Gauge testing?

The PO of my 79/5 had replace the fuel gauge only to have both the temp and fuel gauge needles spike then fail to work. My guess is that when he reconnected the wires to the gauges he connected the +12 volts to the wrong terminal on the fuel gauge causing the voltage regulator to go bad. Could it be something else? I have found a site that does give info on bench testing using 12v and an ohms meter and was wondering if it’s possible to test a fuel, temp gauge in the field using just the ohms meter? If I could what would be the procedure?

 

Chuck, the FSM shows how to test CJ gauges in place (at least, my '77 manual does). They use a Jeep tool that replaces the sender load. If you have the manual, look in the electrical section under "Instrument Cluster and Instrument Panel Components". That shows the voltage measurements on the back of the cluster. This will diagnose whether the CVR (constant voltage regulator) is working or not.

If the voltages are right, then you check to see if the guage needles move as they should. The Jeep tool can be replaced with cheap 1/4W carbon film resistors that you can get at Radio Shack for a couple bucks. Alternately, you can momentarily ground the sender connection to see if the gauge needle moves. I stress momentarily, because if you leave the sender wire grounded for too long you risk ruining the gauge. With the sender wire off, the guage will indicate all the way cold/empty. Ground the wire and the needle will swing across. To check the accuracy of the gauges, you need the resistors to sub for the sending unit: 73R empty/cold, 10R full/hot, with a couple values between.

 

A few helpful facts: 12V goes to the I terminal on the fuel g. The CVR is in the fuel g and sends regulated voltage to the temp g via a flat jumper. The voltage at these terminals should be about 5V and pulsate every second or faster. If the PO connecteds 12V to either end of this jumper, he probably cooked both gs. The fuel g has a S terminal that goes to the fuel sender via a pink wire. You can tap that wire at the connection the rear harness, if you want to test the g. The temp g has an A terminal that goes to the temp sender via a purple w/trace wire.

 

Tim, thanks for the quick reply. A few weeks ago you gave me a web site to purchase a FSM for a Willys pickup and a 79CJ. I already received the Willys and will order the CJ FSM tomorrow morning as the Haynes I do have shows me nothing on the gauges.
I was still wondering if I could field test the fuel and temp gauges without having 12v present using just an ohms meter. The reason why I’m asking is because if I’m at a bone yard and fine gauges I would like the opportunity to field test there before paying for them. Thanks for the help.

 

Ok, I'd expect that the temp g is simply a galvanometer http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/galvan.html . The usual failure would be an open coil, so it'll either have some low resistance (good) or infinite resistance (not) from the CVR connection to the sender. The fuel g has some goofy electromechanical (?) CVR and I don't know how this would measure. I think the 5V (regulated) post to the sender should measure like the temp g.

Measuring the coil for open should weed out the most common failure mode.

 

Tim, I checked the continuity of two spare speedo sets I had in my garage, one was toast the other had a good power path that existed in both gauges (fuel, temp), now I have back up. Sometime this week I will check and replace if needed the fuel and temp gauges in the 5.
The site you supplied had an interesting example of a non-ohmic resistor by connecting 120v across a pickle and producing light similar to a sodium lamp. My son thought that was pretty funny. Again thanks for your help.