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Old 02-01-2008, 12:52 AM   #1 (permalink)
stevemedcroft
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Easy ways to troubleshoot a bad temp gauge?

I have a faulty temperature guage. Here are the symptoms exactly:

- With no key in the ignition (engine off) the gauge is pegged all the way to the right (past the 240 degree mark to the stops)

- With the ignition on and the Glow light on (ten seconds right after start and whenever it cycles on after that) the temperature gauge reads 100 degrees (needle moves almost all the way off to the left side of the gauge)

- Immediately once the glow plug controller cycles off, the temperature gauge pegs all the way off to the right again.

- No matter how long I run the truck, the same things happens.

- It did this when I first got the truck (2 months ago) and continues to do it even though I have done some colling system work (meaning I know everything in the cooling system is in working order despite not knowing the temperatures.

- I have looked for a bad fuse and found none.

- I bought a replacement temperature sensor but the one in my truck has a single pin (shaped like a small headphone jack - and a mate for the connector that runs to it) and the one I bought (which is supposedly for my truck) has two flat pins. It could be that the radiator was swapped from some other vehicle when I bought the truck (the evidence of that is missing brackets, dried coolant stains on the inside of the grill and an obvious recent water pump replacement).

- I have followed the wiring from the temp sensor to the harness where it disappears. There is one manual splice but it is single-wire to single-wire twisted together and covered in electrical tape. The lead then connects to a yellow wire on the harness and flows into the firewall.

- All the other gauges work (speed, fuel, Voltage) except the oil sensor - which is also pegged all the way on and never changes no matter what happens.

My question is this - what are two or three simple things a novice can do to try and troubleshoot this problem? Or, should I by a meter and learn how to read an electrical chart (daunting proposition)? Or, should I just by an aftermarket temp sensor and mount it under the dash and leave the dead one in my dash (although I would like to learn how to resolve these issues and run the truck as designed)?

Your recomendations?
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Last edited by stevemedcroft; 02-01-2008 at 12:53 AM.
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Old 02-01-2008, 07:29 AM   #2 (permalink)
GenBiltstein
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You have a short somewhere along the circuit going to the gauge. Gauge is working fine. Sending unit is fine. Probably ign switch.
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Old 02-01-2008, 01:26 PM   #3 (permalink)
0lee
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Quote:
Originally Posted by stevemedcroft View Post
- I bought a replacement temperature sensor but the one in my truck has a single pin (shaped like a small headphone jack - and a mate for the connector that runs to it) and the one I bought (which is supposedly for my truck) has two flat pins.
What you got may be the sensor for glow plugs, HPCA and high idle which would be installed on the passenger side on top of the engine.

Quote:
Or, should I just by an aftermarket temp sensor and mount it under the dash and leave the dead one in my dash (although I would like to learn how to resolve these issues and run the truck as designed)?
Well, I would install mechanical temperature and oil pressure gauges. Keeping the stock gauges is nice, but they are not very accurate. In some sense, the mechanical gauges help you more than the stock ones to "run the truck as designed" because unlike the stock gauges, the mechanical ones will actually show you what's going on. That is extremely helpful once a problem comes up like a coolant thermostat stuck open or overheating. A thermostat stuck open can go unnoticed for a long time with the stock gauge, but you would see it more or less immediately on a mechanical gauge. If you have a problem with overheating, you would see that on the mechanical gauge, but with the stock gauge, you would have to find out if it's the gauge first. At 215F or more, I would pull over to let it cool down --- but what temperature would the stock gauge show then? 205? 210? 200? 240?

Can you be sure that the stock gauge tells you that your oil pressure is 10 or 15 PSI instead of 20? A mechanical gauge does. And 10 or 15 is too low ...


You can try to take the stock gauges out of the dashboard and put the mechanical ones in instead. That's probably easier with electrical ones, though.


For the oil pressure gauge, check if the wire is still connected to the pressure sender. The pressure sender is hidden at the back of the engine, about where the vacuum pump is. If the wire is connected, the pressure sender may be broken.

For the temperature gauge: The sender is on the drivers side close to the front of the engine, sort of under the first injector --- you seem to already have found it.

