I have a 1989 Trooper II with a 4ZE1 engine. A couple of days ago, I replaced the fuel sender. Fuel gauge needle was not moving. Gas tank had 5 gallons of gas.

Since fuel gauge works with a regulator that the temp gauge has inside, I cleaned the contacs with 1200 grit sand paper and sprayed the whole gauge with contact cleaner.

After this cleaning, I went to the gas station and got the tank full. ( that was the previous fuel gauge problem, jejeje, lack of gas) .Now I have a problem with the temperature gauge. Needle doesn't move at all. Oil pressure and fuel gauges are working fine.

In order to test temperature gauge, I took the temperature gauge sender's cable and grounded it, and needle went full scale. I decided to buy a new sender with my surprise that it didn't work.

When I test this sender when stone cold, it reads around 400 ohms and when hot after a short trip, reads around 35 ohms and needle still didn' t move at all.

Well, instead of jumping the sender's cable directly to ground again, in order to emulate the sender I installed a 10 ohm resistor with the sender cable and ground it. This time, with the 10 ohm resistance, the needle reached half scale.

I did a bench test with the same results. See attachment.
Since I have a 10 ohm resistor moving the needle half scale, how will it move with 35 ohms? Where is the problem then?

 

The sender's cable has has 32 ohms resistance with the key off and cable removed. Tried to check voltage but it varies. With key on using a test light, it turns on and of like a christmas tree light.
Is voltage supposed to be like that or steady like other SUV?

I am asking because I did the same test in a 1999 Montero Sport which it has a 1 cable sender and gives me a steady 9 volts. I know it is a different truck but I wanted to compare in order to try to fix my Trooper's temperature gauge.

If I ground the cable, needle moves to the red hot mark. Sender's resistance varies from around 440 ohms when cold to around 30 ohms when it reaches normal operating temperature.*

Please, if someone here can measure sender's voltage and tell me those nembers, will be appreciated. By the way, my Trooper has a 4ZE1 engine. Thanks in advance.

 

No one? By the way, some people tell me it is a faulty thermostat. I remember when I bought this trooper, it didn't have the thermostat, but at least, temp needle was at the lowest white mark. When thermostat installed, needle reached 1/4 scale.

I took the thermostat out and needle just don't move as before.

 

Installed new thermostat and again another new sender with no luck. The lowest ohms the sender reach at normal temp (the top radiator hose hot, could not hold more than 2 seconds) is 35 ohms. Needle moved from its rest position way back to the left, and reached just the beggining of the white mark.

Still cannot understand why it doesn't reach the 1/4 scale like before.

 

By the way, I have two gauges subpanel, the original one and the another bought from a friend. With this same setup, ( new thermostat, new sender) , oil and fuel sender works fine on both but the original one the needle barely moves from its resting zone, the one I bought needle moves the beginning of the white.

On the other hand, on both assy, when grounding the sender's cable to ground, the needle go pass beyond the Hot red mark.

 

Uncanny for me.

 

Here is a drawing explaining my problem.

 

Damn! And I live in a tropical zone and the sender doesn't reach the 20's ohm zone to move the needle close to 1/4 to 1/2 scale where it is supposed to be.

 

Well, I'll forget about this issue since I installed an aftermarket gauge with it's corresponding sender. It is working fine. Normal operating temp is around 90 C ( 194 F) which is for me, a normal temperature. ( thermostat opens at 85 C and it is a new one).

It is a $ 20.00 dollar gauge. Not bad for the price. Here is a pic.

 

I'm actually having this problem now on my 87 Trooper. Power is reaching the Gauge correctly, I'm sure the sensing unit cable is good and if I ground the sending unit cable gauge goes full hot. It seems like the problem is a mismatch between currently available sender units resistance vs stock. Wonder if anyone would know the stock values for some temperatures.