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Water Temp Gauge Doesn't Work??

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Dave1

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Winter Springs, (Central) Florida
After a recent fresh 350 engine installation in my 1986 GMC 3/4 ton 4WD pick up truck, my stock dash mounted temp gauge quit working. I put in a new water temp sending unit and still no temp registers on the dash gauge. Is there a way to test the gauge with volt ohm meter to see if it is defective?

Just curious, is there a difference in the water temp sending units for a truck that has a gauge and one that has an idiot light? I thought maybe they sold me the wrong sending unit and that is why the gauge is not working.

If the gauge itself has gone bad, how tough of a job is it to replace it with a factory stock replacement gauge?

Thanks for any info.

Dave1
[email protected]
 
Dave1 said:
After a recent fresh 350 engine installation in my 1986 GMC 3/4 ton 4WD pick up truck, my stock dash mounted temp gauge quit working. I put in a new water temp sending unit and still no temp registers on the dash gauge. Is there a way to test the gauge with volt ohm meter to see if it is defective?

Just curious, is there a difference in the water temp sending units for a truck that has a gauge and one that has an idiot light? I thought maybe they sold me the wrong sending unit and that is why the gauge is not working.

If the gauge itself has gone bad, how tough of a job is it to replace it with a factory stock replacement gauge?

Thanks for any info.

Dave1
[email protected]
Click to expand...

Wayulll... make SURE the green wire to the sender is connected, and not melted through ... it runs by the exhaust manifold and can get zapped pretty easy. You are playing with the sender on the left side of the engine, right? :)

To test, pop the green wire off, and with the key on, touch it to ground briefly ... the temp gauge should move. If not, could be wiring, could be the gauge.

The gauge is easy to replace. Taking the dash apart is a PITA. You'll need to remove the black plastic cover, i.e. the part that goes across the cluster and the radio. For this you can do without removing the dash pad.

Then you remove the clear plastic cluster cover, the metal thing underneath, and then the gauge will come out.

Gauges rarely go bad. Wires and senders aer more likely; either bad wires, or miswired.

If the grounding thing above doesn't work, yank the dash, then check the wiring to the gauge with an ohmmeter.

And if you do need a gauge, lemme know -- I have a handful and LMC gets too much for them :) [I get essentially beer money plus shipping]

-- A
 
OK, here's what I learned today. As the engine coolant temp rises, the temp gauge sending unit applies resistance to ground . The resistance causes the temp gauge needle to move on the scale. More heat equals more resistance to ground applied by the sending unit which causes the gauge needle to move further. At some point when the heat gets excessive the sending unit applies full ground to the lead which in turn causes the temp gauge needle to peg at or past 260.

Yes, you can test the gauge by recreating the above scenario. Take the lead off the sending unit, turn key on, have someone watch the dash mounted temp gauge, and touch the lead to an engine ground. The gauge needle should peg. If so, the gauge apparently is OK.

Apparently I have a bad sending unit and will replace it tommorrow and see what happens. Hopefully all is OK. Then I can move on to other problems that need addressing.

One thing I am not sure of, is there separate and different temp sending units for a guage or idiot light. If there is a difference, maybe I got the wrong one. A friend says there is no difference.

Thanks for the feedback.
Dave
[email protected]
 
Last edited:
Dave1 said:
One thing I am not sure of, is there separate and different temp sending units for a guage or idiot light. If there is a difference, maybe I got the wrong one. A friend says there is no difference.
Click to expand...

Well... LMC doesn't seem to list them, urk.

Kragen/Schucks/Checkers says they are different, but it's easy to tell them apart. The one for gauges has the round terminal:

TS6.jpg


and the one for the idiot light has a spade lug:

TS78.jpg


Now, I don't recall if you mentioned the year of truck you're working on; some of the newer ones do have multiple senders, some for the gauges and some for the computer, etc.

You ARE talking about the one on the left side of the engine, between #3 and #5 cylinders, IIRC, with a green wire, right? :D

-- A
 
My truck is a 1986 3/4 ton GMC pick up with an "M" series 350 engine, T-400 auto trans, 4.10 gears, etc.

The hole into the block where the original temp sending unit was located is on the drivers side of the engine down below #3 cylinder sparkplug. It is a fairly large hole, big as your thumb. The temp sending unit I got from local GMC dealer is the spade type exactly like in your photo and it does fit the hole perfectly. The wire lead that connects to the sending unit has the spade type female connector on it. It sounds like a match but I do have a gauge so maybe not.

