I have a unique speedometer problem.
When i turn on my headlamps, the speedometer/odometer goes dead.
Had the cluster tested, speedometer is fine.

Has anyone experienced this before.

Thanks.....JK

86 300 SDL Turbo

Sounds like a grounding problem. With the extra load from the lights in the cluster it probably collapses the circuit.

Grounds are brown wires. Check them for corrosion and the pins for cracks.

-J

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X2 You might want to run an additional ground wire from the cluster just to make sure you've got a good ground.

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Welcome to the forum!

did you know, that JK on most forums means Just Kidding?

there is a connector above and to the right of the accelerator on my SD when it comes loose, i lose all of what you are. ya might wanna look there also.

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External ground didnt fix the problem.
Still loose speedometer after turning on headlamps.
All other gauges remain working.

OK time to start trying some weird stuff. Disconnect your headlight bulbs and then see if the problem persists. If it does it's likely circuit/wiring related. If it doesn't it's likely power related. When you hook things back up do one side at a time and check. More data wlll hopefully lead to a solution.

I don't have an electrical diagram so I can't see what else is in the circuits that might be interfering, but that would be god to check too.

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Current Stable

• 380SL (diesel)
• Corvette C5
• Manx
• Baja Bug
• F350 Powerstroke
• Auburn Boattail Speedster replica

Unplugged headlamps, didnt fix the problem.
Speedometer still dies when headlamps are on...actually dies on either of the two switch positions when tuned clockwise.

JK

86 300 SDL Turbo
96 BMW Z3
96 Ducati 900sscr

Ok we can pretty safely rule out anything related to the source of the problem being power draw. Because it happens without any appreciable draw, it most surely has to be something related to wiring - a short or a ground.

I don't have a wiring diagram for your car, but assuming that the '85 and '86 were similar in design, the only thing that is common with the cluster where the speedo is and the lights is the high beam indicator.





You didn't specify whether it only happens when the high beams are on, so you should check that. And it's worth doing a couple of things. First pulling the high beam indicator light. This won't help if the short is in the power side of things, but it would if it was on the ground side - it's worth a try. Second pull the fuse F9 (or the equivalent one for your car that handles one of the high beams and the indicator lamp). I would also pull (one at a time) the other fuses F7, F11 and F13 to see if the problem is associated with one of these circuits. Also try high and low beams for each of the configurations to see if you can find anything that is different that will provide a clue. As a last resort you could cut the WT/BU wire but certainly pulling the fuse F9 (or whatever it is on yours) would have the same effect, so save that for absolutely last.

That's all I've got for now...

If you've got an electrical manual, I could look further.

Attached Thumbnails

 

__________________
Current Stable

• 380SL (diesel)
• Corvette C5
• Manx
• Baja Bug
• F350 Powerstroke
• Auburn Boattail Speedster replica

Went through the related fuses, no luck. Doesnt matter if high beams are on or off.
One interesting thing happened, after disconnecting the headlamps (which made no difference) I reconnected them, the car sat overnight (rained really hard), Next morning the speedometer worked, with the lights on, for a short period of time. It was sort of erratic, needle bounced around, but it worked. Then it went dead again.

JK

Does anything else not work when this condition exist? Like fan motors?

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Everything works except speedo/odometer.