Topic: Temp gauge and temp sending unit

[Sig 65]

This one has me stumped.  My temperature reading was always reading low on the gauge but finding the engine (6cyl) is running fine but the gauge is off. I replaced the sending unit and it read right for a day, dead center on the gauge when hot. Then next time I drove it shows cold, even lower the the original unit. I bought another sending unit but it shows the same thing. After extensive research and testing I find everything seems to work right, the sending unit, wiring and the gauge itself but it still reads almost at cold.

Here is what I have done so far.

1. I checked the gauge by grounding the wire to it and it swung up to high as it supposed to do.
2. With key on i put a test light on the wire and it pulses, meaning the wiring and the resistor is working.
3. I put an ohm meter on the sending unit cold and it read around 310 ohm and when I boiled it in water it went to about 30 before water boiled,  (I found some info it should read 49 ohm at 180 degree F) so it seems to work properly.  I even tried 2 sending units.

Still it reads almost at zero and moves very little. So I don't know where to go from here.  Any ideas?  Can this be a ground issue at the sending unit?

[Bob Gaines]

This one has me stumped.  My temperature reading was always reading low on the gauge but finding the engine (6cyl) is running fine but the gauge is off. I replaced the sending unit and it read right for a day, dead center on the gauge when hot. Then next time I drove it shows cold, even lower the the original unit. I bought another sending unit but it shows the same thing. After extensive research and testing I find everything seems to work right, the sending unit, wiring and the gauge itself but it still reads almost at cold.

Here is what I have done so far.

1. I checked the gauge by grounding the wire to it and it swung up to high as it supposed to do.
2. With key on i put a test light on the wire and it pulses, meaning the wiring and the resistor is working.
3. I put an ohm meter on the sending unit cold and it read around 310 ohm and when I boiled it in water it went to about 30 before water boiled,  (I found some info it should read 49 ohm at 180 degree F) so it seems to work properly.  I even tried 2 sending units.

Still it reads almost at zero and moves very little. So I don't know where to go from here.  Any ideas?  Can this be a ground issue at the sending unit?

that is what it sounds like. Add a additional ground to the sender and test it. De sure to run a ground from something other then something bolted to the intake.Let us know what you find out.

[midlife]

Aren't the sending units supposed to be from 73 to 13 ohms?

[GT500KR]

I just went through a similar issue with my Bronco, Only it was reading extremely high, then normal. Was very erratic. I would look down at the gauge occasionally, and it would be almost pegged. Went through replacing senders, checking gauges, etc. It turned out to be the Constant Voltage Unit on the back of the dash. If you have another one laying around, it might be worth a try.....

[Brian Conway]

     Not exactly apples to apples but by way of general information.  These are the readings from my 65 289 car 180 degrees and 28.6 ohms.  The scale you are using that includes 310 ohms is not familiar to me.  Brian

[jwc66k]

The scale you are using that includes 310 ohms is not familiar to me. 

Metric ohms?
Jim

[Sig 65]

So I did as suggested and put a ground wire from the sending unit while it is in the engine block but still it barely moves, just from cold to the first line.  Then I took the unit out, connected it to the gauge wire and ground and put it in a pot of boiling water... gauge read dead center!  Somehow when the sending unit is in the engine it does not work properly.

Stumped again....

Is it possible the coolant does not reach operating temperature?  If I get one of those lazer temperature meter what should the temperature read at and where should I read it at.

ps.  I did install a 180 degree F thermostat. (and I am pretty sure it is facing the right way...  ;o)

[Bob Gaines]

So I did as suggested and put a ground wire from the sending unit while it is in the engine block but still it barely moves, just from cold to the first line.  Then I took the unit out, connected it to the gauge wire and ground and put it in a pot of boiling water... gauge read dead center!  Somehow when the sending unit is in the engine it does not work properly.

Stumped again....

Is it possible the coolant does not reach operating temperature?  If I get one of those lazer temperature meter what should the temperature read at and where should I read it at.

ps.  I did install a 180 degree F thermostat. (and I am pretty sure it is facing the right way...  ;o)

Out of curiosity did you do as I suggested and ground the sender to something that was not on the intake manifold like alternator, exhaust manifold etc. ?

[Sig 65]

Bob, Yes I did use a wire to ground it and tried couple of places, engine and frame.  That suprises me that it did not work.

WT8095, No I did not use any teflon tape.  There is some material on the threads but my understanding is that they are conductive (?) and since I have removed it several times that material has really worn down.

I cleaned all ground connection I could find, Battery, altenator, etc.  Is there a specific ground wire that could be bad?

[Sig 65]

So I did one more try today.

1.  With the sensor out, I connected it and ground to battery neg. Then I put the sensor into pot of boiling water and gauge needle went up to almost center.

2. Put sensor in engine block, connected it and also with the same ground wire as before and needle just barely went up to first line on gauge.   

Getting rather frustrated.

[GT500KR]

So you've ruled out the constant voltage unit / gauge regulator?

[midlife]

I'll refer back to my original statement: this sending unit has the wrong ohms readings. 

If you put it in boiling water (212*), the needle should deflect much more to the high side than the center.  Normal operating temperature range (180-192) should place it in the center to left center.  It all holds together: you have the wrong range for the sending unit.

[Sig 65]

I think I have ruled out the constant regulator.  I get a pulse back with test light and grounding the sending wire puts the gauge on high, and all other gauges work properly.  Once I lost the ground to the regulator and all gauges went whacky.  Also boiling water makes the gauge go center.  I have 3 sending units and as far as I can tell they are the right ones.  What puzzles me is that it worked perfectly for the first drive.  Yesterday I also added a grounding wire from the engine to the firewall.  Maybe I just order a regulator and replace it anyway to see.

[Brian Conway]

     A number of the members here and on other forums as well have had this problem.  There seems to be little confidence, rightfully so, in the aftermarket/reproduction temperature sending units.  In most previous discussions, along these lines, the most often successful suggestion is to locate an original Autolite or Motorcraft sending unit.  This page from an MPC will give you the info you need to begin this search.  The part C3 or C6 and the SW number are your guidelines and may be the solution to your current dilemma.  Brian