Backfeed through dash dimmer switch

[SlinktRR]

I have an electrical issue I have not been able to sort out. Whenever I turn headlights on, I see all the gauges start to peg right (high). If I set the dimmer switch to off (no backlighting), the gauge needles stop. I have put a ground wire from the instrument cluster to the steering column. Should I run one to the block or battery or core support? Change the dimmer switch? Any ideas would be appreciated.

 

[davek]

only happens with the dimmer switch? I would have it rebuilt. Hows your dash harness?

 

[SlinktRR]

I have new engine and dash harness. Who rebuilds the dimmer switches? Wonder if there is a good reproduction switch too.

 

[LowBikeMike]

Devin Duke
Wagoner OK

devinism on this site. Does a great job, reasonable and quick turnaround

 

[SlinktRR]

I saw the thread on FBBO about bypassing the dimmer thumbwheel, I'm going to try that this weekend and see if it allows me to run the dash lights without pegging the gauges. Will look at a rebuild too.

 

[JG71B]

I saw the thread on FBBO about bypassing the dimmer thumbwheel, I'm going to try that this weekend and see if it allows me to run the dash lights without pegging the gauges. Will look at a rebuild too.

Not sure on yours but I know 71-4 dimmers suck a lot more juice than one would think. I keep mine at a minimum but enough to see everything

 

[SlinktRR]

Ok I bypassed the dimmer switch after cooking the turkey yesterday. Curious results. Now my gauges read correct (they were reading low before, maybe 3/4 of the correct reading) when the headlights are off. The dimmer is bypassed so I have the thumbwheel all the way down. When the headlights are on, the gauges are fully backlit and the needles start to peg right again.

On one hand I'm happy that the gauges read correct, but I can't drive at night or I risk blowing the gauges.

The bypass still requires you to go through the dimmer switch you just jumper the harness connector to bypass the rheostat. So the dimmer switch may still be the source of the backfeed. Not sure what to try next.

 

[440+6]

Did you try unplugging the dimmer switch and see what happens when you turn the headlights on?

 

[devinism]

Devin Duke
Wagoner OK

devinism on this site. Does a great job, reasonable and quick turnaround

Thank you for the plug.

Did you unplug the dash light dimmer switch completely? You need to, to take it out of the loop.
It really sounds like you may have a ground back feeding through the headlight/parking light circuit. I see that your harness is new, but never assume the wires are in the plastic connectors correctly (it's happened). M&H, or Painless?

 

[SlinktRR]

I plan to do this tomorrow, I didn't know you could completely disconnect the dimmer and still run the lights. It's a year one sourced wiring harness which I think is M&H. Good quality harness but I didn't double check the wiring color coding for the instrument panel.

I have the laminated wiring diagram for 1970 and the area I have been looking to troubleshoot next is the instrument gauge connector. I'll report back.

 

[SlinktRR]

So the dimmer does not appear to be the problem. With it disconnected there is no effect. So I'm looking at the instrument lamp circuit. I notice on the wiring diagram that from the instrument panel connector on the dash this wire goes back to a common splice which leads to the fuse panel, a time delay relay, the ignition switch and the radio connector. Anyone have an idea how to troubleshoot?

 

[devinism]

So the dimmer does not appear to be the problem. With it disconnected there is no effect. So I'm looking at the instrument lamp circuit. I notice on the wiring diagram that from the instrument panel connector on the dash this wire goes back to a common splice which leads to the fuse panel, a time delay relay, the ignition switch and the radio connector. Anyone have an idea how to troubleshoot?

All I could think of:

Undo the dimmer bypass. Don't plug the dimmer in yet. Turn on PARKING lamps first. Do you still have gauge lights? If no....Turn on HEADLAMPS. Any gauge lamps on?
If the answer is no then read on.

Plug the dimmer back in, as that does not appear to be the issue.
1) Are there any modifications to the electrical system? (Ignition, radio, lighting, alarm, time delay, ammeter bypass, etc.).
1a) Any wires get pinched upon assembly?
1aa) Does it still happen with the engine compartment side light harness unplugged at the firewall bulkhead?
2) Do you have your old harness? That way you can compare the wire colors on each connector. It will take time, but may be worth it.

 

[SlinktRR]

Thanks Devin! With the dimmer disconnected, no dash lamps come on. I did a series of tests with the dimmer connected fully again and light harness disconnected at the bulkhead, the problem is the same. So the backfeed must be somewhere in the under dash wiring. Can I use a multimeter to test backfeed?

No mods or added electrical components except for an aftermarket Bosch tach connected to the dash lighting fuse panel for lighting, ammeter is connected and works properly. Radio is disconnected so my next step is to take out the glove box, ash tray and radio to trace all the wiring in this area for the dash lighting circuit. I discarded the old harness because the insulation was dried and cracked unfortunately.

 

[devinism]

Thanks Devin! With the dimmer disconnected, no dash lamps come on. I did a series of tests with the dimmer connected fully again and light harness disconnected at the bulkhead, the problem is the same. So the backfeed must be somewhere in the under dash wiring. Can I use a multimeter to test backfeed?

No mods or added electrical components except for an aftermarket Bosch tach connected to the dash lighting fuse panel for lighting, ammeter is connected and works properly. Radio is disconnected so my next step is to take out the glove box, ash tray and radio to trace all the wiring in this area for the dash lighting circuit. I discarded the old harness because the insulation was dried and cracked unfortunately.

Maybe disconnect the tach light wiring, and maybe also the tach, just to see if it makes a difference?

 

[SlinktRR]

Ok after a whole holiday season plus futzing with this problem, I finally found the source.

***Drum roll*** Classic Industries dash limiter

I thought it just wasn't allowing the gauges to read correctly (i.e. protecting the gauge from 12V but reads too low). So I kept it on the car.

I finally bought the RTE dash limiter this month. When I pulled the old limiter out the ground tab just fell off from the spot weld. I should say spot "weld" because it was just not connected, presumably broke when I put it in or when I removed today. I put in the RTE limiter. Fuel gauge comes up immediately and needle steadies accurately. I turned head lights on and the needle dropped a bit, as they indicate in the instructions. All gauges work now. I turned the dimmer switch to full blast and the fuel needle doesn't skip a beat.

If you have this problem, reach to the folks in Springdale Arkansas and buy this part. Solid state limiter is the way to go, worth every penny. The movement of the needle is worth the extra expense over the OEM replacement.

 

[devinism]

Glad it was 'simple'.
2nd one I've heard of