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Dumb question time - where is temp sending unit ?

Hmm there is a break 74 to78 79 up to 87. I just went though a mismatched sensor gauge. My issue was late model heads use the 3/8 npt, but is the wrong resistance. My gauge would barely move like yours. I finialy took the correct sensor, which is 1/2 npt, and turned it down and rethreaded to 3/8 npt. My gauge now read at the 1/4 mark idling in the driveway with 165* at the tstat measured with infrared. I can live with that, would probably read just under half with a 180* tstat.

Screenshot_2021-03-24 Functional Tests of Factory GM Electric Gauges.png
above stolen from http://forum.73-87chevytrucks.com/smforum/index.php?topic=25339.0
 
I really should not get involved with this. First of all, I'm just starting turkey season, so I'm foggy at best.
Second of all. I don't know exactly how those gauges work, as I don't think I ever worked on a GM. Lots of others, not GM. But, I do know how different gauges work, so I can throw that in here.

Most older gauges used heated bi-metallic strips. Jeep, Ford, not sure about the really old GM stuff.
They had a strip of two different metals soldered or welded together with a thin piece of heater wire wrapped around it. As the resistance changed, the wire got hotter or cooler, and the metal strip bent since the two metals expanded at different rates.
Not instant reading, but usually fast enough. They would all go to the lowest reading when the power was turned off, and had to have a voltage regulator in the line.
Usually ran off about 9 volts.
The old Jeeps with the round cluster, had the voltage regulator as part of the gas gauge. It powered the gas gauge and the temp gauge.
If the cluster lost its ground reference, both gauges and both sending units would burn up in seconds.

Most of the European gauges were D’Arsonval movements. They were the same as a voltmeter, and had a hairspring inside to hold them at 0.
Very fast reading, you could see them go to the low scale almost instantly when you turned the key off. They also require a voltage regulator, and if they did not have some kind of delay circuit, the gas gauge would move as the gas sloshed in the tank.

Then, you had some that had a magnet attached to the meter arm, which was dampened. It would stay where ever it was when the power went off. Others had small springs that returned the meter to 0 when the power went off. The needle was moved by an electromagnet controlled by the resistance of the sending unit. Also had to have a power supply.

Then, you have what the newer Fords have, and probably what GM had a long time.
A magnet or ferrous part on the needle, with two right angle coils in series with the sending unit tapped in. One coil tries to 0 the meter, the other tries to peg it.
Since both run off the same voltage source, the difference between them stays the same no matter the voltage. The difference is controlled by the sending unit. And the difference controls the position of the needle. So no voltage regulator needed.
A few companies added a small hair spring to return the needle to 0 when the power went off, but most of them just had some drag on the needle shaft to dampen the swing.

So, with most GMs, if they have no spring, and I don't think any of them do, the needle moves somewhere according to the current ratio between those two coils, and stays there when the power is off.
If you have the gauge out, you can tap it sideways and the needle will move and stay where it stops with no power.

If you have power going through those two coils to ground, when you ground the sending wire, one coil gets all the current, and the needle swings all the way over one way.
When you open the sending wire, the coils are wound in such a way that the other coil has the stronger field, and the needle swings all the way the other way.
There is friction in the bushing/bearing that the needle shaft runs on to dampen any minor variations, and make the needle stay where it is when the power is off.
If that bearing should be loose, it might act as a spring and cause the needle to swing to a certain location when the power is removed.
I've never seen it, but it could happen.
In your case, it almost sounds like the wiring is backwards. I would have to sketch out the circuit to see what would happen, since its beyond me to do in my head right now.
But, I think reverse voltage on the meter would cause what you are seeing with the meter reading 0 under power and temp when off....... Or maybe the sending unit and ground swapped, or..........

@rampage can do it in his head. So can I in about 4 weeks......

Sorry if I confused everybody. Its almost 8 o'clock, and I need to crash. 4 o'clock comes early for an old guy like me.....
 
