Copyright 2018 - Andy Taylor. Used with permission.
Since the ammeter is separate, and almost never fails, we will consider the Oil, Temp and Fuel gauge. Electrically, all three are identical, with the exception that the temp gauge has a six volt reducer to supply the three gauges voltage. These react to a ohm load from 33 ohms (full sweep) to 240 ohms (minimum sweep).
The six volt line voltage (the three terminal harness made from black wires) is critical to the accuracy of these gauges. The reducer in the temp gauge, is a thermal-electric type point setup, and is a crude way to reduce voltage. The points are old, and will sometimes stick, sending 12 volts to the gauges, causing all three to peg out, or worse, fry them. When you see that tiny wisp of smoke come through the gauge face, its too late!
Install a 12V to 6V solid state reducer, and by- pass the crude points reducer. Pull the red ACC wire off the right side of the temp gauge, use this wire to supply voltage to the reducer. The 6 volt side goes to the black three terminal harness.
If all three gauges are inoperative, its likely the reducer in the temp gauge has failed, or the ACC line to the key switch has lost power (unlikely). Using a 6 volt flashlight lantern battery, and some test clips, apply six volt+ to the black three terminal harness and the terminal of the battery to a good ground on the car, and see if the gauges operate. If so, install a solid state voltage reducer and you are done.
Never throw out a temp gauge just because the reducer is bad; the original service manual suggests replacing the gauge. The fact is most temp gauges will still operate on the 6 volt side, even if the reducer is bad.
Getting to the gauges on the Marathon dash is somewhat easier if you remover the speedometer. 69- 71 had a removable plate on the bottom of the dash. A-11 Superba's and A-10's are easy; just pull the dash panel.
First make sure you are showing six volts to the right terminal of all three gauges (viewed from behind the steering wheel). A Volt-Ohm meter is a must when trouble shooting the gauges.
If any separate gauges do not work, first, (using a jumper wire) ground the LEFT wire (viewed from behind the steering wheel) (sending unit terminal) behind the gauge. The gauge should slowly rise and peg out. If not, the gauge is bad. If gauge pegs, proceed below .
Sending unit colors are: fuel (orange), temp (green), and oil pressure (blue).
Using a jumper wire, ground the sender wires at the sender location; the gauge should peg. If so, the sending unit is bad. If the gauges peg from jumping under the dash, but not at the sending unit location, then the sender wire has an open somewhere.
A pegged gauge usually indicates a shorted to ground sender wire, or too high a voltage at the gauge.
If all gauges read higher than they should, the supply voltage is too high (over 6 volts).
To check the senders, set your ohm meter to accommodate a 33 to 240 ohm range. Then operate the engine. Depending upon which sending unit you are testing, oil pressure should drop the ohm reading, same for the temperature; more fuel should also drop the reading.
Most standard boat senders (Faria / Beede brand) will work for these gauges.
NEVER EVER, APPLY VOLTAGE TO A SENDING WIRE! To do so will destroy the sender.
These gauges are thermal-electric, the winding reacts on the thermal blade via temperature. This causes the thermal blade to contract, causing the indicator needle to move upward. See below.
Showing the reducer part of the temp gauge. this works similar to the gauge operation above, except it causes the points to make and brake.....this type of reducer should never be used anymore, as solid state reducers are far Superior. See below.
Showing gauge wiring layout from rear of dash. The little black box is a solid state reducer I installed.
These reducers can be found on eBay for as little as $3.
Here is a gauge wiring diagram showing how to install the solid state reducer. This spplies to 60 through 72 Checkers with the 5 gauge dashboard,all models.