So last night it went from bad to worse.

I checked the tach light to see if it was still working properly and it was not. Now, when I pull the headlight switch, instead of the gauge lights turning on, the high beam indicator turns on. When I pull the turn signal stalk the high beam light turns off. The tach gauge lights do not go on at all.

Could there be a problem with the contacts in the turn signal stalk?

[arbita1]

Originally Posted by wwest

On the tach the case ground is through the multi-pin connector, grounded internal to the case. On the back of the tach you should see a small round copper washer spot-welded to the case. Inside that washer is a "stem" that should be solidly connected to the copper washer. If it isn't a quick solder job is in the works.

I just noticed your edit. I'll check this first. Thanks.

[arbita1]

It's fixed.

Wwest...the post that you mentioned is not the ground though. It's actually just a support for the gauge face plate. However, your response got me thinking about how the gauge body is ground, which is what led to me finding the problem. So thanks anyway.

The gauge body is actually grounded through this screw.



On the circuit board inside the tachometer, I indicate where each spade connection from the back of the tachometer goes to.



The ground is on the bottom left. You can see there is another circuit that connects to the ground. (I drew an arrow next to it.) This is attached to the post that goes back and makes contact with the screw in the back of the tachometer. You can also see the post wwest was talking about in the bottom of the photo.

The problem was that a piece of the solder (or whatever it is) on the board flaked off. So there was no connection anymore. It must have been hanging for a while and that's why tapping the gauge used to fix the problem. Finally it fell off and caused the problem above. So I resoldered the connection and it works perfectly.

Thank for the help.

[dictoresno]

Originally Posted by schumicat

I had to run a separate ground wire to get the tach illumination to work (spade connector under screw on the back with wire run to screw on chassis in trunk).

im having a similar issue now with my 86 911. it seems like toggling the high beam switch makes the tach "lose its ground". if I manually ground it, everything works perfectly (high beams, fog lights, blue indicator and illumination).

is running a new ground to the tach ok or is there some other short that needs to be tended to. it seems like the left side illumination is flaky and dims/shorts out easier than the others. I can yank around on the wires and get them to work. but the minute I turn the key and turn on the headlights and trigger the high beams, the gauge lights turn off. high beams work fine. so I don't think its the turn signal stalk. where do those bulb power wires terminate to? all the Bentley diagrams say is to "interior lights". id like to chase down where they go.

I did test pin 6 on the tach harness which is the ground wire. I was able to probe the 12V wire on the clock next to it and hook to the ground on the tach plug and it was able to get ground connection the whole time. so I have no idea why this is happening.

someone else mentioned in another thread that if my tach is working, then the ground shouldn't be the issue. perhaps one of the illumination bulbs is loose fitting in the tach housing? I will try to add an extra ground to the back of the tach housing and see if it helps. ill just need to find a new ground point under the dash.

[911obgyn]

Its a ground issue.

[dictoresno]

I solved my issue by jumping the ground from the clock to the tach with a double wire. everything works fine now.