Fordum said:
I really should not get involved with this. First of all, I'm just starting turkey season, so I'm foggy at best.
Second of all. I don't know exactly how those gauges work, as I don't think I ever worked on a GM. Lots of others, not GM. But, I do know how different gauges work, so I can throw that in here.

Most older gauges used heated bi-metallic strips. Jeep, Ford, not sure about the really old GM stuff.
They had a strip of two different metals soldered or welded together with a thin piece of heater wire wrapped around it. As the resistance changed, the wire got hotter or cooler, and the metal strip bent since the two metals expanded at different rates.
Not instant reading, but usually fast enough. They would all go to the lowest reading when the power was turned off, and had to have a voltage regulator in the line.
Usually ran off about 9 volts.
The old Jeeps with the round cluster, had the voltage regulator as part of the gas gauge. It powered the gas gauge and the temp gauge.
If the cluster lost its ground reference, both gauges and both sending units would burn up in seconds.

Most of the European gauges were D’Arsonval movements. They were the same as a voltmeter, and had a hairspring inside to hold them at 0.
Very fast reading, you could see them go to the low scale almost instantly when you turned the key off. They also require a voltage regulator, and if they did not have some kind of delay circuit, the gas gauge would move as the gas sloshed in the tank.

Then, you had some that had a magnet attached to the meter arm, which was dampened. It would stay where ever it was when the power went off. Others had small springs that returned the meter to 0 when the power went off. The needle was moved by an electromagnet controlled by the resistance of the sending unit. Also had to have a power supply.

Then, you have what the newer Fords have, and probably what GM had a long time.
A magnet or ferrous part on the needle, with two right angle coils in series with the sending unit tapped in. One coil tries to 0 the meter, the other tries to peg it.
Since both run off the same voltage source, the difference between them stays the same no matter the voltage. The difference is controlled by the sending unit. And the difference controls the position of the needle. So no voltage regulator needed.
A few companies added a small hair spring to return the needle to 0 when the power went off, but most of them just had some drag on the needle shaft to dampen the swing.

So, with most GMs, if they have no spring, and I don't think any of them do, the needle moves somewhere according to the current ratio between those two coils, and stays there when the power is off.
If you have the gauge out, you can tap it sideways and the needle will move and stay where it stops with no power.

If you have power going through those two coils to ground, when you ground the sending wire, one coil gets all the current, and the needle swings all the way over one way.
When you open the sending wire, the coils are wound in such a way that the other coil has the stronger field, and the needle swings all the way the other way.
There is friction in the bushing/bearing that the needle shaft runs on to dampen any minor variations, and make the needle stay where it is when the power is off.
If that bearing should be loose, it might act as a spring and cause the needle to swing to a certain location when the power is removed.
I've never seen it, but it could happen.
In your case, it almost sounds like the wiring is backwards. I would have to sketch out the circuit to see what would happen, since its beyond me to do in my head right now.
But, I think reverse voltage on the meter would cause what you are seeing with the meter reading 0 under power and temp when off....... Or maybe the sending unit and ground swapped, or..........

@rampage can do it in his head. So can I in about 4 weeks......

Sorry if I confused everybody. Its almost 8 o'clock, and I need to crash. 4 o'clock comes early for an old guy like me.....
Click to expand...

LOL
I’ve never really had to mess with the gauges and I don’t know exactly how they’re setup internally so I’m not even going to attempt it and confuse matters. :deal:
 
Replaced gauge, printed circuit board, replaced part of wire thru engine compartment.....no go.


I have no idea, everything else works in dash as it should.

Going with aftermarket I guess.
 
This problem is fixable, and probably fairly easy. My problem, other than turkey hunting, is that I cannot find any information on how that gauge is wired internally.
Let me ask a couple of questions, and try to get back after the answers:
How many pins on that gauge? It seems some had three and some had four. From what little I can find out, the ones with four pins have to have a resistor between the top and bottom pins. Usually 90 ohms. They are scarce, but available.
I have no idea what that resistor does. Some say its a trim resistor to tweak the readings, but that makes no sense.

