Temp Gauge (short?) Problem

[SpiderGuy]

The Needle on the Temp Gauge rises steadily until it gets past the 190 mark then it suddenly pegs all the way to the right as far as it can go. I suspected it was a short in the Gauge so I replaced the Gauge with a new one but it does the same exact malfunction. (so much for the old "Throw New Parts At It" technique) I had previously replaced both of the Temp Sensors in between the spark plugs with new ones, so probably not the fault. Why are there 2 and what does each one do?

Does anyone know if there are relays or particular sensors that could be the problem, or what I can check ?
Seems like the Radiator Fan should be coming on at some point triggered by a switch? relay? sensor? at about the temp that the gauge is shorting out.

I connected power and ground to the Radiator Fan and the Fan does work just fine, but I have never yet seen the Fan get switched on automatically.

I see there is a wired switch/sensor? at the bottom of the radiator and one at the fitting at the top in between water hoses in front of the timing belt cover, but not sure what they do, or if one of them could be the cause of the problem?
Thanx if anybody can help guide me to a solution. (electrical stuff is a real weak point in my bag of skills)

[PaulC]

First photo is the fan switch to turn on the radiator fan.
#2. Not sure what that's for as my 79 doesn't have it, just a plug.
#3.. Can't tell from the photo but the one with a Grey/black wire is the overheat switch to peg the gauge and turn on the red lite. The other is the sender for the gauge.

[18Fiatsandcounting]

To add to what Paul said above, that sensor at the top of the front coolant outlet is the coolant temp sensor (CTS) for the ECU of the fuel injection system. It doesn't have anything to do with the gauge.

And, as he notes, one of the sensors at the top of the cylinder head is for the temp gauge, and one is a redundant sensor of sorts that pegs the gauge when the engine is in danger of seriously overheating. These sometimes get switched around, but you can disconnect them one at a time to tell the difference. The temp gauge sensor is the one that causes the temp gauge to drop to zero when the wire is disconnected.

The CTS can fail, but this results in erratic engine performance and not a problem with overheating. Since your issue is overheating, there are several possibilities (in order of likelihood): 1) there is a large air bubble in the coolant system, 2) the radiator is gunked up and the coolant flow is not very good, 3) the temp is fine but the sensors are bad, or 4) the fan switch (bottom of radiator) is not working (these don't often fail, but accumulated crud in the bottom of the radiator can make them come on way too late).

#1 is solved by burping the system (many posts on this forum on how to do that), and #2 just involves taking the radiator out and having it cleaned and running a hose of water through the rest of the system to flush out the gunk.

To test theory #3, disconnect the wire (once you determine which one it is) from the "redundant-pegs-the-gauge-sending-unit" and see how your temperature behaves. If still too hot on the gauge, back to #1, #2, and #4. If disconnecting the wire solves the problem, you need a new sending unit. These do fail, although it's not that common.

-Bryan

[SpiderGuy]

Thanx guys for the good info. I forgot to mention that it is not overheating, runs cool so far even without the fan coming on.

Now that you showed me what each part is I will check them out as advised. Looking forward to getting everything sorted out and working right. I've driven it around the neighborhood for about 50 miles so far and it is real fun to drive, really puts a smile on your face and on some people who see it . Hoping to get everything working good enough so I can have the confidence to take it up into the Rocky Mountains on some trips. Maybe even drive it to the Top of Pikes Peak this summer

Thanx again Guys Brian and Paul very helpull advice

Keith