1978 spider cooling fan wont turn off

[carman1957]

Out of no where the cooling fan won't shut off. The engine is cool. I disconnected lead from rad temp switch and it stopped. I can't find the relay. I noticed the temp gauge in the car shows full hot. The car is definitely cool.......

Any ideas
Thank you
Carman

[juvius]

it's a bad temp switch. it's stuck closed. when it is closed, it will send a negative to the relay. the relay is switch by the negative input. once the relay receives the (-) from the temp switch, it then sends power to the fan. that's why the fan shuts off when you unplug the temp sensor.

they are around $20 and shipping new. i have a used one that you can have for shipping

[carman1957]

That's what I thought. The temp gauge shows full hot and pinned. That's weird.the car is cool.
Thx
C

[juvius]

A whole different temp sensor runs your gauge needle

[carman1957]

Do you know where the cooling fan relay is?
Thx
Carmam

[juvius]

Cooling fan relay is on the bottom drivers side corner of the radiator

[carman1957]

I thought that was the radiator fan switch....Is that the same unit that you say is stuck closed that needs replacing?
Thx
Carman

[juvius]

I misread. It's on the drivers side engine bay. Follow the wires off the switch. One goes to the fan and the other to the relay

[zachmac]

Call me crazy but I don't think there is a "relay" in the fan circuit. A hot lead goes directly to the fan at all times from fuse 9, even key off as fuse 9 is fed directly from the alternator / starter hot lead.

The temp switch (cooling fan thermo switch 104 on the wiring diagrams, is not a relay as it is simply opened or closed by temperature, not a separate trigger circuit) in the lower driver's side of the radiator interrupts the ground path to the other side of the fan when cool. Hot, the switch closes and completes the circuit to the fan. If the switch fails closed the fan will run until the battery dies. Since unplugging the switch stops the fan the switch is stuck closed. If you have a multimeter a quick check of the resistance across the radiator switch should show little or no resistance, verifying it is stuck closed.

The temperature gauge as mentioned uses a completely different circuit with no connections in common with the fan circuit. If it is pegged high you have: 1) reversed wires, 2) a bad temp senor, 3) a very hot engine, or 4) a bad gauge.
The sensor is one of the two in the spark plug valley, can't remember which. If you disconnect the wires to the sensor and ground it you should get the gauge to deflect. The 78 wiring diagram also shows a water temp thermo switch 18/2 in parallel with the temp gauge, so maybe BOTH sending units feed it? Any one know?


It may sound weird, but you have two seemingly related but independent problems.

[juvius]

The diagram I'm looking at has a relay for the coolant fan

[aj81spider]

Curious over the disparity I went to my files where I have stored all kinds of useless stufff (like the wiring diagram to a 78, even though I own a 74 and used to own an 81). The wiring diagram I downloaded from Brad Aritgue's site for a 78 doesn't show a relay. Don't know what the right answer is, but I can say that my 81 didn't have a relay.

[juvius]

Curious over the disparity I went to my files where I have stored all kinds of useless stufff (like the wiring diagram to a 78, even though I own a 74 and used to own an 81). The wiring diagram I downloaded from Brad Aritgue's site for a 78 doesn't show a relay. Don't know what the right answer is, but I can say that my 81 didn't have a relay.

you're right... it looks like they did away with the relay in later cars... i was looking at my early diagram since it's my only hard copy... so i think 74 is the last year for the relay then it's totally ran off the temp switch and straight power to the fan like you said. either way, with the fan not cutting off, it would be either a stuck cooling fan temp switch or a grounded out wire between the fan and the temp switch.

sorry for the confusion.

[seabeelt]

Early cars had the relay. Kind of like putting the headlight upgrad in your car. Since the fan draws high current they used a relay to send power direct. However, one of the differences that I know of is that early cars were wired to a "key On" fuse and only run with the car in the run position (powers the relay). Later year cars are wired to an Alyays On" fuse and will run even with the key off - hence hearing the cooling fan run after you shut the car off.

[juvius]

looks like another reason for the relay in the early car is that the fan, horn and inspection light are all ran off the same wire.

[zachmac]

you're right... it looks like they did away with the relay in later cars... i was looking at my early diagram since it's my only hard copy... so i think 74 is the last year for the relay then it's totally ran off the temp switch and straight power to the fan like you said. either way, with the fan not cutting off, it would be either a stuck cooling fan temp switch or a grounded out wire between the fan and the temp switch.

sorry for the confusion.[/quote]

I wouldn't lie intentionally and I always make sure I am looking at the correct wiring diagram for the year since they changed so often. PLUS I just rewired my 78 throughout eliminating tons of wire and useless stuff so I am pretty "current"" on 78 wiring.