Thermostat Always Cold

[nolimitssjca]

My thermostat always reads cold. I have replaced the thermostat and the sending unit (the brass thing and the black screw in thing). How do I check the wiring? aasume there should be voltage or continuity or???

 

[Bigg_billy]

Does your heater blow hot inside the car, it isn't that unlikely to buy a new thermostat that doesn't work properly....

Bill

 

[BrooklynBay]

Ground the wire to see if the gauge moves all of the way over to the hot position.

 

[nolimitssjca]

The heater and AC works normally. I've also replaced the heater core due to a leak.

Which wire needs to be grounded? From the sender or on the cluster...????

 

[BrooklynBay]

The one which is connected to the stud on the sending unit. It has an angle shaped end which looks like an "L".

 

[nolimitssjca]

Grounded the wire to the block. No movement.

 

[Bigg_billy]

You checked all your fuses right?

 

[nolimitssjca]

Yes. I checked the fuses. They all work.

 

[Bigg_billy]

I'd try a new thermostat, that one sounds defective.........

 

[nolimitssjca]

I've replaced it twice.

 

[nolimitssjca]

Does anyone know how the electrical work on this? Shouldn't I be able to make the needle move inside the truck somehow? If it failed that test then the wiring would be suspect.

 

[budwich]

The information that was provided is correct. Grounding the wiring that goes back to the instrument cluster will cause your needle to read high (with your key in run)... since the sensor has lower resistance as it gets warm in "normal" operation.... a ground is as low as you can go. If the needle doesn't move, then you need to ground closer to the instrument panel.

Are you certain that you are looking at the right sensor... not sure but there are two one for the computer and one for your instruments.

 

[nolimitssjca]

I have replaced both sensors. The sensor on the left is one wire and easy to ground. The other sensor is under the fuel rail and much tougher to get to. The wire I grounded was the exposed sensor on the left. Assuming this is the correct panel sensor, where does the wiring go back too? I mean is it direct to the panel, or does it run through another point or two before getting there?

 

[budwich]

The one that feeds your instrument panel (not your engine computer) is a single wire unit (at least in my 96 drawings), so you only need to ground that wire and it should show up as a full scale (hot) readings. If it doesn't, as I indicated you need to then ground back at your instrument cluster (same color wire) and see if that shows.

Also the run doesn't go directly back as it goes thru a splice that is a "takeoff" to the message center (if you have that option).... which could also be your problem (ie. broken splice).

 

[nolimitssjca]

I'll pull the instrument panel and check that next. Thanks for the helpful info.

 

[gijoecam]

Before you do that, do you have a message center in the center console? If so, unplug that and re-run the checks.

At this point, I suspect a bad gauge. If grounding the sending unit wire with the key on doesn't cause the gauge to move, odds are the gauge is toast. According to my schematic, there are three connectors and a splice (splice is for the message center) between the gauge and the sending unit. The first connect is, of course, the wiring on the back of the IP, the second is C115 which is along the top edge of the firewall, almost dead-center in the hood opening, and C148 is buried down in the lower driver's side of the engine compartment. It should be a R/W wire all the way.

Oh, and we can rule out the fuse as the same fuse feeds almost all the gauges on the dash.

-Joe

 

[budwich]

Unplugging the message center is unlikely to help. We are trying to find a ground back at the instrument panel.... a ground that isn't travelling thru the message center to get to the instrument panel.... it is "also" travelling to the message center. So if the message center is working, that would indicate a problem from the splice to the instrument panel (including the gauge). If the message center is also NOT working then that would indicate a problem in or near the splice.

 

[nolimitssjca]

I don't know what the message center is (So I doubt I have one). I this in the dash too?

 

[budwich]

I don't have one either in my 96 but from the drawings (and other discussions), the MC is in the console area... I think you would know if you had one.... so you probably don't. It doesn't matter anyway... other than helping you diagnose your problem (as I indicated previously). However, the splice in your wiring may still be there and could be an issue but its more likely that your problem is back at your instrument panel... either at the connector or the gauge itself. Again do the ground test at the instrument panel and you will quickly eliminate alot of guessing.

 

[nolimitssjca]

Thanks for the detailed help!