Need Electrical Troubleshooting Advice (long)

I'm hoping one of the electrical geniuses on this board can help me identify an intermittent fault that has been nagging me.

I am almost sure it is ground-related, but I am going crazy trying to indentify the culprit.

The car is an 82 SC. Sometimes, but not all the time, when I turn OFF my rear defogger, the speedometer dies. It eventually comes back to life within anywhere from a few seconds to a few days. Sometimes all by itself, and sometimes when I rapidly cycle the defogger switch. I cycled it most of today, though, and the speedo's still dead.

Here are some other things I noticed:

The defogger switch activates a clicking noise when it is turned off (either pushed in or turned back to the left). The noise seems to come from above the gauge cluster near the windshield, sounds like a relay clicking, but I thought the only realy for the circuit was in the engine compartment. If anyone knows of a related component in that area, please let me know where it may be and what it looks like.

A "click" can be heard through the stereo when the defogger switch is turned off and when the wipers activate. These are more pronouned on AM stations. The radio also makes a loud electical click when it is turned off. I've been told before that such sounds indicate a possible general ground problem with the car's system, but I've checked and cleaned the ground strap to the battery and transmission.

I've also checked the speedo and defogger circuits, but I can't see any related grounds.

I know there are tons of grounds behind the dash, but what could cause the problem to sometimes fix itself?

I'd really appreciate anyone's opinions or advice.

Thanks,

Olivier
82 SC

does your radio and defroster make the loud click with your ignition on but the engine not running?

Yes, they both make the noise with the engine off.

Olivier

Over the years corrosion builds up on the mechanical connections of electrical circuits.

The first and cheapest thing you can do is to unplug and replug all circuits with spade connectors, and loosen and re-torque all the bolt and screw type connections. If you come across one that is excessively corroded or rusted, you should clean them real well and apply a corrosion barrier. Of course check all of your wire ends also.

You need to perform this mission with the battery disconnected.

The gauges come out real easy. I would pull the clock and see if you can determine where the clicking is coming from. Unless you are the original owner there is no telling what kind of modifications have been done by POs.

This would be my plan of attack, intermittent electrical problems that fix themselves are more often then not the result of a bad connection somewhere.

Good luck

Olivier,
I've been looking at my Bentley SC wiring diagrams for the last half-hour trying to get a clue from your description of the car's problems.
I can't see how you could get the symptoms you describe *if* things are wired as they should be. But there is no telling what you might find behind the dashboard due to the dreaded Previous Owner(s). I would suggest a look back there, comparing it with the actual wiring diagram. And while you are looking, you may see some poor connections you can repair.

------------------
Doug
'81 SC Coupe (aka: "Blue Bomber")
Canada West Region PCA
members.home.net/zielke/911SC.htm

oliver,
There is a ballist resistor bhind the clock, yet above it...more like a fusable link. At least I think your year would have it. I remember the wiper having ground on it, and perhaps the defro. is grounded/linked through it also. I could be way off, but have a look...maybe it's worth a look.

S in VA

Hello

Speedo is speedometer not tachometer.

Speedo has the same power suppley then the other instruments indipendend from the defogger switch. Also there is no direct ground beetween them. Speedometer signal goes separate but has one common ground at the speedo.

The resistor is a condensator for the wipermotor.

Just open you enginelid and look on the left side relayboard. There will be a big silver relayunit witch is the rear defogger relay. unscrew the lash an pull it out.

What happens then ?

Do you have a heatet windscreen ?

What happens if you go to the secound switch position at the defogger ?

Grüsse

Thank you very much for the ideas, but nothing has worked so far. I pulled the gauges and I am fairly convinced the wiring is all according to original factory specs.

Here's what I found:

The clicking was from a standard round relay mounted in the metal cowling under the gauges. I pulled it and tested it by switching it with one that is known to be good, so that doesn't seem to be the problem.

I only see one relay on my wiring diagram, and I always assumed it was the double relay in the engine compartment. I pulled that one and the front relay still clicks. I pulled the cover off of the dual relay and could observe the relays working. Relay 1 activates when the switch is pulled, and relay 2 activates when the switch is turned to the right either in the in or out position.

The rear window does heat in both positions. I think some of the wires in the grid are bad, but I don't thank that should have an effect on this problem.

I also think I eliminated the ground as an issue. I ran a temporary separate ground to see if that was the problem, but the same symptoms were present.