Do you have any kind of multimeter? We need volts and ohms.
Do you have the old sending unit? If you have both, we can rig something up I hope.
I had a couple more questions, but I'm so sleepy I can't even remember them.
I don't suppose you have any kind of electronic parts? Rhostats, resistors, things like that?

Hang in there, I feel sure it can be fixed. An aftermarket gauge will still have to be wired in, and I think this one can be fixed easier, unless you go with a mechanical gauge.
 
The old gauge and new one have 4 pins out the back, both have the ceramic resistor or whatever on the back..

I ordered the sending unit that was recommended on page one.

I have multimeter. I don't have any other "electronics" like you mentioned.

My theory is the old sending unit failed and napa is wrong range for my truck.

Also, the new gauge in box had a reading, so I'm going to ignore key off/ power off readings, like mentioned, its irrelevant
 
Just so I am clear. The gauge still doesn't move or barely moves at operating temperature? If so you need the early pre 79 sensor. Then you'll need to turn it down on lathe and re-thread to 3/8 npt. That what I need to do to get my gauge to work. I think there is a guy on https://67-72chevytrucks.com/ that offers the turned down sensor.
Or drill and tap the cylinder head to 1/2" pipe.

Wes's Burb build/update/maintance

Towing a single axle box trailer to Lake Havasu tomorrow. maxed load would 1500 pd with gtwr of 3000. I have it loaded light with some garage stuff, dam it's hard to pick stuff in the garage I might not need for a bit. Will be a year or so till I'm full AZ resident. Going to be a good test of...
ck5.com ck5.com
 
Ok. Well I went on Rockauto , put in 1978 Blazer and the sending unit for "gauge" looks like the one that was in my truck, it has the round nub on the end, not a blade terminal.

The Napa one has blade and the one i ordered from page 1 looks to be bladed as well.

So, not sure whats correct.

Off topic question - I have an extra wire sitting by A/C that isn't connected to anything and its in the A/C harness. I believe it was part of the quadrajet but curious.
 
If its a green wire it may have been for a idle solenoid to bump up the idle speed when the A/C compressor kicks on..

Hmm...temp sender wire is green too usually...could it be they may have got "swapped"?...if they would reach its possible..

In my experience as a parts counterman thinks like temp sending units and transmission modulators ,oil pressure senders often were "listed wrong",and you had to match the old part up (and many times engine swaps also ended up having mismatched senders that were incompatible with the gauges)...many times the sender that ended up being "correct" had a different connector (spade vs bullet type or "nail head" than the one being replaced too)..so the wire had to have the matching connector installed..

Most all of my older GM trucks ,I put in a aftermarket triple gauge cluster with all mechanical gauges for water temp,volts,vacuum,or oil pressure...the GM gauges were at best "close but no cigar" as far as accuracy..
 
I give up, tried different sending unit and gauge. Was even swapping them all around. I went as far as dipping sending unit in boiling water and trying to bench test it and yes sending unit had a ground around threads.

Going aftermarket I guess
 
Ok, strange updates and with more questions.

Needed to go to dump and take my 5 x 12 trailer full of brush. Hooked up to K5 and drove there with a few hills and dropping out of OD, it got the motor warmed up and gauge started to move to the middle mark and was reacting as I was driving.

I took 180* thermostat out and put in 195*.,now temp gauge reads just at 210*. The sending unit is in the block..

I don't trust the gauge but could it actually be reading correctly?

Whats the temps these should run at its stock 305cid ?

THANKS
 
Not totally unexpected. I have the factory gauge at the cylinder head and the EFI temp senor in the intake near the thermostat. Generally the head temperature is hotter than the intake reading. So if you have a 195 thermostat, it wouldn't surprise me if the head was around 205 based on my experience.
 
What year is your truck again?

Oem went 195 with computers to control timing and fans. This for emissions.
Imo your 700r4 will much prefer the 180. I run mine with 160. Might be to cold will see if if coolant stays in radiator long enough to give up enough heat.
 
to hot for my liking. esp if the factory controls are not there to safe guard from over heat. Do you still run a cat?
 
The cats will appreciate the 195 stat. They are the reason it was made.
 
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