Voltmeter

[trhains]

Sorry Alan cant open this link, says something about my IP address?

[Othen]

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

The voltmeter sounds fine, they are really simple, so as long as it moves it will almost certainly be working okay.

Run through the checks - find out what is working properly and what isn't and we'll go from there.

Stay safe,

Alan

[Othen]

Originally Posted by trhains

Sorry Alan cant open this link, says something about my IP address?

See if this works:

https://www.volvo-forums.com/index.p...pic=16395&st=0

It just confirms more or less what you have done already.

[trhains]

Morning Alan,
pulled fuse 13, made no difference rev counter and all gauges still work, (speedo, fuel and temp)Although not sure fuse 13 controls rev counter on my car don't know which one does its not noted on back of fuse cover lid.
Last night I played about:-
Key in ignition

Turn side lights on

No instrument cluster illumination, but handbrake warning light has very very faint glow
Centre console lights illuminated and dim with rheostat
All fuses in place
Voltmeter working reading approx 12v

Engine running with side lights or headlights turned on

No instrument cluster illumination, Centre console lights and radio illuminated and dim with rheostat
All fuses in place
Voltmeter working .
Rev counter working but winds down with rheostat

All warning lights work in both situations
All gauges work but no illumination.

regards
Trevor

[Othen]

Originally Posted by trhains

Morning Alan,
pulled fuse 13, made no difference rev counter and all gauges still work, (speedo, fuel and temp)Although not sure fuse 13 controls rev counter on my car don't know which one does its not noted on back of fuse cover lid.
Last night I played about:-
Key in ignition

Turn side lights on

No instrument cluster illumination, but handbrake warning light has very very faint glow
Centre console lights illuminated and dim with rheostat
All fuses in place
Voltmeter working reading approx 12v

Engine running with side lights or headlights turned on

No instrument cluster illumination, Centre console lights and radio illuminated and dim with rheostat
All fuses in place
Voltmeter working .
Rev counter working but winds down with rheostat

All warning lights work in both situations
All gauges work but no illumination.

regards
Trevor

Okay.

Did you try removing the two connections for the voltmeter to set the system back to the way it was before you fitted it? If so what was the result?

It looks like the instruments are wired to the lighting rheostat (they should not be). Pull fuse #16 and see if the instruments (plus handbrake light etc) go off (they should not, but it sounds like they probably will).

I'll see if I can find a wiring diagram for your car (I'll ask LS). It sounds like someone has wired it up incorrectly (lighting rheostat to the instruments).

Would I be right in thinking the car is new to you?

[Othen]

Originally Posted by Othen

Okay.

Did you try removing the two connections for the voltmeter to set the system back to the way it was before you fitted it? If so what was the result?

It looks like the instruments are wired to the lighting rheostat (they should not be). Pull fuse #8 and see if the instruments (plus handbrake light etc) go off (they should not, but it sounds like they probably will).

I'll see if I can find a wiring diagram for your car (I'll ask LS). It sounds like someone has wired it up incorrectly (lighting rheostat to the instruments).

Would I be right in thinking the car is new to you?

Further to that above, I've found some wiring diagrams for a 1985MY car, which should be more similar to yours.

First: still disconnect the voltmeter - try just the light first - see if the rev counter and lights go back to normal. If they don't then disconnect the sensor and see if that makes it go back to normal. If either of those works we have found the problem and just need to fix it. So do that first.

Next the lights, here is the diagram:



First: is the voltmeter light connected where I have drawn it, and where did you earth the light? It is possible there is some interference to the instruments' earth return, so make sure it has its own separate earth onto the chassis.

Then pull fuse 16, see what goes out (it should be all the lights you see listed in the box). See if the rev counter behaves normally without fuse 16 (I think it might well). Replace fuse 16.

Now, the instrument panel, if you have not found the fault with either of the above then I think it will be somewhere on this diagram:



So, first pull fuse #8, the rev counter should stop working (it has its own power supply on the later cars, I don't know why). Note what the tachometer does. Replace fuse 8.

If you have not found the fault by now it is somewhere around where I have put the asterisk. See the input from the rheostat runs to the lighting for the tachometer - there must either be a crossed wire to the power feed, or (more likely I think) there is some back feed to the earth return. Maybe you mixed up a wire or dislodged a wire when fitting the voltmeter.

I think the most likely candidate is the earth return for the voltmeter light (so do the first bit first - disconnect it).

Good fortune, there should be enough above to find and fix the issue. Be methodical and change one thing at a time - see what changes.

Dinner time for me (sausages).

Alan

[trhains]

Morning Alan, thanks for all the info you sent yesterday, I had some gardening jobs to catch up with so had a day off from the car. Will get onto it today.
Ive had the car 4 yrs and never had any electrical problems till now, other than the odd bulb.
When all this lockdown thing started I lifted the front carpets to get to the access holes for the cills so I could clear them out and get some wax oil in them, but not sure how anything electrical could have been disturbed. I also took the cassette player out to clean the head but thats just a plug connection so not sure if anything was disturbed.
Thanks for the diagrams and the suggestions you have given me, I will try and get my head round them after breakfast and then make a systematic start on trying to resolve this issue.
Thanks once again for your help and interest and I'll give you an update later on,
regards
Trevor

[Othen]

Originally Posted by trhains

Morning Alan, thanks for all the info you sent yesterday, I had some gardening jobs to catch up with so had a day off from the car. Will get onto it today.
Ive had the car 4 yrs and never had any electrical problems till now, other than the odd bulb.
When all this lockdown thing started I lifted the front carpets to get to the access holes for the cills so I could clear them out and get some wax oil in them, but not sure how anything electrical could have been disturbed. I also took the cassette player out to clean the head but thats just a plug connection so not sure if anything was disturbed.
Thanks for the diagrams and the suggestions you have given me, I will try and get my head round them after breakfast and then make a systematic start on trying to resolve this issue.
Thanks once again for your help and interest and I'll give you an update later on,
regards
Trevor

Okay, good fortune in finding the fault.

