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temp gauge adjustment

KirsL

KirsL

Diesel maniac
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The coolant temp gauge wasn't working on my Chevy so I started testing to see what was wrong. When I would remove the sender wire with the key on the gauge would bottom out. If I grounded the wire the gauge would max out. So the gauge does work. So I get a new sender for the truck. Engine is a 6.5 turbo diesel, truck is a factory diesel with full gauges for an 82.

The low mark is 100 the high is 240. Well the gauge works now but reading it gives are off. They read high. Truck was at 180, has dual 180 t-stats, but the gauge was reading just below the 240 mark :eek1:. I check the actual temp with a temp gun pointing at the t-stat housing. So basically I'm wondering if there is a way to adjust the gauge to read correctly :thinking:? Or would that require the gauge being rebuilt or replaced?
 
Well as far as I can tell the cluster is the original and has never been disassembled. I had it removed from the truck as a unit when I replaced the wiring harness. Harness is a brand new one built for me by M&H Electric Fabricators. The gauge always read fairly accurate with the original 6.2 diesel, with in least 5 degrees.

Is there a way to test the sender when not in the truck? Basically can you test it the same way you would a t-stat or is there a different method?
 
You should even be able to test it on the truck with a multimeter.

For the life of me I've not been able to find it lately, but in SOME GM manual (Camaro or truck from the 80's-early 90's) one of the pages listed the corresponding values you should get while testing resistance through the sender, at different temperatures.

Gauges read 0-90 ohms if that helps.

Did you use the same sender from the 6.2 in the 6.5? (if possible) Either way, you didn't use any sort of thread sealer (tape, goop, etc) on the sender did you, as thats a no-no for this reason.
 
I have a full service manual for the truck. I'll look through it to see if those values are listed.

Yep that helps, gives me a starting point at least. I'm guessing 0 ohms is low temp and 90 ohms is high temp?

All senders are the stock style 6.2 ones. Other then being a little more displacement and a turbo its really the same engine. So all senders and locations are the same.

Nope, didn't use any thread sealant. Think the sender came with a single wrap of teflon tape, but that came off as it was tightened.
 
People have posted that it takes VERY little teflon tape/thread compound to mess with the senders. I'm a bit skeptical of that (they are, after all, pipe threads), however if there is a CHANCE there is tape on it, removing and ensuring the threads are clean would be something I'd do if I were having this problem since others have stated it has been an issue.

Opposite. 0=max, 90=min. Numbers=resistance to ground, so shorted to ground resistance is 0, open (wire disconnected from sender) would be infinite resistance.
 
You can take the sender and put it in a pot of clean boiling water with an ohmmeter hooked to it and get the resistance when it is at 120 degrees. Of course, you need the table of resistances to figure out what it should be.
Failing that, if you have someone who has a truck like yours whose gauge seems to be right, you could take the wire off his sender and hook the ohmmeter across his sender while it is in the truck at operating temp to get an approximate value.
Small inaccuracies can be fixed by either carefully bending the needle, or putting a resistor in the circuit.
The amount you are off though seems excessive. I suspect either the wrong sender or a defective one.
If there are any numbers on the senders, you might compare the old one and new one. Just because the box is right, does not mean the part in the box is the right on.

J.
 
Fordum said:
Small inaccuracies can be fixed by either carefully bending the needle, or putting a resistor in the circuit.
Click to expand...

These needles are plastic, they can't be bent can they?

Obviously off this much and working fine prior its not the needle position, but if it were...?
 
<These needles are plastic, they can't be bent can they?

Obviously off this much and working fine prior its not the needle position, but if it were...? __________________>

Yep, now that you mention it, I think they are. I was thinking about another model. However, I think the plastic ones are press on on a smooth shaft. So you could carefully twist it on the shaft.
But, like you said, odds are if it was working before its probably not bad now.
I'm still leaning towards a bad/wrong sending unit.

While Teflon tape would cause problems if it interfered with the ground connection of the sending unit, it can't make a ground any better than perfect.
So, since the higher the resistance the lower the temperature, Teflon tape increasing the ground resistance would make the gauge read lower not higher.
Doesn't mean tape is not causing a problem, just not making it read high.

If you find a known good sender on someone elses truck, you could measure the resistance of it at a known temprature, and then grab a resistor from Radio Shak of that resistance and hook it between the wire from your gauge and ground and it should read the same.
If not, you can try to adjust the gauge.

J.
 
OK finally had a moment to check on this. I measured temp with and inferred gun on the t-stat housing, read 170.2 degrees. The ohms reading at the sender was -60.0. That's with one lead one the battery positive and the other on the senders terminal. When I try and use the battery negative to the senders body the meter goes a little nuts. It wont give me a stable reading when hooked up that way :dunno:.

The gauge itself when at that same temp still sits just below the 24 mark on the gauge. It slowly works its way up as the truck warms up and goes down if the truck is off a while. So I'm thinking its a sender issue.
 
OUCH!, is your meter still ok? When using the ohms function, you MUST NOT have any other source of voltage in the circuit!

To check the sender in the truck, you need to disconnect the wire that is hooked to the sender, and measure the ohms between the empty post and the body of the sender. That will tell you the actual resistance of the sending unit.

Your meter has a battery inside that supplies the calibrated voltage to determine resistance. Any outside source will make the reading unusable, and will often damage or fry your meter.
Modern meters are a little more forgiving than older ones, but they have their limits when on ohms.

I saw a nice Simpson 260, mirrored scale, rolltop case, FRIED when a guy tried to check the battery voltage of his car with it set on ohms.....
Not a total loss though. I got the case....

Try checking it with nothing hooked to the sender and see what you get.
Of course, we still need a chart that gives the correct resistance.

You know, I just might have access to a sending unit like yours. A friend of mine's 350 is in pieces in my shed, and it has a brand new sending unit in it. Not sure if it is the same or not, but I'll check it out.

J.
 
You can measure voltage to figure this out. The sensor body to block should be 0V when the engine is running, as should the block to B-.

To measure the sensor resistance, disconnect the plug on it.
 
The meter is fine, its an older digital that will on occasion give unstable reading. Usually when not hooked up correctly. OK so I'll have to check the ohms from the body of the sender to the terminal. All measuring was done with the sender wire unhooked. Didn't even think of checking voltage. I'll look into that as well.

Thanks
 
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