Last night I did do the ground the lead test and the gauge needle did peg so it appears the gauge is working. Now, I just need to find the correct sending unit.

The GMC dealer parts man showed me several sending units that were listed for this truck/engine and most were of much smaller diameter and would not fit the hole in the block without the use of a reducing bushing. I think I will call the dealer again today and see if he can determine if there is a different GM sending unit for a gauge that has a male spade connector on it.

Thanks for your help.

Dave
 
All GM senders for our vintage (and later) on just about all apps are the same, with gauge of course.

If you've got a wrecking yard, it's a piece that rarely fails...

There were two different thread sizes used. If you get the small hole sender, you can get an adapter, but it's just easier to find the right one. Say a 1974 and a 1987 I believe should net you both sizes, one of which will be right. IIRC the later heads use the smaller hole.
 
There are TWO temp senders?! I have an 1985 350 and i was having the problem of the temp needle spiking all the way up to 260+ then going down to the regular range. I also have a little red LED on the left side of the steering wheel that recentley came on when I went 4x4 one time. It has now since then turned off. I have no idea what that light does. The only temp sender that I know of was on the side of the thermostat housing. Should I have changed the other one too? Do i have an other one in the block?
 
dremu said:
Well... LMC doesn't seem to list them, urk.

Kragen/Schucks/Checkers says they are different, but it's easy to tell them apart. The one for gauges has the round terminal:

TS6.jpg


and the one for the idiot light has a spade lug:

TS78.jpg


-- A
Click to expand...


Is this always true? I just bought one as well and I told them I wanted one for the gauge and it came with a spade terminal instead of the round terminal. Both the so-called gauge one and the idiot light one had spade terminals. My gauge still doesn't show a temperature but I can vary it using a potentiometer to ground so I know the gauge is good. Maybe this is the reason why in the past I resorted to going junkyarding to find a sender, eh? Kind of pisses me off since I paid $12 for it.
 
I *THINK* at some point GM went with a different connector, but for our vintage I believe all were that weird nail head connector.

In my previous post when I said "the same", I meant the correlation between the resistance of the sender and the reading on the gauge, not that they were externally identical.

To my knowledge, all temp *switches* utilize the spade connector.
 
Doesn't the resistance go to zero as the temperature increases?

When I was at the auto parts store (CarQuest) they showed me the other style sender (switch for idiot light) and it was a can instead of a cast chunk of brass.
 
Yes, resistance to zero, you ground the wire, everything else good, it will peg the gauge. Remove ground, should go full cold.
 
OK, just making sure I'm not crazy. I couldn't remember what it does when you touch it to ground but figured it was like every other thermistor with a sending unit that uses one wire. Someone probably already said it in the thread... don't have much time to read while at work sometimes.
 
Yesterday afternoon I stopped by the local parts house and went thru the listing of temp sending units with the counter guy. I was surprised to see so many listed. We found the one listed for my year/make/model but it was the wrong size thread i.e. way too small.

After some research we found a large size sending unit with a spade type connector that was specified for a gauge. It was about $10. Took it home, made the installation, and WALA I have temp.

Seems odd the GMC dealer could not figure this out. I will be returning his $20 sending unit first chance I get. The sending unit I bought yesterday is made by Standard Motor Parts and the part number is TS-71T.

Thru this situation I have learned there are two locations a sending unit may be located on one of these GM 350 engines. My sending unit is located in the side of the block right below the #1 & #3 cylinders. It could have been located in the thermostat housing. The sending unit for the thermostat housing is the smaller diameter unit. The one that goes into the side of the engine block is the large diameter unit.

Hope this info and experience helps others.

Now, on to my distributor installation/timing problem.

Dave1
 
sweetk30 said:
check for the ground strap to the block from the firewall area.
Click to expand...


sweetk, I'm glad I saw this. I was cleaning the engine compartment yesterday and noticed the ground strap from the firewall to block is no longer attached to the block. It is just hanging from the firewall. Where does it attach to the block?
 
joe dirt said:
Where does it attach to the block?
Click to expand...

Usually in the back of the head, but you can put it anywhere as long as you get a good clean connection between the firewall and the block.

Scott
 
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