"When I restored my '60 Plus 4, I changed the wiring to a negative ground system. My heat gauge sits on hot from the moment the engine is fired up. I have gotten conflicting information, some stating this is normal and others saying the needle should start at cold. Please please set me straight. If my heat gauge is incorrect by showing me hot from start up, could my reversing the wiring polarity to negative be the reason the gauge starts at hot?
Thanks, David"


Bill Fink


The polarity will not affect the operation of your water temperature gauge, which depends on the bending of a bimetal strip as it is heated.

Plus 4 electric temperature gauges normally sit on "H," but swing to "C" as soon as they get powered up by turning on the ignition.

To check gauge operation, first make sure you have a good ground between the radiator and the chassis.
Then, to check the operation of the gauge, temporarily ground the wire to the temperature sending unit in the top tank of the radiator to see if the gauge swings to "C." If it doesn't, the wire inside the gauge has probably come unsoldered from the terminal at the back where it connects to power or the sending unit. The gauge can be removed from the dash, the cover swung open, and the wire re-soldered to the post.

Good luck.


Gerry Willburn

Readings on the temperature gauge are independent of battery polarity.

The temperature sender on the Plus 4 is, essentially, a temperature sensitive resistor with a POSITIVE temperature coefficient.  That is, as the sender gets hotter, the effective resistance goes up.

With the ignition off, no current flows through the gauge and it reads HOT. When the ignition is switched on, the resistor is at a low resistance value and the gauge moves to cold as current flows through it.  As the engine (and the sender) heats up, less current flows and the gauge reads hotter.  A very simple concept.

If it stays at hot all of the time, the problem is probably an open in the line, either in the wiring or a ground.  Common problems are corrosion (causing high resistance) between the sender and the radiator or a lack of ground to the radiator.  A 10 ohm resistor from the sender to a good ground should make the gauge read cold.  If it does, the sender or its ground is the fault.  If it does not, the problem lies with the wiring or the gauge.

Is there 12V at the green wire on the gauge?  If yes, repeat the 10 ohm trick on the other terminal of the gauge.  If it does not read cold, the gauge is bad.  If it does, look for a broken wire between the gauge and the sender.

Did you replace the sender?  I have seen senders that read backwards.  I am not sure what models they are for (Plus 8?).

The system (in the Plus 4) is notoriously inaccurate.  Once assured that it is working, let the car warm up to "Normal" (check this with a candy thermometer) and see what the gauge reads.  It probably reads hotter than normal.  By selecting the proper resistor value (probably between 400 and 100 ohms) to put in parallel with the sender one can make the gauge read "normal."  The gauge will no longer be linear but one is only looking for relative readings anyway (Is it hotter or colder than normal).

Hope this helps,


John Worrall

It is normal for the gauge on a +4 to go to HOT WHEN THE ENGINE IS NOT RUNNING!!  When you first start the engine it should move to cold & then gradually up to normal as the engine warms up. Looks as if you should check the wiring on the guage, including the EARTH (ground). Alternatively, you could have either a faulty guage or a faulty temperature transmitter at the radiator end.

With best wishes,
John Worrall