Electrical gurus...need help with speedo gauge lights

[J-RAD]

OK, here is the problem...

Out of the blue I just started experienced a problem with the speedometer gauge when the headlights are "ON" on my '82 SC.

When the headlights are "ON" and the ignition is in the 'ON' position, the speedometer light goes dark...all other dash lights stay lighted as normal. In addition, it appears, that when driving, the speedo eventually starts to read low...it still registers a speed but 60mph may only register as 40mph for example. When the headlights are "OFF" the speedo registers speed just fine.

In addition, when the ignition key is "OFF" but the headlights are "ON", the speedo gauge will light up correctly. If I leave the headlights "ON" and toggle the ignition key back and forth between "OFF" and "ON", the speedo light will go on and off accordingly.

I'm thinking there is a short somewhere but where? Could it be an issue with the ignition switch? As I mentioned, this just happened out of the blue. TIA!

[RoninLB]

is this a DME ?

[HawgRyder]

More likely to be a bad ground on the speedo.
Bob

[filou]

you need to test several circuits as a matter of fact. First, i would do what's it's called a voltage drop test at the ignition switch. you need to get electrical diagram for your car and go from there. It seems, depending on the description of symptom that you are having a drop of voltage when the ignition key is on and lights on. Drop of voltage are due mainly to a high resistance somewhere in the circuit (old wire, burn out contact, especially a worn out solenoid for starter). Do you have a digital voltmeter ? i use a cheap one that does the trick.
Study your diagram first, notice the components for each circuit then go to the car and try to locate them. Tidious work but that the only way and a lot of patience. Good luck and let us know what happen.
truly, Filou.

[Shawzy]

Do a few more tests -what happens when:

- high beams activated?
- light switch is in first position (i.e. not off, but not all the way out)
- key is turned to start or crank position?

I don't know off hand how the above are directly related, but it may isolate some things and give you more to look at.

Because this is sudden, it is likely a simple problem. My tendancy in these cases is to get way too complicated, and I always get burned (read, I get too happy with the multimeter and wiring diagrams).

If you have the Bentley def check out the wiring diagrams for ignition/lights etc. You may have a lead grounding out on something nearby, or a lead has popped off. Sounds like a ground issue to me though.

Before you pull the speedo, check out wiring at the light switch - I've had wires pop off when poking around there at something else. There are a lot of connections there, but you may see something obvious.

Have you or others poked around under the dash or behind instruments in the last 6 months? I have disrupted things by accident while working on other projects.

Good luck keep us posted

[J-RAD]

Prior to this I have not done anything under the dash in recent months.

I did pull the headlight switch and I actually have an extra one that I swapped out and still came up with the same results. Theory: it's not the headlight switch or any of it's connections.

To answer a few of Shawzy questions:

When I describe the ignition swith as "ON" that means the first position (accessories) and/or start/crank position...both produce identical results.

High beams don't seem to work now either....hmmmm...may or may not be related.

I agree that it must be a ground somewhere and it's probably a very easy fix but I'm less than skilled at reading electrical diagrams...

[J-RAD]

Problem solved.

[Shawzy]

Afraid I can't help much without diagrams in front of me. I guess I should start sneaking the Bentley into the office....

BOTH high beams don't work - can you flash them with col switch? As opposed to not being grounded, sounds like they are not getting power. This suggests: light switch, col switch, ignition itself (fuses should be fine).

This could be an isolated problem but not likely. Once you get highs working again other stuff may pan out.

Fact that speedo is wonky when problem exists (not accurate) is odd. Likely a 12V or GRD issue as your trans wiring sounds ok.

Do you have cruise control? If so, try pulling the connector off the brain.

Again, without diagrams I can't see how everything is connected damn it...sitting here with my hands tied it feels.

Others with more experience will chip in soon hopefully.

Don't let diagrams scare you like they did me at first - jump in. Like reading a map once you get the hang of it.

Good luck

[filou]

Just a advice, Unless you are going to study your electrical diagram, don't even try to "try" to fix your electrical problem because all it takes it's ONE bad mistake and your can kiss your ignition circuit good bye !!
Ok, i am a little heated up but really, be very careful with electrical problems. You can create more problems but "playing" with it.
Good luck !!!!!

[RoninLB]

I don't have a guess..

but I would start jumping a new ground into the suspect circuit.

then disconnecting wires from sw., taking notes of everything, then jump the input to the output.

I glad I have only to think about this one..........Ron

[Ciaran]

I know that this is an old thread but does anyone know what the solution to this problem with the speedometer/ light was? I have the exact same symptoms on my Carrera 3, and it would be great to know where to begin to check.

Any ideas would be much appreciated.

Ciaran

[jinos]

Does anyone have an idea on this issue? I still have the same problem and been researching this for awhile. Many thanks.