Dashboard gauge tester for older vehicles

[zgerbic]

I thought I would share a little test fixture I made some decades ago for testing dash gauges. I have used it on old Fords, Chryslers and VWs that have senders that provide a 73 ohm resistance when empty/low temp or 10 ohm resistance when full/high temp. It can be used to indirectly test other things as well, such as the dash voltage regulator.

In the diagram I provide two fixtures. Figure A is the one I use and allows me to test three positions on a gauge accurately. Figure 2 tests only high and low marks but uses a cheap common SPST switch.

Dash Gauge Tester.jpg

While it is true that you can test a gas (or temp) gauge by opening the connection to the gas sender (empty) and then shorting the connection to a gas gauge (full), you can still get tripped up by a bad dash voltage regulator, an out of cal dash gauge or a defective sending unit. The switch in Figure A will help assure things are actually working right.

If you remove the connection from the gas sending unit and connect the switch to the gas sending wire and ground the other switch terminal, the three switch positions should put the dash gauge at the empty, half full and full marks. If they are all a little high or a little low, the dash voltage regulator may be bad or a bit out of spec. To be sure, put the switch in place of the temp sensor and see if the marks are correct or off a bit. If the needle is also a little high or low, the dash voltage regular is likely bad. If the needle lines up closely with the marks, then the gas gauge is out of calibration. The same tests can be done in reverse to check the temp gauge.

If both gauges read correctly with the test switch, the dash voltage regulator is ok. If both gauges are equally high or equally low, the dash voltage regulator is bad or a little out of spec. If one is right on and the other is off, then the gauge that is off is out of cal and the voltage regulator is ok.

Once you are sure the voltage regulator is ok and the gauges are ok, the only component left is the fuel or temp sensor. The fuel sensor is the easier of the two to test. If you can take it out of the tank, you can just measure the resistance of the sensor with the float up or down. It should be pretty close to 10 ohms and 73 ohms, or it is bad. Alternately you can check the resistance when the vehicle is almost out of gas and then when full. The temp sensor is a bit harder to test but with some hot water and a thermometer it can be done.

The switch based fixtures I show are pretty cheap and easy to make. I have a couple of 18" wires with alligator clips for connections, and the bottom of the switch is encased in silicon seal to prevent shorts. When you need a test, it can be done in a couple of minutes without fumbling around, and you know the test is accurate and probably where the problem is.

[Gray035]

This looks really useful. I'd like to make one of these to troubleshoot my gauge cluster and sending units soon. I'm only a novice concerning automotive electrical systems.

What type of resistors are appropriate? Is the best approach to solder them in-line on the wires?

[zgerbic]

Gray035,

I would just use carbon composition or carbon film resistors, probably 1% or 5%, with a wattage rating of 1/4W. The resistors are really cheap but it may be hard to just buy one each of the three. You may need to buy a resistor assortment for about $8 to $10 to get the three needed. The switch does not have to be a high current switch as it passes a very low current, less than an amp. I would solder the resistors to the back of the switch as physically shown. I would rap the insulated wire hooked to the 10 ohm resistor around the insulated wire coming out of the center terminal to give the wires some physical strength in case you accidentally pull on them while connected. Once you are sure it works I would encapsulate the bottom of the switch in silicon to fix the wires in place and keep the terminals/resistors from shorting against the metal in the truck.

If you buy an assortment keep in mind that you can put two or three resistors in series to add up to the value you want, or you can put resistors in parallel to get some odd values. For example, for the 13 ohm resistor you can put a 22 ohm and a 33 ohm in parallel to get 13.2 ohms*. Two 100 ohm resistors in parallel will give you 50 ohms*.

* a simple parallel resistor calculation
(22 X 33) / (22 + 33) = 726 / 55 = 13.2

(100 X 100) / (100 + 100) = 10000 / 200 = 50