Temp Gauge Problem
Thread Starter
Pro
Joined: Oct 2000
Posts: 558
Likes: 22
From: Vancouver, B.C., Canada
Temp Gauge Problem
The gauge on my 72 has quit working. It was working last fall when the car was put in storage. Can the sending unit be checked someway wih an Ohm meter?? Any other checks I can do??
Former Vendor
Joined: Aug 2006
Posts: 76,656
Likes: 1,847
From: Jeffersonville Indiana 812-288-7103
St. Jude Donor '08-'09-'10-'11-'12-'13-'14-'15
Yes you can use an ohm meter to check the sender. What is the guage doing?
If the dash unit is missing a ground it will peg to the right, if the dash unit has a grounded input signal it will peg to the right and if it is missing input signal it will go to the left.
Willcox
If the dash unit is missing a ground it will peg to the right, if the dash unit has a grounded input signal it will peg to the right and if it is missing input signal it will go to the left.
Willcox
Le Mans Master
Joined: Aug 2006
Posts: 7,411
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From: mount holly NC
2025 c3 ('74-'82) of the Year Finalist - Unmodified
2019 C3 of Year Finalist (appearance mods)
Gauge test.
With the key on remove wire from sending unit: gauge should go to cold.
Now ground wire: gauge should now go to hot.
To bench test gauge apply12 volts to bottom quick connect and ground middle: needle should go down.
Ground to top makes needle go past high.
PG.
Now ground wire: gauge should now go to hot.
To bench test gauge apply12 volts to bottom quick connect and ground middle: needle should go down.
Ground to top makes needle go past high.
PG.
Safety Car
Joined: Sep 2007
Posts: 4,443
Likes: 326
From: Indy Indiana
get you a autozone digital multimeter....
could be open in the wire, as was suggested, to test, ground the sending unit wire to see if the gauge pegs hot.. if it does then test your sending unit by testing the ohms decrease as the engine goes from cold to hot. put ohmeter lead one on the tip of the sending unit, and lead two on any engine ground and record ohms as you start the car and let it come to operating temp.. the ohms should drop...
if the ohms do not drop then you have a bad sending unit or a bad ground..
there are exact specs out there to what the ohms should be at what temp., but that is a calibration issue.. here, all you need to do is prove the sending unit is working
could be open in the wire, as was suggested, to test, ground the sending unit wire to see if the gauge pegs hot.. if it does then test your sending unit by testing the ohms decrease as the engine goes from cold to hot. put ohmeter lead one on the tip of the sending unit, and lead two on any engine ground and record ohms as you start the car and let it come to operating temp.. the ohms should drop...
if the ohms do not drop then you have a bad sending unit or a bad ground..
there are exact specs out there to what the ohms should be at what temp., but that is a calibration issue.. here, all you need to do is prove the sending unit is working
Former Vendor
Joined: Aug 2006
Posts: 76,656
Likes: 1,847
From: Jeffersonville Indiana 812-288-7103
St. Jude Donor '08-'09-'10-'11-'12-'13-'14-'15
If the needle is to the left you are missing input signal. (ohms).
First make sure the wire is on the sending unit, then inspect the wire to make sure there is not a break in it somewhere.
If you go to the sending unit you can test it for ohms with a regular multi meter. Cold it should be around 375 or higher.
As far as there being an accurate scale on these things. . . There is nothing that is 100 percent accurate but you can get in the ball park with
220 deg - 70 ohms
200 deg - 90 ohms
160 deg - 140 ohms
140 deg - 250 ohms
100 deg - 340 ohms.
Willcox
First make sure the wire is on the sending unit, then inspect the wire to make sure there is not a break in it somewhere.
If you go to the sending unit you can test it for ohms with a regular multi meter. Cold it should be around 375 or higher.
As far as there being an accurate scale on these things. . . There is nothing that is 100 percent accurate but you can get in the ball park with
220 deg - 70 ohms
200 deg - 90 ohms
160 deg - 140 ohms
140 deg - 250 ohms
100 deg - 340 ohms.
Willcox
Former Vendor
Joined: Aug 2006
Posts: 76,656
Likes: 1,847
From: Jeffersonville Indiana 812-288-7103
St. Jude Donor '08-'09-'10-'11-'12-'13-'14-'15
I doubt it will be the sending unit. I think if you test the output you'll find it working. Most of the time when you are missing signal it's a wire problem between the dash and the sending unit.
If you purchased a mutlimeter or have one. Ping (test for continuity) the wire from the sender to the dash. I think you'll find it has a break in it somewhere.
If you do not know how to do this, post again.
Willcox
If you purchased a mutlimeter or have one. Ping (test for continuity) the wire from the sender to the dash. I think you'll find it has a break in it somewhere.
If you do not know how to do this, post again.
Willcox
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Le Mans Master
Joined: Jun 2008
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From: Rockledge FL
here is what you do because i had to do the same thing
to test the dash wiring go to radio shack and buy the right combination of resisitors to give you 200ohms (based on the same chart already posted)
one side to the wiring one side to ground with the ignition on
the gauge should read 200.
then if that works its the sender (my case)
the senders from local parts houses did not work at all.
testing them in boiling water and an ohm meter would show them to be way off when they reached 200 degrees.
i bought a replacement from lectricunlimited.com and tested it and as soon as the thermometer in the boiling water reached 200 degrees the ohm meter said like 94 ohms.
to test the dash wiring go to radio shack and buy the right combination of resisitors to give you 200ohms (based on the same chart already posted)
one side to the wiring one side to ground with the ignition on
the gauge should read 200.
then if that works its the sender (my case)
the senders from local parts houses did not work at all.
testing them in boiling water and an ohm meter would show them to be way off when they reached 200 degrees.
i bought a replacement from lectricunlimited.com and tested it and as soon as the thermometer in the boiling water reached 200 degrees the ohm meter said like 94 ohms.
Thread Starter
Pro
Joined: Oct 2000
Posts: 558
Likes: 22
From: Vancouver, B.C., Canada
I fixed the problem!! It was the sending unit. The core had completely delaminated from the shell. I guess the heat/cold cycles killed it. I bought a replacement and so far it seems to be working as before. The grounding suggestions for the gauge worked great. This was a new one for me.
Thanks for all your help/suggestions.
Thanks for all your help/suggestions.
