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Unlikely to be a problem...but where is the main engine ground connected? On my '88 it goes to the engine block where the fuel pump used to be mounted on the older vehicles. Wouldn't hurt to run a small jumper wire from the cylinder head that the sending unit screws into, to wherever the main engine ground is connected. If that doesn't help, remove the jumper wire.

My ground goes to a stud on the intake manifold straight to the neg. side of the battery and it also has two wires that ground there , a tan wire and a black wire. I took them off and cleaned it up. The stud had forma-gasket ( the hard kind ) on it , After cleaning it up I replaced it with black RTV gasket maker. ( not sure if this stud goes into a water jacket or not and if so I am not sure if the RTV will work out ok )
 

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name

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After cleaning out the threads for the sending unit with a drill and a bore brush, I replaced the thermostat ( 195 ) and temp sending unit and the gauge now reads 1 tick higher than it did before. I can also see a more prominent swing in the gauge when the thermostat opens and closes.

The sending unit looks the same as the old one but the box is labeled a "switch" instead of a "sending" unit, I am not sure what difference that makes. ( if any )

What does that valve in the old thermostat do, and what do the tan and black wires in the previous post go to ?
 

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Erik the Awful

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Typically a switch is a digital signal for a dummy light or cooling fan control. A sending unit is an analog signal for a gauge or the ECM. Unfortunately, the person printing the labels rarely has the experience of installing the part nor an understanding of the circuit.
 

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The stud had forma-gasket ( the hard kind ) on it , After cleaning it up I replaced it with black RTV gasket maker. ( not sure if this stud goes into a water jacket or not and if so I am not sure if the RTV will work out ok )
The stud probably goes into the water jacket. RTV Silicone will likely seal the cooling system, but it's not the "best" product to use. RTV Silicone will shred on removal, little chunks will fall into the coolant and get circulated.

Better to use an actual thread sealer, like Loctite/Permatex 592. Fabulous stuff for sealing threads--tapered or straight.

After cleaning out the threads for the sending unit with a drill and a bore brush, I replaced the thermostat ( 195 ) and temp sending unit and the gauge now reads 1 tick higher than it did before. I can also see a more prominent swing in the gauge when the thermostat opens and closes.
Sounds great. Temp gauge may still be a little low, though.

The sending unit looks the same as the old one but the box is labeled a "switch" instead of a "sending" unit, I am not sure what difference that makes. ( if any )
As Erik the Awful said, a "switch" is intended to be on/off. Generally drives a dash warning light that would either be "out" or "full brightness".

A "sending unit" is variable-resistance, for driving a dash gauge that has a range of motion for the indicator needle.

A "sensor" can be either a switch or (more commonly) an analog sending unit which might be a variable ground, or a variable voltage, but implies that it's signal goes to the onboard computer instead of a "gauge" or "light" on the dash. (Not always true--but usually true.)

What does that valve in the old thermostat do, and what do the tan and black wires in the previous post go to ?
Looks like a "Jiggle Valve". Lots of import cars use those in the thermostat. Nice feature. The valve is open when the engine is shut off. Makes it nice for refilling the coolant after cooling system service; once the system is full with the air bled out, it doesn't make any difference any more.

But unlike boneheads that drill bigass holes in the thermostat flange to "let the air out", the jiggle valve CLOSES as soon as the water pump starts moving coolant. So there's no un-metered flow out of the engine and through the radiator that slows engine warm-up.

Robertshaw thermostats have a tiny opening stamped-into the flange, allows air to escape the engine during cooling system refill, but is so small that it doesn't meaningfully slow warm-up. Cheaper to stamp the flange, than to install a jiggle valve.

Typically a switch is a digital signal for a dummy light or cooling fan control. A sending unit is an analog signal for a gauge or the ECM. Unfortunately, the person printing the labels rarely has the experience of installing the part nor an understanding of the circuit.
Yup. And they likely don't speak English anyway.
 

Erik the Awful

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I work in supply chain, managing parts. Trust me, the people I work with speak English (ish), but don't always understand the terminology. We had bearings being refused by the warehouse because the vendor marked them "each" instead of "set" and I had to explain why it was important that they are marked with the correct unit-of-issue.

If you get a bearing marked "each", it's not supposed to come with a bearing race. If we accept bearing sets marked as "each", what's to stop the vendor from saving a few bucks and sending bearings that are actually "each" when we need bearing "set" that comes with the race? When you're dealing in million dollar contracts, the semantics absolutely matter.
 

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As Erik the Awful said, a "switch" is intended to be on/off. Generally drives a dash warning light that would either be "out" or "full brightness".

