Can't figure out why cooling fan doesn't come on

[sbsspider]

Hi

I'm new to the forum; but have had my 76 Spider for a few years and love it. I believe Spiders are the best coolest cars that are still inexpensive and parts are readily available and also inexpensive.

Anyway, I am trying to figure out why my cooling fan won't come on when the temp. goes up to the red (over 100 C).

I have done the following:
- replaced thermo switch
- replaced fan relay
- checked wiring between fan motor connector and relay
- I verified I have voltage coming to the thermo switch
- I verified that the bottom of the radiator is getting hot (no air lock)
- I verified that the fan does come on when I bypass the switch (Note: Previous owner hotwired the fan to the ignition so it came on with the ignition. However, this would blow Fuze I after just a few minutes. I took out this hotwire).
- The proper color wires are going to the proper places per the wiring diagram.

That's it - I don't know what else to check. I think it's a wiring issue; as the previous owner removed all the smog equipment and there are wires just hanging in the engine compartment. I have verified that the correct wires are going to the thermo switch and to the fan motor connector and back to the relay. My understanding is that when the thermo switch opens; it grounds the circuit and starts the fan. There is ground from the thermo switch and fan motor connector that goes to the side of the engine compartment.

HELP. I don't know what to do next.

Thanks,

Scott S.

[vandor]

You should not have voltage coming to the thermo switch, as it grounds the relay.
Apply 12V to the post of the relay that goes to the fan. Does it come on? If not either the fan is bad or the wiring between the relay and the fan has an open.

[spider2081]

You should not have voltage coming to the thermal switch, as it grounds the relay.

I think if the switch is in the open position. (engine not hot enough to close switch) voltage should be measured on one side of the switch. Voltage should also be measured on both sides of the fan relay coil when the thermal switch is open. Also the other side of the switch should be connected to ground. So if the switch is open and the meter is connected to the switch terminals voltage should be measured across the thermal switch in the radiator. This check tells you that power is applied to the relay coil, wiring between the relay coil and the thermal switch is most likely good. It also tells you the ground for the thermal switch is good. If the thermal switch closes ( Engine temp above 195)the ground passes through the switch to the relay coil. Now the relay will still have voltage one side of the coil and now a ground on the other side of the coil.

[DieselSpider]

Short in the harness or potentially a bad fan motor overdrawing if it blows the fuse when you bypass the switch. When the engine is cold and you connect a volt meter to both terminals on the switch you should get a reading of over 12.8 volts with the engine running then when the engine is warm enough to require the fan then the voltage should drop to zero since the switch will be closed. A battery test meter showing amperage draw will display how much current the fan is drawing.

[wizard124]

Test the switch.

[sbsspider]

Thanks for the replies! Although it is a holiday weekend, I did some additional investigation for a short time. When I connected the two thermo-fan switch wires together; the fan came on with the ignition. Now, I think this normally tells you that the thermo-switch is bad; but my switch is brand new and I'm not ready to believe it yet. I also noted that I have NO power to the light blue wire that goes directly to the fan motor connector (comes from connector 13; then from the fan relay) when the ignition is on. I think I read somewhere it said that this light blue wire should be hot with the ignition on. Is that correct??

I verified continuity of the light blue wire all the way back to the fan relay. From the wiring diagram; the fan relay gets its power from the violet jumper wire from the horn relay (my horn does work) - Eventually coming from Fuze A; Circuit A. Thus, if the light blue wire should be hot with the ignition and it's not; I'm thinking maybe the violet jumper wire from the horn relay (which is bent at a 180 degree angle) is bad?

Am I thinking correctly here; or should I just believe that my new thermo-switch is indeed bad?? Or could it still be something else??

Thanks,

SS

[spider2081]

If when you connect the wires at the fan switch in the radiator together the fan runs, I believe the cars wiring is working correctly. Your issue is the switch is defective or it is not getting hot enough to switch on. I would recommend you purchase a laser temperature gun and check the temperature of the radiator at the fan switch.

My diagram shows the radiator fan relay has 2 voltage sources. the "hot all the time" violet wire comes from fuse A. it is the voltage that will be switched to the fan motor when the relay is engaged. The other voltage source to the relay is from fuse 10 it is a light blue/black wire. This wire is hot in the "start and run" position of the ignition switch. This wire powers the relay coil. The ground for the relay coil is provided by the fan switch in the radiator. One side of the fan switch in the radiator and the fan motor are wired to ground. When the fan switch closes (195 degrees) the ground is passed to the relay coil. The relay energizes and the voltage from fuse A is now connected to the fan motor turning the fan on.

So if when you connect the 2 white/black wires at the fan Switch together the fan runs all of the wiring, both fuses and the relay have checked good.

[divace73]

is this the bottom of the radiator much cooler than the top when it starts to overheat?

If you so have air in your system and the fan will never come on....

[76was124]

or should I just believe that my new thermo-switch is indeed bad?? Or could it still be something else??

Is it possible when trouble shooting with your new switch some of the wiring was wrong and you had 12 volts and Ground connected to the two wires of the switch? If so , this would have created a short when it turned on, and damage it, You mentioned a fuse blew?

To test just the switch, disconnect all the wires to the fan switch. Connect the two leads from the fan switch to an ohmmeter or continuity tester and get the engine warm enough that the temp switch should turn on. Be careful of the spinning parts(belts etc) while measuring.

If there is no continuity when hot, the switch is bad..

[80spiddy]

I had the same issue. Bad ground to sender. Attach a new ground wire from sender to fender ground pod.