For both gauges: They depend on voltage. When the glow plugs are being used, the voltage can drop and cause a low reading.

That your temp gauge goes all the way to the right probably means that the wire going to the sender is grounded somewhere, either because the sender is broken or because the wire is damaged. I would check the temp sender with an ohmmeter, if the resistance is always 0 or close to 0, I'd think that the sender is broken. I'd also check the voltage on the wire going from the sender to the gauge, at the connector that goes on the sender. I would expect to see 12V there; it it's much less, I would think that the wire is somewhere damaged and grounded.
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Old 02-02-2008, 12:23 PM   #4 (permalink)
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Olee just told you what it took me days/weeks to figure out. The only thing missing was what you thought was the temp sender, is possibly a low coolant sensor that funky pin connection. Check your dash bulbs .

When I was cleaning and tracing wires one of the test stated, was to ground it wit a 90 ohm resistor which brings the gauge to about 1/2 way up. ONLY FOR 1 WIRE GROUND SENDING SENSORS. This seemed to work. And lastley when replacing 1 wire senders don't put to much sealant as to lose the ground.
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Last edited by nunderwier; 02-02-2008 at 12:24 PM.
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Old 02-02-2008, 03:12 PM   #5 (permalink)
0lee
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There's a low coolant sensor in the radiator, passenger side, at about 1/2 the height of the radiator.

The coolant temp sender had a pin sticking out with a circular plate on top for the connector.
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Last edited by 0lee; 02-02-2008 at 03:14 PM.
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Old 02-02-2008, 03:48 PM   #6 (permalink)
stevemedcroft
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Thanks all. Got stuck working today and will hopefully get an hour with the truck this evening. I'll trace the right sensor and let you guys know.
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Old 02-02-2008, 04:10 PM   #7 (permalink)
stevemedcroft
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Gauge wire just not connected - but there's more...

I snuck away for a minute and located the temp sending unit (I was looking at the low-coolant sensor in the radiator before).

The problem was that the wire to the gauge was not connected to the sending unit at all. I think.

When I got under the truck, there was a wire extending from the harness that contains the glow plug leads (the green wires with the female spade connectors on the end) that seemed to be the one intended for the sensor. This 'extra' green wire coming out of the end of the harness was not connected to anything. It too had a spade-like connector but it did not seem to mate to the sending unit (and in fact, came apart).

That lead had some exposed wire near the connector so I wrapped it around the sensor and tried the key and the temp gauge was no longer displaying incorrectly. This wrapping is obviously no solutions so...

My final question on this is is the connector on that lead replaceable - snip and crimp with something I can get at a hardware store?
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Last edited by stevemedcroft; 02-02-2008 at 04:11 PM.
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Old 02-02-2008, 05:16 PM   #8 (permalink)
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sure...but go to your FAPS...(Favourite Auto Parts Store)...not a hard ware store
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Old 02-02-2008, 05:42 PM   #9 (permalink)
0lee
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Quote:
Originally Posted by stevemedcroft View Post
The problem was that the wire to the gauge was not connected to the sending unit at all. I think.
same as mine was ...

Quote:
This 'extra' green wire coming out of the end of the harness was not connected to anything.
Afair the wire that goes on the sender is brown ... green wires are for the glow plugs.

The connector goes on the sender in a 90 degree angle.

Quote:
My final question on this is is the connector on that lead replaceable - snip and crimp with something I can get at a hardware store?
If you get a new sender, it will probably have a standard connector like the glow plugs have. You can use a crimp connector for that; these connectors are available in parts stores. If you need many of them, you can get them cheaper at home depot.
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Old 02-03-2008, 08:43 AM   #10 (permalink)
GenBiltstein
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Olee quoted:
Quote:
Well, I would install mechanical temperature and oil pressure gauges. Keeping the stock gauges is nice, but they are not very accurate.
You got that right. Even though this is not part of this initial problem of this thread.
Next to idiot lights one of the worst things is stock gauges. Not calibrated, sensors not calibrated, gauges share a common ground for lighting.
I especially hate it when people tell me that they have those gauges that tell them when to change the oil.
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