Overheating, thermo fan not working, cause is ignition switch

[frustrated]

Hi folks,

Just thought I would let you know the solution to my overheating VP.

Intermittently, the radiator thermo fan wouldn't come on, and the engine would overheat.

The clue to the solution is that when this happened, the internal heater fan didn't work either. Checking the circuit diagram showed that the common link is the ignition relay near the driver's feet. This in turn is controlled by a sliding contact in the ignition switch on the ignition lock barrel. When I took this off and opened it up, there was evidence of poor electrical contact to the spade connector that goes to the pink wire (the ignition relay). I'm going to clean the contacts now and reinstall it, and am 99% confident that this was the problem.

[UPDATE: I was wrong! The real problem was the warning lamp check relay, see a few posts down.]

It would also have stopped the side-mirror adjusters and the reversing lights. So... if your thermo fan won't come on, check to see if you side-mirror adjusters work, and if they don't you might have the same problem!

[frustrated]

Hm... well, I was wrong.

I put the car back together after cleaning up the ignition switch, then went for a drive, and after ten minutes the internal heater fan stopped working (and the A/C wouldn't turn on), BUT the side-mirrors still worked. I wiggled the key, and it had no effect (which it probably would have if the ignition switch was a problem). It came good by itself after a couple of minutes.

So, I looked at the ignition relay. Coil resistance was good (80 ohms). The only unusual thing was that when ramping up the voltage on the coil with a test power supply on the bench it would make one clicking sound at about 5V and another at 6V, whereas another relay made a single click. Contact resistance (with 6A current flowing) was fine.

Funny thing was, the internal heater fan kept running when I unplugged the "blower heater" C. Beats me. [UPDATE: this relay is only used when the blower is turned fully on; it is unused for the low and medium fan settings.]

Anyway, I swapped the ignition relay with the one next to it (read window demister, I think) and we will see what happens.

The strange thing is that the car definitely has a problem with the side-mirror motors occasionally not working, which points to the ignition switch. So there could be two (or more) semi-related problems.

Now you know how I got my user name.

[frustrated]

Me again.

OK, I'm closer to solving this. I wired up 4 LEDs, one to each of the four pins of the ignition relay (soldered to the pins themselves, in such a way that the relay could still be inserted into its socket), with the negative side of the LEDs going to earth through a 4.7k resistor. This acted as a simple visual indication of the voltage on each pin, so that when the problem occurred, I could easily track it down.

And this afternoon, I discovered by accident that if I pressed the rear window heater button, the negative side of the ignition relay coil would increase in voltage (the LED got brighter) to the point where the ignition relay cut out (i.e., the voltage drop across the coil must have been less than a few volts). I poked around a bit where the internal fuses were, and suddenly the problem went away. Maybe there was a bad contact on one of the fuses.

So, it looks like there is a poor electrical contact somewhere on the negative side of the ignition relay coil.

[anfo]

give it a rub with a bit of wet and dry check the snugness of the fit in the socket

[frustrated]

Anfo - thanks for the advice.

The good news is that I've found the problem. 100% definite this time. It was the warning lamp check relay. This relay is wired across the starter motor, and illuminates all the warning lamps on the dash when you start the car - just so that you know they are working. Normally the relay is not energised, and the normally-closed contacts on the relay provide the negative return line for (amongst other things) the rear window heater relay and the ignition relay. The ignition relay in turn is part of the circuit for the engine fan relay.

I took out the warning lamp check relay (in the engine bay) and found that by tapping it gently I could make the normally-closed contacts make and break contact. So this is 100% definite confirmation of the problem. Even when closed, the contact resistance wasn't great (the contacts dropped 0.2V at 6A). After taking the relay apart, cleaning the contacts, and twisting them slightly to change the contact point, the voltage drop was only 0.01V at 6A. I also swapped in the fog lamp relay, which is the same type, and unused in my car.

So, the end result of all this is that if your engine fan isn't working AND at the same time your internal blower isn't working, THEN you are likely to have a problem with either the ignition switch, ignition relay, warning lamp check relay, or the wiring between these.

[immortality]

well done mate. most auto sparkies would have given up ages ago