TEMP GAUGES....both not working!

[smallie9]

I have a 66 Coronet with two temp gauges, the stock one in dash and an extra under dash, both electrical gauges. The sending units are new for both (each has own unit) and all the electrical in the engine compartment and under dash is new.
The stock dash gauge will move ever so slightly as the engine warms, then drop to cold and never move after that.
The under dash gauge will slowly move to 250 plus and stay there.
Any thoughts or ideas on what the problems may be would be appreciated.

 

[Ron H]

Sounds like grounding gremlins - they can be a beeitch! Since your wiring is all new, this rules out some common things with messed up wiring from adding stereos and such over the decades. Since it's only the temp gauges, check your sending unit and wiring as a start if you haven't already. Check all connections to make sure they're tight. Check the unit ground. With 'two' gauges I'd disconnect the added one and see if this affects the dash one and v-v as a check. Could be another ground wire may be needed checking this with a jumper first.

 

[Jeff Brundage]

Make sure your cooling system is purged. You didn’t mention where they were located but air pockets can cause them not to read properly. Double check with heat gun. Also depending on what type of sending units they are, teflon tape can prevent them from making ground connection.

 

[smallie9]

OK, I removed all teflon from threads, checked sending units and they are working. Checked continuity from water pump housing to engine block, and other items on engine (thought gaskets may separate (aluminum) water pump housing from engine continuity) all good. Started engine and made sure no air in system and water pump and thermostat are working.

With engine up to temp, the in dash gauge did move to just the "C" on the gauge and stayed put, and I removed the plug and it dropped. I feel it is working now, just not reading correctly so maybe a new gauge is in order.

I did the same with the under dash gauge and it responded the same,only it still goes to max temp reading. This gauge worked fine previously to me relocating it. I feel they're may be some more grounding concerns I need to track down or improve on here.

I did see some improvement with your guys help so thanks. I will keep plugging away, it seems like I am on the right track now.

 

[Sixpactogo]

You can't have too many grounds. If you have an digital Ohm meter, see what kind of reading you get between Neg post on the battery and the water pump housing. How about the dash frame itself? Grounds and bad connections are the biggest electrical problems with our old iron.

 

[Ron H]

If the grounding checks are looking good and adding them as suggested - yep can't have enough of them on our old cars as posted - (I think I need to do this for my gas gauge and hope not more), if you have a laser temp gun, or can borrow one, see what the gun is indicating for engine temp to have a known check on what it is. It's suggested that this check be made at the thermos. Could be the dash gauge is faulty though wouldn't think the odds are that great it is, but ya never know to rule things out. Also curious as to the 'two' sending units you have. Assume the dash is connected as stock; but where is the under dash one located? Reason I ask is wondering if it's close to some area of the motor that get's hot - near exhaust manifold, etc.