Apologies to resurrect an ancient thread, but I noticed that after doing some stuff in the interior of my car, I do not have any illumination to the speedometer gauge. (But the other gauges and lights in the interior work as they should when the lights are turned on.) Is there a way to determine if I have a short somewhere, or would there be a problem with the speedometer gauge itself in this situation?

 

If there are no lights working in only the Speedo, it's likely a bad +12V or the common ground on that instrument. I've attached the schematic so you can see the wiring (lower left). Power for the individual lamps comes in on Pin 7 and they are grounded via Pin 3. If the speedo itself is working then its ground should be good, which leaves the input power as the main suspect. Note that the same tie point also supplies the Clock lights so this might help isolate the problem.

The solder mount connector pins on the back of all the instruments is a common source of these electrical gremlins. Google "Porsche 964 Christmas tree" to get an idea. Removing the instrument and re-flowing the solder on the pins is the usual solution.

Good luck!

Jim

 

Interesting! I removed all five gauges because I wanted to fit new rubber circular O ring things (mine were looking yucky). So maybe with just the tension of pulling the black push-on connectors, I inadvertently dislodged some of the solder around one of the pins? I did look at the back of the speedo last night, but I was mainly looking for a broken connector, which I did not see. I'll look again tonight.

What does this mean exactly? If the clock gauge is working and illuminated, can we determine that the ground is good? Want to understand what you're saying.

 

If the clock lights are OK then 12V is coming from the fuse to the dashboard area but not getting to the speedo lights. This points more to a bad connection on the 12v going into the speedo. Also, since the speedometer mechanism shares a common internal ground connection with the light bulbs, if the speedo works your ground must be OK.

If you were doing work on the gauges, the chances of cracking a solder joint on one of the gauges increased significantly, so that's where I would look.

Hope this helps.

Jim

 

Well, I did my best to capture the solder joints I assume you're talking about, and although I'm no electrical engineer, they seem fine to me? None of the pins seem loose against the chassis/PCB, either :/





One other note: the gauge cluster dimmer (964-618-104-01) (located just above the fuel gauge and oil gauge) is not functional at all. After turning on the main light switch, all the gauges and interior lights turn on (besides the speedo obviously), but the dimmer has no function. I turn it, but nothing happens. I even unplugged the switch itself, but the lights stay on (except for the speedo). When I removed the illumination control relay (964-618-103-00) (located inside the dash behind those two left-most gauges), I get no interior gauge illumination, which is how things should be.

Could the dimmer potentiometer be a problem? Wouldn't that affect all the gauges? Shouldn't removing it affect the chain and cause there to be no lights? The car is on jackstands and wheels removed in anticipation of refinishing the calipers, so not really drivable right now (to confirm whether the speedometer functions at all).

Do you suggest just re-flowing those solder joints on the speedo? Send it to North Hollywood Speedometer (a ≥$500 endeavor, I'm afraid)?

 

OK, some interesting stuff.

Based on what you have checked out and my look at the schematics there may be more than one problem. First, the speedo lights. Since these lights don't come on at all but other lights on the same circuit supplied by the same voltage source are working, there seems to be only 2 possibilities:

1 - All of the bulbs in the speedo are burned out.
2 - There is a break in the wiring between the junction in the +12V source (where it splits to go to the other instruments) and the bulbs in the speedo.

Assuming you have checked the bulbs, that leaves a circuit break. From experience I can tell you that it is not always possible to see an open circuit in a solder connection. A tiny crack caused by 35 years of vibration along with a bit of corrosion can cause a nearly invisible open circuit. If you have a soldering iron or a friend who does electronic work, it is an easy and relatively risk free job to re-flow the solder on these connectors. Using a medium hot soldering iron (350 to 400 C) and some flux core solder you just melt a bit of new solder around the base of each pin until it flows around it. This only takes a few seconds and should ensure a good connection. It's a good idea to let it cool a bit between each pin as you don't want a lot of heat buildup on the inside of the gauge.

As for the dimmer not working, this is a two part circuit: there is a potentiometer (the rotating **** on the dash) that adjusts a transistor circuit to vary the voltage to the light bulbs (I've attached the schematic page that shows this at location K1). If you are familiar with how the Porsche schematic diagrams work, you can trace this circuit to the other page I sent. This is a separate problem that could be the wiring between the adjusting **** and the electronic circuit or the circuit may have failed in the "ON" state. I don't know exactly what this looks like but it probably means some fiddly troubleshooting behind the dash.

Good luck.

Jim

 

I know it's not that, because I checked the bulbs, and they are good

Alrighty! I will do just this. I have my iron at work, so will just reflow all five of those pins. Thanks for the tip on the potentiometer. Will check more into that as well.

One last, somewhat unrelated question: on my ROW car, when I have the main light switch on, and the instruments are are lit up, when I pull the defrost button (to activate), or the foglight button (to activate), the light on that pull switch goes out. But when I push it back in (to deactivate), the light comes back on. Is this the expected behavior? I can't see why it would be, but thought I would ask. I really appreciate your help with these electrical gremlins

 

This is not the right behaviour, just tested it on my C00 1991 C4. When I pull the defrost, the illumination in the switch becomes stronger / brighter. Same with fog lights and rear fog light.

 

What do you think's going on in my situation?

 

Maybe bad ground somewhere?

 

Any tips on hunting that down? 😢

 

Do you still run the original bulbs or did someone upgrade to LEDs at some point?

 

Not sure if you meant to ask me or this guy. I am just running the original bulb type. LEDs seem kind of too blue/white for me personally.

 

So, about the behavior of the lights in the defroster and fog light switches, I took a quick look at the schematic and also checked the behavior in my car. When the ignition is on and the headlights are switched on, if these switches are OFF, the bulbs in the switches will light up at the intensity set by the brightness adjustment on the dash. If the switch is turned ON (pulled out), contacts within the switch remove the power coming from the dashboard dimmer and apply power that is coming directly from the battery. In my case, since my cabin lights are adjusted to mid-bright, this causes the switch bulbs to double in brightness. Turning the fog lights or defroster off causes the bulbs to go back to mid-bright.

If you get no light at all with the switches on, it looks like you have another problem to solve since the full 12V power doesn't seem to be getting to your switches. According to the schematic this is supplied by 15A fuses #14 (fog and back-up lights) and #28 (defroster) in the front fuse box. The illumination bulbs are separately powered from the actual fog lights and defroster elements which are powered by their own relays in the Main fuse box. I've attached the schematic page for the Fuse box.

If you don't have it already you should try to find a download of the Porsche 964 C2 and C4 Service Manual, Volume 7 of which is the full Electric Wiring Diagrams. I'd attach it but the zip of whole manual is 146 MB.

Jim

 

Sigh. Man. It's like instead of solving one electrical gremlin, I'm just discovering more of them! I have an update on the speedometer situation. I re-flowed solder around all those pins, and I still get no illumination on that gauge. Does that mean there's something going on in the speedometer itself at this point?