Lastly, I pulled a long relay (looks like a flasher unit) which is just behind the oil temp gauge. I didn't start or run the car, but I couldn't figure out what the relay (or flasher) was for. All the lights, flashers, turn signals, etc. worked. The extra-long rectangular relay had an adjusting screw inside it. Any ideas what it controls? The connections were (if I can remember): G, 31, 15, 86?, 87?. I couldn't find anything on the wiring diagram that matches those connections.

Thanks again, this one is driving me crazy!

Olivier
82 SC

ohecht,

There is indeed a relay, at least there is on my 73, almost directly behind the oil level gauge. In my car, this is the second round gauge from the left. I would pull out this gauge from the interior of the car far enough to reach your hand through the hole. You should be able to see the relay(it's a round one in my car and is the same type of relay that operates my air conditioning system.

In addition, my turn signal relay resides nearby.

I am not sure if this is the rear defog relay, but I just replaced this relay less than 3 days ago as preventative maintenance. If this is the defog relay, it sounds like the culprit to me! Let me know if this fixes the problem.

Bill

------------------
William Armentrout
1973 911T
2.7 carerra rs specs
www.geocities.com/william_armentrout

I swapped it out with a good one today, I will see if the speedo works on the way to work tomorrow. I think the realy was fine, however, because it worked fine in the horn relay location.

Olivier

Well, today on the way to work I had the defroster on stage two (out and to the right). When I quickly turned the knob left and pushed it in, the speedo came back to life! As expected, but I was hoping to solve the problem. FOr now I will just only turn off the defogger after turning off the car to prevent it from happening again.

I am still baffled by the connection.

Thanks to everyone for their help.

Olivier
82 SC

oliver,

Don't give up yet. It's possible that when you press the switch, or move it, you could be disturbing a wire ever so slightly to cause it to disconnect.

If your speedo is working erratically, than it's possible that the ground wire may be the problem.

If you follow the wiring diagram, the red/bk (12V +) is the power wire to the unit. From there power power goes to the speed indicator dial. On the other side, there are three wires on one terminal. The br/re goes to the speedo sensor, the br/ goes to the tach, and the other br/ goes to ground. The green wire comes from the other side of the speedo sensor to the speed indicator dial

Check and see if any one of these wires are being rubbed or in contact with your defogger switch. If not, than take the red/bk and use another 12V source which comes from the ignition switch and see if this doesn' solve the problem.

Steve

Thanks, I'll give that a shot. It does seem to be related to the electrical action of the switch rather than the physical movement. The car also has cruise control, which I need to look into, because it hasn't worked since I've owned the car. The troubleshooting I did on the cruise control electronic unit indicates the clutch switch may be bad or out of adjustment.

Olivier

Hello

Just clean all your fuses and twist them in the holder.

Next step is to loosen the Ground on the trunkfloor. It is a 13 mm nut driverside beetween brakesystem and fuelpipe. many brown wires go on there. Just loosening a bit spray on solvent free electric cleaner and retighten.


VDO tempostat unit sits behind the cardboard right to the brake servo unit. Just pull the big socket.

Did you pull the defoger relay ? Results ?

Grüsse

Everything was the same with the defogger relay pulled (the same clicking relay sound from behind the dash and the inoperative speedometer)

I will check the ground location you mention.

Is the VDO unit you are referring to the silver "oversized pack of cigarettes" unit that controls the cruise control? I have examined that and used the factory manual troubleshooting steps which indicated a clutch switch out of tolerance. I have yet to investigate that possibility further.

Olivier

Hello

The heatet rearview mirrors could be involved. They have the same ground post.
Also speedo and mirrors have the same pwer source on the starter switch.

Also check your cigarette lighter hanging on the same ground

Clutch switch is under the center consule engaged by the clutch cable. It is the same switch like used on the early cars brake light.

Did outpluging the VDO unit change something ?


Generally i still focus on what can avoid geting the signal from the speedsensor on the rear. One is that the power side fails the other is the feedback loop is interfered to one long signal ( insteat the triggering ).

As all cables go trough the main wire loom to the rar i would just run a long speackerwire from the open rear inspection lid to the speedo to shoot out if the wireloom is involved.

Or use a Led hanging in beetween the posts on the speedo to indicate if the triggered signal is coming.


Grüsse

Thanks for the hints, I will definitely try those. I hadn't thought about the rear view mirrors, maybe the intermittent nature of the problem is moisture or condensation related. Next time it happens, I will determine once and for all if it is the power, ground (probably not) or input causing the speedo problems.

Olivier