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73-87 Chevy _ GMC Trucks => Interior & Equipment => Instrumentation => Topic started by: sonicbluezx3 on September 10, 2012, 09:54:35 am
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Okay I have a question for you guys....
First of all, let me explain a little bit. My truck (1986 K10 Shortbed) originally came with a 4.3L V6 engine. The guy I bought the truck from swapped in a 6.2L diesel engine. He used the original wiring (set up for the gas engine) and everything worked fine. I just did a total overhaul of the body, and in doing so, I put a diesel cluster in. Now, the temp gauge pegs out as soon as I turn the key on ignition. Now I know my truck isn't running hot like that because I didn't even have it running when the gauge was pegged. To make things more complex, I replaced the temp gauge that was in there with another one because I accidentally broke off the needle. The thing that makes me think it isn't the gauge is that the gauge that I broke the needle off of pegged out as well. The only one that worked was the one from the cluster for gas. I know the temperatures are different on the two, but it's only by 20 degrees.
My question is.... is the wiring different? Is it likely that I have a faulty gauge? Or sensor? I just don't want to start throwing parts at it until I can figure out which part it is, and I don't know much about these gauges.
Thanks for the help!
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The sensor in the block must match the gauge in the cluster. Since you said you had a setup that worked, just replicate it. The Diesel and Gas Gauges are not the same.
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Since both gauges pegged, you know the wiring is ok. There's a signal getting to the gauge. So concentrate on the sensor.
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So he could have kept the sensor from the gas engine and used it in the diesel?
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yes.
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I asked him and he said the sensor is what was in the 6.2... Im wondering if i should just replace it and see if that fixes it since they are cheap.
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I should also mention that when i shut off the truck/key, the gauge stays pegged out.
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If that is what he used I would try that, either way they need to match.
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Right.... I'm saying he used the temp sensor that came with the engine (so the temp sensor is for a diesel) and he used the temp gauge that was in the gasser cluster. So I'm not sure if it would be the temp sensor.
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Generally a gauge will peg like that if there is a ground problem (I.E. a wire grounding to the body where it shouldn't be)
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Yes. The grounded wire would complete the circuit, and with no resistance the gauge would peg.
The gauge is actually a voltmeter. It reads temperature because that's the system it represents. When the engine is cold, the temp sensor has high resistance. but as the engine warms up, the sensor responds and loses its resistance, allowing more voltage to go to the gauge. When a gauge is pegged, it means the sensor has lost all of its resistance OR there is a grounded wire, either of which will peg the gauge.
My guess is that the gauge stays pegged because it has overridden its operating range because of the low resistance and high voltage. there's a little spring in the gauge and the voltage causes it to expand because it's bimetallic-there's two different metals- so they react to the voltage and drive the needle higher.
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So does that mean i should replace the gauge?
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Sounds like you should check the length of the wiring to make sure it's not cracked and grounding somewhere on the body.
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I'm not sure if this makes a difference, but I have the battery disconnected and it still pegs. I will check the wiring tomorrow night when I get home from work and see.
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There is no difference in sending units between gas/diesel v6 or v8, the only difference is one unit for gauges and one for idiot light dash.
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I just replaced the sensor but still have the problem. The gauge doesnt move at all, so im going to get a new one coming. Im pretty sure that'll fix the problem. I did try to replace the needle too... So i might have screwed somethig up while doing that.
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I'm not an electrical wizard by any means but it seems that you should be able to test this with a digital volt meter before you burn up another gauge. If you have a grounding short in the wire all you're going to do is roast another gauge even with a new sensor. Since the sensor sends variable voltage based on resistance related to heat it seems that with a cold engine your volt meter would read a very low voltage when connected in place of the gauge. As the engine temp increases the volt meter should read an increase in voltage. If you connect the volt meter and turn on the key with a cold engine but read a high voltage then that should point to a short in the wiring which would explain what you are roasting your gauges.
