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zoegrant
11-18-2009, 06:05 PM
When I start up my 50 commander the temp guage slowly over 30 seconds creeps up to midway between cold & hot ..then when it hits midway it snaps up to hot in a split second...this is all with a cold engine.......does this sound like a heat senser or an internal gauge problem ? ..John

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Jeff_H
11-18-2009, 07:02 PM
I think that symptom points to either a bad gauge or the gauge is not getting a ground.


Power from the Key switch goes to the IGN terminal
on the gauge and splits to 2 coils inside the gauge.
Coil 1 is pulling the temp gauge needle to the right (hot) and Coil 2
is pulling it to the left (cold). There is also
a spring pulling it to the left too. The 2 coils are not the same
resistance so the "cold" coil (coil 2) has a weaker pull on the needle than the one going to the sender.

When things are working correctly, the sender has a high resistance
when its cold and low resistance when its hot. So, the current in
coil 1 is lower than coil 2 when cold and the needle is pulled to
the cold side. As the sender gets hotter, its resistance drops and
more current flows in coil 1 and it overpowers coil 2 and the spring so the needle is pulled to the hot side.

If coil 2 is open (broken) or the ground to it is lost, then coil 1 is
the only actor. With no power applied to the gauge, the spring keeps
the needle to the left, but when power is applied to the gauge,
coil 1 will have current and quickly overpower that spring without coil 2 there to keep it back.

I hope this makes some sort of sense [|)] I made a little diagram to
put with this using puctuation charactors but all the spaces I used got stripped off when I previewed it so I deleted it.

If the '50 gauges are like the '53-'55 ones, you can measure the resistance of each terminal to the car body (ground). Do that w/o any wires attached. You should expect to measure about 60 ohms from the ign terminal to the sender terminal and about 95 ohms from ign to ground. I suspect that ign to ground will measure open and its either broken inside or no ground.


Jeff in ND
http://i156.photobucket.com/albums/t5/ee-engineer/devilstowerthumbnail.jpg
'53 Champion Hardtop

52 Ragtop
11-19-2009, 09:48 PM
My 52 did that, slowly up to normal temp, then BANG fullhot. I replaced the sending unit, same thing, replaced the gauge, and it's now fixed!

Jim

Milaca
11-19-2009, 11:29 PM
My Hawk does this also. I even replaced the gauge but with a used unit and it does the same thing. So, I guess I need to purchase a new unit. Nice explanation of how this stuff works, Jeff. I am electrically ignorant but have some understanding of it now. Thanks :)

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Brent's rootbeer racer.
MN iron ore...it does your body good.

Jeff_H
11-20-2009, 09:03 AM
Forgot to mention the other possibility is there is a short to ground in the wire that goes to the sender.

Jeff in ND
http://i156.photobucket.com/albums/t5/ee-engineer/devilstowerthumbnail.jpg
'53 Champion Hardtop