Stay safe,

Alan

[trhains]

Morning Alan a few answers in brackets to the suggestions you gave me:-

First: still disconnect the voltmeter - try just the light first - see if the rev counter and lights go back to normal. If they don't then disconnect the sensor and see if that makes it go back to normal. If either of those works we have found the problem and just need to fix it. So do that first.

(Voltmeter totally disconnected, made no difference rev counter still adjusts with rheostat)

Next the lights, here is the diagram:


DIAGRAM

First: is the voltmeter light connected where I have drawn it,
(yes connected to green wire to clock)

and where did you earth the light?
(scotchlok connector to earth for voltmeter then earth to chassis)

It is possible there is some interference to the instruments' earth return, so make sure it has its own separate earth onto the chassis.

Then pull fuse 16, see what goes out
it should be all the lights you see listed in the box. See if the rev counter behaves normally without fuse 16 I think it might well. Replace fuse 16.
(Yes rev counter behaved as it should no adjustment with rheostat, just bulb fail light illuminated)

Now, the instrument panel, if you have not found the fault with either of the above then I think it will be somewhere on this diagram:


DIAGRAM

So, first pull fuse #8, the rev counter should stop working
(No it still worked and adjusted with rheostat)

It has its own power supply on the later cars, I don't know why. Note what the tachometer does. Replace fuse 8.

Working my way through checking cables for damage but nothing found yet, cant get instrument cluster out as I cant remove knob to rheostat to get to screws for instrument cluster.

Do you think the rheostat could be dodgy as I have seen comments on other forums about bi-passing it to give full brightness lights as lights are pretty dim anyway?

I really appreciate your help in this matter
regards
Trevor

[Othen]

Originally Posted by trhains

Morning Alan a few answers in brackets to the suggestions you gave me:-

First: still disconnect the voltmeter - try just the light first - see if the rev counter and lights go back to normal. If they don't then disconnect the sensor and see if that makes it go back to normal. If either of those works we have found the problem and just need to fix it. So do that first.

(Voltmeter totally disconnected, made no difference rev counter still adjusts with rheostat)

Next the lights, here is the diagram:


DIAGRAM

First: is the voltmeter light connected where I have drawn it,
(yes connected to green wire to clock)

and where did you earth the light?
(scotchlok connector to earth for voltmeter then earth to chassis)

It is possible there is some interference to the instruments' earth return, so make sure it has its own separate earth onto the chassis.

Then pull fuse 16, see what goes out
it should be all the lights you see listed in the box. See if the rev counter behaves normally without fuse 16 I think it might well. Replace fuse 16.
(Yes rev counter behaved as it should no adjustment with rheostat, just bulb fail light illuminated)

Now, the instrument panel, if you have not found the fault with either of the above then I think it will be somewhere on this diagram:


DIAGRAM

So, first pull fuse #8, the rev counter should stop working
(No it still worked and adjusted with rheostat)

It has its own power supply on the later cars, I don't know why. Note what the tachometer does. Replace fuse 8.

Working my way through checking cables for damage but nothing found yet, cant get instrument cluster out as I cant remove knob to rheostat to get to screws for instrument cluster.

Do you think the rheostat could be dodgy as I have seen comments on other forums about bi-passing it to give full brightness lights as lights are pretty dim anyway?

I really appreciate your help in this matter
regards
Trevor

Hi Trevor,

I'm sorry to hear you still have a problem -let's start thinking through this.

So - from the information you give above:

a. If you disconnect the voltmeter completely (the gauge and its light) the rev counter problem persists, so from this we know the issue is nothing to do with the voltmeter. Either it pre-existed and you had not noticed it, or something has been inadvertently disturbed when the voltmeter was fitted. That is important as we now know we are not trying to fix a problem with the voltmeter.

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).

From what you have told me we may conclude there is a short somewhere between the output from the rheostat and the rev counter, probably its earth return or its signal input. Let's see if we can narrow that down, so try this next:

Disconnect the signal input to the rev counter - this is really easy, just go to the -ve side of the ignition coil, find the -ve terminal (they both look the same, but the -ve one will not have a wire leading to the ignition module attached to it - rather it should have just one wire, probably black) and pull the connector off. The rev counter should stop working. If it doesn't and still reacts in any way to the rheostat then we will know the short circuit is to the tachometer signal wire. Replace the wire to the coil after.

So far so good? The fault still needs fixing: from the output of the rheostat to the rev counter signal wire, or to the instrument itself. Finding it is just a matter of tracing the wires along (difficult I know) and checking for the fault - a multimeter would be handy to check the output at any connections.

Could the rheostat be faulty: even if it is, we know there is a short somewhere between it and the rev counter that needs fixing anyway. Shorting out the rheostat will just mask the problem, so I'd say fix it first.

If you want to short out the rheostat that will almost certainly make your instrument lights a bit brighter because even at full output it will have some resistance. You can do this anyway if you like - just run a wire (about 1mm^2 is plenty, it won't be carrying much current) from the input to the output side of the rheostat. The chances are the rev counter won't register at all (it will be like the rheostat is turned up fully), but that might be an interesting test in itself).

Some owners do bypass the rheostat because the dashboard lighting is pretty poor (especially compared with modern displays) - in fact I was going to try it on my 244 the next time I have a reason to pull the center console apart (or I just run out of jobs - it is only 4 screws). By all means try it, but I'd say fix the cross talk between the lighting circuit and the rev counter first.

Good fortune - and stay alert.

Alan