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Temp sensor, oil pressure, fuel gauge all failing


bootstrapper

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I got this truck for free and only just recently got it running, so now I'm addressing the issues as they pop up.

At first the oil pressure gauge read low. It moves a little when I turn the key, but not into the normal range. No horrible engine seizing though, and I hear it's not a real gauge anyway so I let it go.

Then I noticed the temp gauge would never get into the normal range. It would move a little as the truck warmed up, but still way to the left.

Then I stopped at the station for my first tank of fresh fuel (finally!) and the needle never came back off "E" after 10 gallons of liquid dinosaur bones.

I've searched around and can't figure out if there's a common ground I could check or whether I should start replacing all the sending units individually.

Thanks in advance!
 


RonD

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There should be a ground strap from the engine to the firewall, usually from the head, if this is loose or not there then gauges can read low.

Yes, the oil pressure is just run from a pressure switch not a variable resistor type pressure sensor.
Power for that gauge runs thru a 20 ohm resistor on the back of the gauge, and the oil pressure switch is the ground.
On gauge setups the oil pressure switch(sender) is normally open(N/O), so with key on, engine off the gauge would show 0 psi, because there is no ground,
With engine running and oil pressure above 5psi the switch closes, so gauge is now grounded and because of the 20 ohm resistor it will show just below 1/2 on the gauge.

On idiot light systems the oil pressure switch is the opposite, it is N/C, so light will be on when engine is off, light goes off(no ground) if oil pressure is above 5psi with engine running.

So to test your gauge, find the oil pressure sender, remove it's 1 wire and ground that wire, turn on key and gauge should move to about 1/2, if it doesn't then either 12v power is low or gauge is bad.
If gauge checks out OK, then oil pressure sender could be bad or it has a bad ground on it's threads, maybe some corrosion, remove it and clean the threads reinstall and check it.

Temp gauge is the same type of setup, sender has 1 wire(there is a 2 wire sensor that looks similar, it is for the computer, so not gauge related).
The 1 wire temp sender is a variable resistor, so remove it's 1 wire and turn on key, gauge should go all the way down or all the way up, either is fine.
Then ground that 1 wire, gauge should go the opposite way.
If that happens then gauge is good, sender or thermostat is bad.
Thermostats that wear out are suppose to fail open, so engine never warms up.
Upper rad hose should be 200degF when engine warms up, if it isn't then replace t-stat.

Fuel gauge is also the same test.
In the frame rail there should be a 4 wire connector for the fuel tank
Pink/black
Black
Those are the fuel pump power and ground wires

Yellow/white
Orange
These are the fuel gauge wires
Yellow/white is from the gauge, if it is disconnected the gauge should go to full, if you ground this wire, gauge should go to empty.
If gauge responds this way the gauge and wire to that point is OK, wire or sending unit in tank could be bad.
(1988 and older will do the opposite)

Now that there is fuel in the tank try rocking the trunk a few times to free up the in tank float, if truck was sitting with empty tank float could be rusted and stuck at the bottom of the tank
 
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bootstrapper

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Thanks for the great info. I removed and polished all of the ground points I could find in the engine bay and it didn't change anything. I have a temp sender so I'm going to start by replacing that one, but my Haynes and Chiltons don't have pictures of the 2.3 engine. Is that it in the heater hose?
 

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If it has 1 wire then its a sender for the dash board temp gauge.
If it has 2 wires it is the ECT sensor for the computer so it can set "choke mode" when engine is cold.

On the '90 2.3l the sender was just above the oil filter and towards the back of the engine, the oil pressure sender is just above the temp sender

ECT sensor was under the intake more to the front of the engine, but on the same side, drivers side.

Not sure what is on the heater hose?
 

bootstrapper

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Update: at the time of the first post, I hadn't driven it around much more than to check things, so the engine never really got hot. After driving it an hour last weekend, I noticed the temp gauge did actually budge up. I've heard these engines run very cold under normal circumstances, so I'm thinking maybe the gauge is actually working fine.

The gas gauge is still dead. Haven't gotten to that one yet.

The oil pressure gauge is driving me bonkers. Some of the time it's midrange, then it will drop to zero. I'm guessing that's the equivalent of the oil idiot light going on and off, but could that be from the plugged oil intake screen in the pan I've heard about? It seems like there's oil at the top end, because I rarely hear any tapping, and when I shut it off I can hear the sound of it dripping back down. I'll post back again when I check the oil gauge at the sender like you suggested RonD.
 

bootstrapper

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Update: I tested all the gauges according to your excellent instructions.

Temp: gauge good. Replaced sender, will post back what happens.

Oil: gauge good. Still acting erratically--thinking about replacing the switch with a sender.

Gas: gauge bad? Grounded the wires at the frame rail just like you said and the gauge didn't budge. Is there anything else to try before I take off the dash and replace the gauge itself?

Thanks again for your outstanding guidance. This forum is awesome.
 

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As far as the temp gauge, does the heater blow HOT after warm up?
These 4cyl engine do not generate alot of heat, so the thermostat is important in keeping the engine coolant temp up above 200degs so you get good lubrication and best MPG.
Thermostats are suppose to fail OPEN, so engine doesn't over heat, you should have a 192-195deg thermostat, so engine should take no longer than 5 minutes to get up to 200deg, maybe 8min. if its below freezing outside :)
200-220deg is just below 1/2 on most gauges.

Sender mod for oil pressure is a good idea, hopefully it is a gauge issue not an oil pressure issue.

If you ground and unground the yellow wire going to the Fuel gauge with Key ON gauge should move.
There is an anti-slosh circuit on the backside of the fuel gauge that is known to cause problems.
Good read here, with pictures :):
http://www.therangerstation.com/forums/showthread.php?t=140669
 
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bootstrapper

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I never thought to check the heat until today when it finally got chilly. And no, it doesn't get hot. So I guess it's thermostat time. Only thing is, I've been getting a little light pinging recently, which I would have thought would be a hot engine. Maybe it's a lean mixture instead. I'm trying to figure out if any of these issues are connected or if the truck just has a lot of issues :)
 

bootstrapper

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I'm assuming the oil problem is the gauge and not the pressure, because the gauge drifts up and down throughout a drive. It's the on-off type, though, so I figure that if the pressure were actually low then it would just stay low. It seems more like a wonky resistor that is giving out random voltages.
 

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Start engine cold and remove rad cap.
There should be no flow in rad with t-stat closed.
Feel rad hose coming out of t-stat housing it should stay cold, not slowly get warmer.
With working t-stat that hose will stay cold until coolant in engine gets to 190deg, then t-stat opens and hose becomes hot very fast.
 

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Temp: gauge good. Replaced sender, will post back what happens.
I'm looking to replace my sender too and mine's located in the same spot. What's the easiest way to get to it? I've tried underneath, I can reach it but can't see just because of where it's located. I've thought of removing the tire as a possibility also. How did you do it?
 

bootstrapper

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I ramped up the front wheels but didn't take anything else off. It just took me a little bloody knuckles to get the wrench around it but it'll come. Just beware of the niagara falls of coolant that follows.
 

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I'm doing a flush tomorrow so there won't be anything coming out (hopefully), thanks.
 

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