Temperature gauge non-operational

[Laserface]

I have an 81 Rabbit pickup. The coolant gauge never moves during normal operation. I have a Bentley and went through the troubleshooting tips, but still nothing.

Bentley says first to check thermostat, and I have proper heat in the cabin. Move on to the gauge/sender.

Then it says the gauge should rise if the wire from the sending unit is grounded to chassis. When grounded, the gauge rises and eventually pegs out. This indicates a faulty sending unit. So I replaced the sending unit with no change. Both sending units (temp gauge and glow plug light) show nothing on the temperature gauge, but both properly operate the glow plug light.

Thats all Bentley has. The other paragraph is to troubleshoot gauge that stays at the high end of the dial.

The gauge is ok (per Bentley's test, not during normal operation), and the sending units work, but the gauge stays at rest.

Why doesn't this work? Hopefully I'm overlooking something simple.

[ORCoaster]

Are there different sending units for the gauge?  Proper resistance under temp sense to show on the gauge.  Shorting it out just means you have zero resistance. 
A Proper measurement on the gauge with proper temperature takes the right amount of resistance to the gauge at that temp.

Only thing I can think of is measure the sending unit as it warms up and see what it starts at and where it ends up.  Then check against what it should really be.  Isn't there an ohms number in the Bentley?

[TylerDurden]

I'd measure the resistance between the sender's terminal and the block. If the sender is mounted in the water flange, corrosion may be interrupting the connection.


This is a reading from my MK2 sender:



The one-wire sender from the MK1 may have a similar temperature/resistance profile.

[fatmobile]

Measure voltage on the connector without it connected,..
 and then with it connected.

[ORCoaster]

I think Tyler hit it.  I now remember that the connection between ground and the sender tends to be a problem.  Most MK 1 and 2 electrical gremlins can be tied to a bad ground.  Back off the sender and clean the connections on both sides.  I bet you see the fix.


Excellent graph by the way TD.  IR temp readings? 

[TylerDurden]

Ja, IR thermometer and a pan of water.

A flaky sender can make you crazy. A real gauge or IR thermometer as backup can save a bit sanity.

[ORCoaster]

Please explain how you measure resistance with a pan of water.  Temps from it maybe but that comes from IR thermo no?

[TylerDurden]

Well, I forgot to say I had a package of freeze-dried multimeter, leftover from Y2K. 

[ORCoaster]

Obviously a very resourceful person.  Not only well prepared but well deployed.  Freeze dried goods shouldn't go more than a few years.  The burns and freezer cycles just degrade the product.  I hope those readings on the resistance were unaffected.  Looks fairly smooth and logical. 

Good work even with old and cold equipment.

[homerj1]

I found this site helpful as I had a similar issue
http://www.toplessrabbit.com/forums/viewtopic.php?f=27&t=2042

[Laserface]

Thanks for the replies guys. Right after I posted this, I pulled the motor to replace a leaking rear main seal and replace the clutch. I'll update this when I start troubleshooting.