C2 67 Temp sending unit

[Terry S.]

We just put the 350hp factory a/c car back together but can't get a reading on the water temp gauge. Tried two sending units (one new and one old) and they either read nothing or rise very slightly. We did put thread sealer on the first sending unit but not the second. Had all the gauges redone and tried a ground wire from the temp sending unit to the water pump which made the gauge peg all the way above the 250 mark. Also installed new sending unit wiring from Lectric Limited. Any help would be appreciated. Thanks

[John H.]

Re: C2 67 Temp sending unit

Have you verified continuity of the sender wire circuit through the multiple connector on the engine side of the fuse block to the gauge? Many bogus temp and battery gauge indications result from poor contact, terminal push-out, or corrosion on the terminals in those multiple connectors.

[Stephen L.]

Re: C2 67 Temp sending unit

The fact that you pegged the meter by shorting the temp sender output wire to ground tells me the circuitry is ok. (As engine temp rises resistance of sender goes down. A "shorted" signal would give maximum gauge indication). Try shorting the sender "case" to ground. You may have some residual sealant in the threads that is isolating the case from a good ground.

[Jack H.]

Re: C2 67 Temp sending unit

I agree, the fact that the temp gauge pegged when the sender wire was momentarily grounded out in the engine compartment suggests the wiring from the dash gauge through the firewall is intact... Here's a situation we don't see very often that CAN result in the symptoms you stated.

If you study the wiring diagram, you'll find a 6-way power node that drives the fuel gage, temp gage and several other circuits (pink wires, see UPC 12, Sheet D2) which gets power via the ignition switch through a fuse.

If one of the other branch circuits connected to this node is defective (essentially 'nearly' shorted to ground), there'll be insufficient B+ at the temp guage for the circuit to run and register readings on the temp gage. The most 'notorious' parallel branch circuit to fail and generate this kind of fault is the gas gage with its Whetstone bridge circuit topology...

Diagnostics are easy. Follow the wiring diagram and disconnect all of the parallel circuit loads EXCEPT your temp gage and see if it suddenly springs to life. Then, reconnect the other parallel power paths from the power node one at a time until your temp gage stops working again. Now you know where the faulty circuit path is...

BTW, if it's the fuel gage circuit, you should see similar 'funnies' with its operation (fuel gage doesn't move regardless of how much gas is in the tank).