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Dumb question time - where is temp sending unit ?

I burped the system, I pulled the sending unit and water gushed out , so no air pocket.

I drove he truck 20 miles, upper hose Hot, lower hose cold. I believe system is operating properly, jus gauge off
 
Well pulled dash, have dash sitting on bench, removed gauge , moved dial to 100, plugged it back in and its reading 160* , WTF!

How can gauge read/move with ZERO power to it.

So , is it gauge or dash issue ? And can't seem to find gauge either
 
Very strange, when the gauge has power it reads cold, when the power is removed reads middle range. With power and grounded sensor wire, results in full sweep. 2 different sensors gauge shows cold after warm up with 180* tstat. Do I have the facts correct ?
I am at a lose to explain the middle range reading with no power to gauge, my 1st thought was wiring a short. That doesn't explain the same results with the cluster on the bench.
Is there a chance the gauge is from a pre 79 dash ? the gauges have different resistance requirements
 
It's not right if the gauge doesn't peg in both directions based on the wire being grounded or open. Something else is up.

It wasn't the sender, it's probably not the gauge either. Only failure point I've seen that's even close to problematic with the gauges is the resistor on the back. I've seen I think one of those that has failed.

I'm not super electrically educated, no clue why it would move with no power, but how are the traces on the back of the cluster?

Three contacts for that gauge at the cluster flexible circuit...positive, ground, and the sender. With it on the bench, should be no continuity between any of those contacts. If my thought process is correct, the gauge not maxing out when the sender wire is disconnected, means there is some sort of ground elsewhere that shouldn't be. Disconnect the sender, run a continuity check on that wire to ground. If no short to ground there, the problem is definitely between the cluster and gauge.
 
How do I run continuity of that wire ?

As far as I know truck was bone stock when I bought it
 
the sensor wire? It seems to be good with the correct test results. try putting a 12volt light bulb between the sensor wire and ground. this should make the gauge move over half the range. I am not sure why the power off reading is abnormal, but it sounds like it works correctly when powered. if the bulb trick works we are back to the sensor and grounds at the motor.
What size threads does your sensor have 3/8" npt or 1/2" npt ?
 
Sounds like the gauge has power with the key off and not on. If it used to work correctly maybe the ignition switch is going bad. If it’s never been right the wiring may be Miss matched.
 
Get a multimeter, set it to continuity, and probe the sender connector with one lead, and ground the other. If it chimes, the wire is the problem. Repeat (or start with) the cluster connected.

A cheap multimeter is less than that sender was, guaranteed. And incredibly useful even when used for the most basic functions.

The gauge does NOT work properly with power. It does not max out when the sender wire is disconnected, which is a failure in that circuit somewhere upstream of the sending unit. The continuity test of the sender wire will (nothing is impossible, both senders COULD be bad and you just coincidentally have another problem on top of that, however incredibly unlikely that is) prove the sending unit wire is or is not the problem. This is assuming one or both of the senders is good, but you don't even need the sender at this point. The sender wire not connected to anything will suffice for further testing. Until the gauge pegs out with that wire disconnected and key on, you haven't found the issue.
 
If i pull wire off sending unit and ground it,,the gauge pegs HOT.

For some reason the gauge goes to 160 no matter what except when actual power is applied running or not
 
Wes Harden said:
the sensor wire? It seems to be good with the correct test results. try putting a 12volt light bulb between the sensor wire and ground. this should make the gauge move over half the range. I am not sure why the power off reading is abnormal, but it sounds like it works correctly when powered. if the bulb trick works we are back to the sensor and grounds at the motor.
What size threads does your sensor have 3/8" npt or 1/2" npt ?
Click to expand...
Not sure what size threads, guessing 3/8" , I think some are miss reading my post.

power on - gauge doesn't move, even after driving for 1/2 hour and idling for awhile

Power off and even gauge cluster on bench - gauge reads 160* with ZERO power going to it
 
badmix said:
If i pull wire off sending unit and ground it,,the gauge pegs HOT.

For some reason the gauge goes to 160 no matter what except when actual power is applied running or not
Click to expand...
With the wire disconnected and NOT grounded, the gauge HAS to peg to cold to indicate the circuit is working properly. Grounding the sending wire is only half the test. The other half, it sounds like, it fails. Correct?
 
With wire disco'd and not grounded it reads 160* I believe, not zero. It only reads zero (100) with power and running or not.

With wire grounded it pegs gauge to 260*

I believe the sending unit wire is good. Don't really know how to figure if grounding. Never had issues with anything else as far as ground goes
 
I'm unfamiliar with the "circuit board" on back. What are symptoms of issues with those?
 
I'm pretty sure its perfectly normal for the dash gauges to "stop" anywhere they please once the ignition is shut off...when its turned back on,they should re-set to the actual setting--like the gas gauge may peg,drop lower.or stay the same when power is cut off,but return to the actual level of fuel when the key is turned back on..Tem pauge may read hot,cold,or remain "centered" with the power cut off..

I've read this in old GM owners manuals and Chilton books have a whole chapter on gauges explaining how the gauge's needle has two coils of wire next to it,one on each side,wound like relay coils,that produce magnetism and make the needle go up or down,depending on which coil gets the most voltage,which is controlled by the sending unit and possibly a resistor across the gauge's terminals.
 
Ok, that makes sense for the off position but what about while running? It never comes off 100 or basically zero
 
So tough to diagnose....especially when the temp gauge used to read accurately (assuming it did!)...it would have to be a wiring,ground or dash gauge issue if it once worked correctly and now wont..but if it never was "right" or accurate,many things could come into play..
Maybe the dash cluster or temp gauge was swapped from another year truck,and doen't match up to your trucks wiring harness (dash clusters vary between years,or if it had a factory tack,clock,location of gauges,and are wired differently on the printed circuit on the back of the cluster that connects everything to the main harness plug,they have different pinouts..

I've had a few printed circuits get partly smoked when a short happened in a wire under the dash and melted across it--the connections on them are finiky too,its just a strip of copper foil,they corrode or fail sometimes,they may develop high resistance with age too.
 
Yea. Well I ordered new gauge and printed circuit and the piece it all connects to, as mine has the screw holes all buggered from years of taking it off. Always had to put bigger and bigger screws in, lol
 
I don't believe the gauge ever worked. It always read 160 and they had the wrong style sending unit, one with a round end, not blade. So really don't know what all has been done to truck or what is correct or not
 
diesel4me said:
I'm pretty sure its perfectly normal for the dash gauges to "stop" anywhere they please once the ignition is shut off...
Click to expand...


Yep, ignition off means nothing with the gauges, fuel, temp, voltage will jump to random angles once you shut ignition off
 
Which gauge did you get. The 80 up? The sender I recommended is for 80 up.
 
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