Cooling

[AlanRabbit]

I'm new to the forum. I just dusted off my Dad's 1978 Diesel Rabbit. It has a lot of cosmetic problems so I'm taking it one step at a time before I put a lot of money into it. I put a battery in it yesterday and today after a quick shot of starting fluid it started right up! I let it idle for a while and could see it was getting too hot and the radiator fan never came on. I suspect there is a thermo switch but I don't know how to trouble shoot the fan. Has anyone done that? Thanks.

[briano1234]

Yep we have been there.

The Radiator Fan is isolated from ground, so there is a separate ground and power to it.
In the case of a 3 speed fan there are 2 12V circuits.

Take the connector off the fan, you will see a brown wire (ground) on the plug as well as a red or red-striped wire.

To the connector on the fan jumper the brown wire to ground, and the other straight to the positive side of the battery. The Fan should spin so watch your fingers. If the Fan doesn't spin, then it is bad.

If the Fan spins then check the radiator temp switch Drivers side low on the radiator for 12V One side goes to the fan and the other goes to the battery. If the battery side dosen't have 12v then you can have a bad relay on the switch. If the Switch has 12V then you have a bad switch.

You may want to get a Bentley or a Haynes manual and get to know the circuit diagrams really well.

Further if you haven't replaced the Negative battery cable do it now. You will need 2 1 from the battery to the frame, and the other from the frame to the engine/tranny. It is easier to buy 2 cables from the autoparts store of your choosing. Just remove yours and measure it so you get the right length.

If you have replaced the cable ends, that is good to do, but usually the cables are corroded internally and after 25-30 years needs replacing.

[AlanRabbit]

Thanks for the help. Just as expected, I have more questions! I determined that the fan works, and there are 12v at the positive on the temp switch. I decided to drain the radiator and flush. After I filled it I started the engine and ran it until it got hot. The fan didn't come on. I felt the radiator and it was actually cold on the temp switch side but hot on the opposite side. Does this mean that the water pump is bad? It seemed to be circulating. There was water returning to the fill tank through the small 1/4" line.

Thanks

Also, how hot does the water need to get before the fan comes on?

[briano1234]

The Radiator being half cold/hot is almost a sigh that it is plugged and probably needs a good flushing/ back flushing. The Water pump is also suspect or the t-stat. You could remove the t-stat and look up the hole to see if your impeller is intact, then using a thin screw driver possibly try to spin it to see if it moves on the shaft.

The radiator switch I usually test with a meter and a cigarette lighter... Hold the Lighter to the base and see if it clicks then measure it with a ohm meter to see if it shorts out.

The water temp is usually 195 on the switch before it turns on.

[jettncab]

Also, how hot does the water need to get before the fan comes on?

All depends on the fan switch you have installed:

• 95-84 switch turns on at 95°C/203°F.
• 102-91 switch turns on at 102°C/215°F.
• 85-80 switch turns on at 85°C/185°F. (This is a low-temp switch.)
• 93-88 switch turns on at 93°C/199°F.