Rad. temp sensor story

Local friend and ’91 syncro Westfalia owner called me last weekend for some help in tracking down an unusual problem. I’ll list the symptoms in point form:
– he had been away for a couple of weeks and the van was parked.
– when he came back the battery was dead.
– his wife noted that the van was making “ticking noises”.
– when he used booster pack to start van, the radiator fan came on and was making a “twig caught in the blades” noise.
– the fan would come on with ignition on.

His first thought was bad radiator temp sensor, so he disconnected it (lower right part of radiator, as you face the van). Even disconnected the fan would come on when ignition on.

I came over at that point and we first dropped the spare tire to look up at what the rad. fan was hitting to make such a noise. Turned out that the 15″ wheels and beefy tired spare had pushed up a section of black plastic tubing (which I am guessing is the brake vacuum line) so that the fan blades would hit it. It wasn’t worn away enough to make a hole, but it was close.

We then started to pull fuses to see why the fan would come on even with thermo switch pulled. Oh, I have to say at this point the van does not have, nor ever did have, air conditioning (a/c complicates rad fan control a bit). We were using the Bentley manual, following the wiring diagram for a 91 westy. Diagram below, you can see how pulling fuse#1 should stop any fan power.

We were puzzled for a bit, thing did not make sense. Also we noted the rad fan had 2 red wires leading to it where above diagram shows red and red/black.
Clearly we were on the wrong track. We looked through Bentley and found a wiring diagram for a 450 W rad fan (in 1986 model tear section).

Ah ha! This looked better.

We found the relay and 50 A fusible link above and to the left of the fuse panel, right above the grounding “crowns”. Remember, still at this time, the rad fan would come on if ignition turned on, even if fuse #1 is pulled and thermoswitch disconnected. Oh and also the wiring diagram shows a blue wire into the thermoswitch where the van had a red/blue wire.

We pulled the relay and lo and behold the fan would not come on, yah! Funny thing was, the relay checked out ok using multimeter and it clicked on and off when energised. Putting the relay back in made tha fan come on, but this time disconnecting the thermoswitch turned the fan off.

Geez, this is long winded and boring tale…

So we bought a new thermoswitch and swapped it in (this is not the time to tell what we found in the rad when we pulled old switch out) and everything worked as it should.

Note the erosion of the switch.

Theory time: we figure that when the van was sitting, the thermoswitch failed and allowed the second stage fan speed to come on. The brake booster vacuum line was worn and the battery drained. My friend’s wife must have heard it at the end of the battery life when the fan was slowly turning and making a clicking noise as it hit the brake booster vacuum line. This long run time caused the second stage relay to stick in the on position. Pulling the relay, the jiggling etc, caused the relay to open, and was then “under the control” of the thermoswitch, and again fuse #1.


  1. Leave a comment

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

%d bloggers like this: