79 308GTB fans don't power up when temp rises

Have you checked the contact connection points. My XKE had the same problem and I unclipped the wires from the fan to the the power source at the connectors, cleaned and roughened the metal ends with a metal file and reconnected them. The fan worked perfectly after that.

 

Remove & short together the two wires on the thermostat on the radiator - if both fans come on the problem is the switch or the contacts to which the wires are connected - its usually the latter - I'm pretty sure there are threads here about replacement thermostat switches if necessary

 

Even if the radiator thermo switch appears to test okay, I would replace it...get one for a 1979 VW Rabbit...about $17. They even make them in two temp ranges...get the lower one.

 

You have a bad relay. Might as well replace them both while you're replacing relays and it won't hurt to clean all of the fuse contacts while you're there as well. Be sure to get the correct relay, they may all look the same and fit in the same hole but......

 

You can jump the relay with a wire and see if they come on. I think it is pin 30 and 87, there will be a diagram on the side of the relay. They are standard bosch relays try A&F in oakland they can get them without "The Ferrari routine of grabing your ankles"

 

The switching device mounted in the bottom radiator tank is a bimetallic thermostatic switch or thermoswitch (not a thermocouple). It should have an abrupt conductivity behavior change with temperature (cold=open and warm=closed).

The internal switching coil of a ...113 Bosch relay draws 140~160 mA at 12.6V (and it decreases a little as the coil warms up). If the engine is running (i.e., the alternator has raised the voltage slightly), it might draw a little more current, but your measurements seem much too low for actuating two ...113 relays -- you should be measuring ~300 mA total flowing thru the thermoswitch wires if you directly connect the GV (yellow/green) wire to the N (black) wire with your ammeter leads.

I think it would be highly unlikely for two relays to fail simultaneously, so if you're only getting 70~100 mA, you've got some high impedance in a connection (or the thermoswitch itself -- if you're leaving that in the circuit when making the current measurement).

The next thing to do would be to use your ammeter to connect the GV wire at the thermoswitch to the "-" battery terminal:

If that works (but touching the GV wire to the N wire doesn't work), the N wire is not well grounded.

If it's still dead, follow Larry's suggestion to remove those ...113 relays and touch a jumper wire from the 30 terminal slot to the 87 terminal slot (either middle 87 or bottom 87 -- it will have a female metal terminal)
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If one fan comes on with AC, you can rule out that relay. I find it hard to believe it's the relays. The things that generally can cause this issue (in order):

1. Thermostatic switch in the radiator, or more often the connectors on it.
2. Relays (but rarely BOTH like Steve said).
3. Fuseblock. On the right fuseblock, the rightmost pair of fuses both get their current through the busebar on the back and the through-rivets. When the rivets corrode, it creates resistance, which makes heat, and the fuseblock melts. Now one or more of the contacts on that bar stops working. It is my experience that 90% of the time a non-functional fan in a 308 has a bad fuseblock.

An injected car always seems to melt the left fuseblock on the fuel pump circuit. The carbed ones always seem to melt the right fuseblock on the fan circuit.

Check power at the bottom of the right two fuses on the right fuseblock.

Then e-mail me for the replacement.

Birdman