I'm sure there's a lot of folks on here that know this stuff way better then me. It just seems that a little testing might save you buying more gauges.
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Good point... I'll have to try that. Although I'm not sure how my wiring would have gotten shorted since I swapped the two gauges over prior to doing anything. I could probably even test this out with alligator clips and the gauge in hand. If the old gauge works with the clips then I'll know it's wiring... if it does the same thing, I'll know it is the gauge. Either way I hope this gets resolved this weekend because I'm ready to drive my truck! Ha.
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Alright I have done some more testing and I still haven't solved the issue. New gauge (and about 3 diff ones from the junk yard), and replaced the sensor twice (thought the first may have been a defect), and nothing. So here's what I did:
I took the gauge out of the cluster and used alligator clips to connect the terminals on the back to the corresponding things (+ to battery, - to chassis ground, and connected the sensor to the gauge in the right spot... the new one had everything labeled as to where it was supposed to go). It still does not move. So I'm pretty baffled and so is everyone else I've asked. The gauge is new, the sensor is new, and the wiring matches up.
The only thing I can think of is that the wire that connects the ignition switch to the sensor wire (it T's in) sends some sort of signal.
Anyone know? If I can't figure it out I'm probably just going to buy some aftermarket gauges and use those in place of the stock stuff. I would rather get this one working though.
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Do you have an ohmmeter?
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Yes
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Testing the sending unit:
With the engine cold:
Disconnect the wire harness from the sender. Zero your ohmmeter (if necessary). Connect one lead of your ohmmeter to the sender terminal. Connect the second lead of your ohmmeter to a good clean ground. Measure the resistance and record the reading along with the approximate temperature of the engine.
With the engine at operating temperature:
Zero your ohmmeter (if necessary). Connect one lead of your ohmmeter to the sender terminal. Connect the second lead of your ohmmeter to a good clean ground. Measure the resistance and record the reading along with the approximate temperature of the engine.
Sending unit measurements should range from greater than 1400 ohms cold, up to ~100 ohms at operating temperature.
Making a substitute "test" sending unit:
Pick up a 5,000 ohm (5K) linear taper potentiometer (#271-1714), a 2,200 ohm (2.2K) 1/2 watt resistor (#271-1121), and a couple of alligator clips from your local Radio Shack. Also, pick up some flexible 18-gauge wire to use for test leads.
Connect the 2.2K resistor across the two outside terminals of the potentiometer. Jumper the center terminal of the potentiometer to one of the outside terminals. Connect a couple of 2' long 18-gauge wires to the two outside terminals of the potentiometer. Connect an alligator clip to the free end of each lead and solder all the connections.
Testing the gauge calibration and wiring:
Zero your ohmmeter. With your ohmmeter connected to the leads of the potentiometer (pot), adjust the pot to 1365 ohms. Without changing the setting, attach one lead of your pot to the signal wire from the gauge. Attach the second lead of your pot to a good clean ground. Turn the ignition ON and observe the gauge. The needle should align with the first (left) line of the gauge.
Disconnect the pot from the vehicle. With your ohmmeter connected to the leads of the potentiometer (pot), adjust the pot to 96 ohms. Without changing the setting, attach one lead of your pot to the signal wire from the gauge. Attach the second lead of your pot to a good clean ground. Turn the ignition ON and observe the gauge. The needle should align with the center line of the gauge.
Disconnect the pot from the vehicle. With your ohmmeter connected to the leads of the potentiometer (pot), adjust the pot to 55 ohms. Without changing the setting, attach one lead of your pot to the signal wire from the gauge. Attach the second lead of your pot to a good clean ground. Turn the ignition ON and observe the gauge. The needle should align with the last (right) line of the gauge.
Edit:
If the gauge needle aligns with the left, middle and right gauge markings as indicated above, the gauge and gauge wiring are functioning properly - look to the sending unit or its ground connection. If the gauge does not respond as indicated, you'll need to follow up with systematic voltage tests.