Join Date: Sep 2000
Location: Shrewsbury, MA, USA
I guess I should have also mentioned that you have to ground the rear mounting plate in order to get the gauge to work right. There is a ground wire in the harness but most people hook it to the front (fuel and oil pressure)mounting plate run a jumper from the front plate to the rear plate if you have the gauges wired this way. The lower terminal on the resistor is the ground connection for the temperature gauge, the upper should be internally connected to the +12V battery lead, and it is insulated from the backing plate with a rubber grommet. Sometime both the upper an lower terminals have these grommet and in this case you need to put the ground jumper right onto the lower terminal to get it to work.
Power comes in through one of the two stab on terminals and goes out to the temperature sender through the other terminal. This is one side of the gauge operating circuit. The other side is through a set of windings inside the gauge connected to the +12V terminal inside the gauge, then through the resistor and out to ground. This second circuit allows for a fixed amount of current to flow through the gauge in a direction opposite the first circuit, that generates a fixed amount of electromagnetic force (EMF). As the temperature sender changes resistance it allows more (hotter) or less (colder) current to flow through the gauge. When there is more EMF generated in the sender circuit than the resistor circuit the gauge moves up scale towards hot. When there is less current in the sender circuit the EMF in the resistor circuit pushes the needle down scale towards cold.
That's why you need a resistor in the second circuit, so that you can balance the gauge properly. Maybe they moved the resistor inside the gauge, but I think you still need it.
Check your ground connection to the gauge, and/or the rear mounting plate.
1969 Indy Pace Car
350/300HP RPO Z11
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