I was chasing down "overheating" issues after replacing a radiator until I found out that I've been misled by a faulty engine temp warning switch. From what I understand, this switch activates when there's high temps on the engine and brings the temp guage to max, regardless what the temp actually is. It's the switch between the spark plugs that has a black-white wire/the one closer to the radiator.
Following advice on this forum, I unplugged the wire to get true temp readings, and found that my cooling system is actually working fine. Now I'm not sure if I should plug back in this faulty switch, or if it's even worth replacing because it had me chasing my tail for a few days for nothing. I read a few members have left this unplugged. What's the forums opinion? Whats the risk? If I leave it unplugged, what's a good way to secure the wire? Also, could this switch activating despite normal temp behavior be indicative of a hidden issue?
faulty engine temp warning switch
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jesc
- Posts: 11
- Joined: Wed Mar 12, 2025 2:08 am
- Your car is a: 1978 Fiat 124 Spider
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18Fiatsandcounting
- Posts: 3860
- Joined: Fri Mar 15, 2019 11:23 pm
- Your car is a: 1969 and 1971 124 spiders
- Location: San Francisco Bay Area
Re: faulty engine temp warning switch
It's pretty common for these overtemp switches to fail over time, and one symptom is a different temperature reading when you disconnect the wire as you found out. They also have the annoying tendency to fail intermittently, in that the needle pegs quickly and erratically when the engine isn't overheating. They are not very expensive and not very hard to replace although you do have to drain some coolant so it doesn't come out of the top of the head when you remove the switch. The socket size for the overtemp switch is usually 22mm, and note it's larger than the size of the gauge sending unit.
The switch simply grounds when it reaches its overtemp setpoint, around 230 oF or so although they're not terribly accurate. Since the wire to it is just a grounding wire, there's no voltage on it, so you can just shove it into a small piece of rubber tubing, wrap it with electrical tape, or get some sort of electrical insulating boot.
The overtemp switch is a redundant system that some people just find annoying so they disconnect it. It would have been cool (ahem) if Fiat had wired this for an overtemp warning light (like the low oil pressure light), but they didn't.
-Bryan
The switch simply grounds when it reaches its overtemp setpoint, around 230 oF or so although they're not terribly accurate. Since the wire to it is just a grounding wire, there's no voltage on it, so you can just shove it into a small piece of rubber tubing, wrap it with electrical tape, or get some sort of electrical insulating boot.
The overtemp switch is a redundant system that some people just find annoying so they disconnect it. It would have been cool (ahem) if Fiat had wired this for an overtemp warning light (like the low oil pressure light), but they didn't.
-Bryan