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Rev Counter and Rear Fog Light Switch

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paul@riccall

PCGB Member
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My 1980 sc has just development a couple of problems :-

1) The rev counter seems to have a mind of its own. I can be sat in traffic, and the rev count will show that I am doing 5000 revs !!. Also the last time it went funny, it completely died, and all the interior lights and radio went off. Is it simply the sender unit that needs replacing, or is it somthing more sinister. I have check all the fuse connections and everything seems ok.

2) How do you change the rear fog light switch. I have taken out the radio to see if you can get at it that way, but it looks as though everything is hidden behind the bulk head. Is it a matter of prizing out the switch with a screw driver or do you need to remove the cardboard partion in the front boot and go in that way. I need to get this sorted otherwise the car is going to fail the MOT as I can't turn the fog lights off.

Any help would be greatly appreciated.

Paul
 
If the rev counter is suddenly behaving erratically the first thing to check is battery voltage, with the engine running you should get about 14+volts at the battery terminals. If other things are misbehaving too then the problem maybe else other than the rev counter. If the rev counter is receiving the correct voltage from the battery then check the sender. The fog light switch from memory is tricky to get to, unscrew the plastic button then unscrew the retaining washer, the fog pushes into the dash then, I'm afraid you have to get to it through the luggage compartment. I think it's more likelt to be the wiring grounding somewhere than the switch at fault. The attachment of the wires to the fogs is pathetic to say the least, make sure the wiring directly at the fog lights is in good order.
 
"I'm afraid you have to get to it through the luggage compartment"
If my old 3.2 Carrera is in any way similar, that might be an understatement. On that you had to remove the heater to get at the switches. Or maybe I just did it the hard way?
 
I've got similar troubles on my 81 SC:

The rev counter "dances" up and down erratically - up to redline, down to sero, generally hhops about
Occasionally it does this in time with the indicator - i.e. add about 2k rpm with each oscillation
Other times it seems to work normally

Other things that may or may not be related:
The oil level bounces up and down like crazy (although this could be a damper not working correctly)
Cruise control doesn't work
Heater fan doesn't work
Handbrake operates the "Brake" light instead of the "Parking Brake" light

I've got one of those solar panels you can plug into the cigarette lighter to trickle charge the battery. If I do this, the speedo jumps up to about 20 mph!
 
Often electrical faults on multiple (but not all) equipment is down to ground fault. The negative connections to the body tend to meet in groups and bad connections here can cause all sorts of strange effects.
 
Good advice for anyone with electrical problemas on an SC is to remove and repalce all fuses with new ones, clean the fuse holders with a copper brush and refit the fuses with a dab of electrical grease. Remove and brush clean the legs of all relays and every other joint and connection you can get to. If that doesn't cure the problems and you are no good with electrics it's time to see an auto-electrician. Old batteries often cause problems for rev counters. Chris it is normal for the oil level to bounce up and down to a degree when the oil tank is being emptied and filled according to engine revs. If you mean it just bounces around completely independently of engine revs then it could be the gauge. I'm not sure what you mean by a damper.
Cruise control has a 'brain' on a pcb so that could be shot, or it may be a vacuum issue, it's not very good anyway.
Heater blower motor working (in the engine bay) could be a failed unit or the switch is not turning it on from the unit between the seats. There is also a fuse involved I think.
The brake light issue sounds like incorrect wiring which considering all the electrical faults you have sounds like someone who doesnt know these cars has possibly messed things up in yours.
 
Of course, that's why you check the battery voltage first, the reading there gives an indication of where you progress to checking next.
 
Thanks bones,

The cleanup operation sounds like a good place to start. According to te history, this car has had two aftermarket alarms in it, but there isn't one there now. I guess there may have been some bodgery going on somewhere.

Damper: I've seen these on fuel tank floats on other cars, Ususally some kind of physical device that stops the float moving up and down too quickly as fuel sloshes about in the tank - like a shock absorber does for the supension?
 
There isn't a damper in the oil tank. With the car on level ground, the engine running at full working temp' and the dip stick reading between the two level markers the oil level gauge should hold steady at 1/3 to 3/4 full (approx). When you rev the engin the gauge dips dramatically, when you come off the revs the gauge may rise quickly back up, seems as if it's bouncing around but it's normal for a 911. If alarms have been in and out of there then you are probably right in checking the wiring over is a good move, keep a fire extinguisher in the car too, you never know with older cars when they are going to set alight.......[:D]
 
The needle bounces when I'm sitting at the lights at steady revs, for example. Must be related to the same problem as the tacho

Good idea re. the fire extinguisher. I was wondering what use to put the cassette storage box to. Actually, I've noticed my TomTom 910 would fit discreetly in there with some kind of bracket contrivance. I've never been entirely happy with attaching it to the windscreen. Sends out all the wrong messages, 911 drivers always knowing where they're going of course [:D]
 
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