Voltmeter

[trhains]

Morning sorry afternoon just, hope you are all well.
Done some more investigating.
Pulling fuse 16 does as you said.
Pulling fuse 8 does as you said
Coil, my coil has 2 x red/white to -ve
and 2 x blue to +ve
I disconnected from -ve and engine doesn't start, connect back up and engine starts.
Confused!
Trevor

[Othen]

Originally Posted by trhains

Morning sorry afternoon just, hope you are all well.
Done some more investigating.
Pulling fuse 16 does as you said.
Pulling fuse 8 does as you said
Coil, my coil has 2 x red/white to -ve
and 2 x blue to +ve
I disconnected from -ve and engine doesn't start, connect back up and engine starts.
Confused!
Trevor

Hi Trevor,

There is no need to be confused. This really isn't very complicated; we'll go very slowly.

So, to be sure, this is what you are saying is happening:

b. When you pull fuse 16 the rev counter behaves normally. So, the interior lighting controlled by the rheostat is interfering with the rev counter, either on the +ve or earth side. This is important as it tells us we have a fault with the interior light circuit.

c. When you pull fuse 8 the rev counter still works (which it should not) and is controlled by the rheostat. This confirms the lighting circuit controlled by the rheostat is powering the rev counter, as the rheostat output is inversely proportional to the rev counter display I suspect its output is somehow leaking to the earth return side of the rev counter, or it is interfering with the signal input to the tachometer (a wire going to the -ve side of the ignition coil).


Richtig, now let's think about the coil:

I'm guessing your car has solid state ignition (no CB points, not that it matters in this context, apart from comparing the wire colours to the most similar diagram I can find). The two blue wires will go to the +ve terminal, so don't worry about them. The other 2 wires (the red/white ones): one will lead to the ignitor box, that one has to be connected to make the car run. The other red/white wire (it is a bit naughty of Volvo to use the same colour, but the electrons don't know what colour sheathing they are moving through) will go to the rev counter. It is in the bottom right corner of this diagram I sent you the other day:



The part marked 'E'. It will probably disappear into a sheath, then into the cabin where the tachometer lives. It is just this one you have to temporarily disconnect to see if the rev counter still works (it shouldn't, but it might if the rheostat is providing a signal). If the rev counter needle moves (at all) when you wind up and down the rheostat with just this one red/white wire disconnected you will know the leakage problem is from the output from the rheostat, if it doesn't then it will be between the rheostat and either the power or earth terminals of the rev counter instrument. Replace the red-white wire on the coil when you are done.

So, by now you will have narrowed down the fault to somewhere between the rheostat output and either the rev counter signal wire (the red/white one you disconnected in the above) or the tachometer power/earth in the instrument cluster. You still have to find the fault, but you are probably down to just a feet feet of wires to check now.

My apology, I rather assumed you would have known the above about the coil, but no matter.

I hope that helps. Just take your time and be methodical, if you get confused re-trace your steps and try again.

Stay aware.

Alan

[trhains]

Good afternoon,
So separated the 2 red/white wires to coil and fit new spade connectors to each,
connected back to -ve side of coil. (one of the red/white is thicker than the other)
Tried starting engine with one or the other disconnected in turn and the engine wont start unless both red/white are connected to coil.
Suggestions for next move?
regards
Trevor

[Othen]

Originally Posted by trhains

Good afternoon,
So separated the 2 red/white wires to coil and fit new spade connectors to each,
connected back to -ve side of coil. (one of the red/white is thicker than the other)
Tried starting engine with one or the other disconnected in turn and the engine wont start unless both red/white are connected to coil.
Suggestions for next move?
regards
Trevor

I’ll have to think about that one Trevor, one of them should just be the sensor for the rev counter.

It might not be for a day or so, but when I get a mo I’ll see if I can come up with an idea.

Stay alert mate,

Alan

[Othen]

Hi Trevor,
I have not forgotten about this - just been busy for a couple of days. I'll get onto it soon.
Stay alert,
Alan

[trhains]

No problem Alan, I really appreciate your help, whenever you get chance is ok. Jobs in the garden have been getting my attention since the weekend.
Thanks once again
regards and best wishes in these strange times.

[Clan]

Originally Posted by trhains

Thanks Alan
I'll give your suggestions a try.
As far as the voltmeter is concerned I bought it on ebay and one of the pics was of it connected up to a 12v battery and it showed a reading of about 12v. It seems to work when fitted as it reads about 12v with ignition on and with engine running about 14.5 and reduces as you apply a load like heater fan or wipers.
regards
Trevor

you know the 3 pin plug on the back of the rev counter housing .... red-black- and green i think? well connect a black wire to the black wire and run it to the chassis somewhere , It certainly cured the problem when i was working on them in the 80's ! I think there is a bad connection on the circuit board but this is a quick easy fix .

[Othen]

Originally Posted by Clan

you know the 3 pin plug on the back of the rev counter housing .... red-black- and green i think? well connect a black wire to the black wire and run it to the chassis somewhere , It certainly cured the problem when i was working on them in the 80's ! I think there is a bad connection on the circuit board but this is a quick easy fix .

That sounds quite plausible Clan, and more or less where we were heading with this investigation.

Give that a go Trevor.

Alan

[trhains]

Thanks Clan and Andy, I will give it a try in the morning and see what happens.
Thanks again guys for your interest and help
regards
Trevor

[trhains]

Good morning gentlemen,
SUCCESS
it worked, fit the black wire as suggested, then last night when it got dusk tried the lights. Everything works, Rev counter as it should - no adjustment with rheostat,
all instruments lights including clock work and all adjust with rheostat.
I really appreciate and value everyones help and suggestions, thank you very much.

Next step will be to connect Voltmeter as original job, should I pick up power for voltmeter light from clock light feed or avoid instrument feeds and power from somewhere else,
regards and thanks again
Trevor