Temp Guage Malfunctioning - Front & Rear Probe?

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Aug 22, 2007
Phoenix, Arizona
My temp guage seems to stay pegged zero and then randomly decides to go all the way hot and then drops back to zero.

Do I need to replace both the front and rear probes? Why do I have two anyway? Both seem to be on the cooling jacket of the head. Are they thermistors?

Cool Cruisers has them for like 10 bucks, but I need to figure out if I should replace both of them.

Any insight greatly appreciated.
GY wire and temps sensor

the 71 schematic shows a green and yellow wire from the temp gauge to the sending unit .Power from the gauge is jumped on the yellow wire from the fuel gauge

The temp sending unit works by varying the resistance to ground and therefore the current to the gauge. grounding the temp sensor wire will peg the gauge hot I think

I would say check the gauge connection---clean and tight
same for the temp sensor

Make sure the temp sensor has a good case ground by cleaning it up, removing rust and dirt etc

You can check the sensor with a multimeter ---just check that the resistance changes smoothly as it warms up and if it jumps around like the meter does change it

After that I might just run a new wire on the outside from the gauge to the temp sensor and see if it works--if it does , install it permanently

Also I have not run into it myself but I have heard that air in the system can do that also

good luck with it:cheers:
I have twospots in my 78 2F head. my oem temp sender was replaced and was constantly hot. I filled the system with the truck pulled up a hill,cracked it open then shut it to remove any air. This worked unitl the truck got hot! I had other issues. I installed an aftermarket unit in the other hole and it seemed to be much more accurate, nature of the best I suppose.
When I put the rebuilt head on I noticed that the oem unit was right next to an exhaust port, when I put the aftermarket sending unit back on I put it in the oem hole ( I think maybe closest to the firewall). The oem unit is now in the forward location and does run hot from time to time, but the aftermarket guage is cool all the time. I think the placement does make a difference, it makes sense. The oem setup with smog stuff had a second temp unit (1979 and up maybe), why this was there is beyond my expertise. I believe there was another unit on the manifold as well. I desmogged so couldn't care less about that stuff but it would be interesting to know about. I say ditch the oem guage and get on that works well. Good Luck
thank you both very much! i will do some further testing and indicate what i find.
There are two temp senders in a 74 head. One is a thermistor for the dash gage. The other is on-off for the emissions computer. Sounds like your gage is connected to the emissions sender.

FWIW, the order the go in from rear to front is:
heater outlet
gage sender
emission sender.

The sender is currently swapped w/ the heater outlet, so there is a possibility that the same person also swapped the two senders.
That makes perfect sense, Jim

Thank you Jim. I know exactly what happened, and this confirms my hunch. My temp guage worked fine before going to the machine shop awhile ago. When I had the valve job done, i'm sure the machine shop put the sensors in backwards.

This seems vary reasonable. BTW...does the threaded fitting the probes sit in differ for each? One is silver in color and the other is brassy looking.
The fittings look different, but I don't know if they are different inside. The threads in the cyl head are the same in all 3 holes. Sounds like you just need to drain the coolant down below the head, then pull all 3 parts out of the head and get them in the proper locations.

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