A "sending unit" is variable-resistance, for driving a dash gauge that has a range of motion for the indicator needle.

A "sensor" can be either a switch or (more commonly) an analog sending unit which might be a variable ground, or a variable voltage, but implies that it's signal goes to the onboard computer instead of a "gauge" or "light" on the dash. (Not always true--but usually true.)

In regards to the temp sending unit :

When I went to pick up the part at Auto Zone, the counter guy let me see the computer screen because there were more than one option to choose from . The option I picked was described as a " temperature sending unit , 1 OR 2 wire " . When I got the part I opened it up to verify it would match up with my pigtail and old unit, and it did. I did not notice the actual wording on the box until after the job was done and I was taking pictures for this thread. I am mentioning this not to throw rocks at the parts guy, but to help out the next guy reading this thread determin if he has the right part or not .

All that being said, does the fact that this part moves the temperature gauge mean that I do have the right part, even though it does not read as high on the gauge as I was expecting, reguardless of the wording on the box. Or do I not have the right part and the fact that it moves the gauge at all is coincidental ?
 

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The stud probably goes into the water jacket. RTV Silicone will likely seal the cooling system, but it's not the "best" product to use. RTV Silicone will shred on removal, little chunks will fall into the coolant and get circulated.

Better to use an actual thread sealer, like Loctite/Permatex 592. Fabulous stuff for sealing threads--tapered or straight.


Sounds great. Temp gauge may still be a little low, though.


As Erik the Awful said, a "switch" is intended to be on/off. Generally drives a dash warning light that would either be "out" or "full brightness".

A "sending unit" is variable-resistance, for driving a dash gauge that has a range of motion for the indicator needle.

A "sensor" can be either a switch or (more commonly) an analog sending unit which might be a variable ground, or a variable voltage, but implies that it's signal goes to the onboard computer instead of a "gauge" or "light" on the dash. (Not always true--but usually true.)


Looks like a "Jiggle Valve". Lots of import cars use those in the thermostat. Nice feature. The valve is open when the engine is shut off. Makes it nice for refilling the coolant after cooling system service; once the system is full with the air bled out, it doesn't make any difference any more.

But unlike boneheads that drill bigass holes in the thermostat flange to "let the air out", the jiggle valve CLOSES as soon as the water pump starts moving coolant. So there's no un-metered flow out of the engine and through the radiator that slows engine warm-up.

Robertshaw thermostats have a tiny opening stamped-into the flange, allows air to escape the engine during cooling system refill, but is so small that it doesn't meaningfully slow warm-up. Cheaper to stamp the flange, than to install a jiggle valve.


Yup. And they likely don't speak English anyway.
Jiggle pin.
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when used on a vehicle 99% of the time the jiggle pin is either at 12 o clock(looking at the engine) or its placed at the highest point (Like in vertical applications) There may be exceptions but Id suspect they are few

on a 3800, for example, the jiggle pin would be at 12 o clock when looking at the engine which places the flange of the thermostat where the thermostat housing wont crush the thermostat(It has an offset base) but this is only for 1999+ blocks. If you were to look at the block at eye level with the spot for the T-stat, you would notice that the block has a slight "raised" area sweeping to 12 o clock making it the "highest" point when put on. Not of the cooling system but so that the jiggle pin, or hole, will let air out past it like you describe. Fun stuff !
 

AuroraGirl

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In regards to the temp sending unit :

When I went to pick up the part at Auto Zone, the counter guy let me see the computer screen because there were more than one option to choose from . The option I picked was described as a " temperature sending unit , 1 OR 2 wire " . When I got the part I opened it up to verify it would match up with my pigtail and old unit, and it did. I did not notice the actual wording on the box until after the job was done and I was taking pictures for this thread. I am mentioning this not to throw rocks at the parts guy, but to help out the next guy reading this thread determin if he has the right part or not .

All that being said, does the fact that this part moves the temperature gauge mean that I do have the right part, even though it does not read as high on the gauge as I was expecting, reguardless of the wording on the box. Or do I not have the right part and the fact that it moves the gauge at all is coincidental ?
do you have a gauge? Then you need a gauge "sensor" or sender which gives a varying reading from cool to warm to then over temp
The important thing is that the overtemp thing would work in an actual overheat. you could use your stove to determine this with some water to find the values if you dont want to overheat your engine to find out.


But a "switch" type of "sender"(I wouldnt use sender but some may) would be "HOT/NOT" in how it works. At overheat temps, it would trigger a light/warning and it would be carefree otherwise

I Would susepct you didnt bleed your coolant of air before Id think the sender which is giving you a varying reading on the dash was wrong or defective.
 

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I'm going to jump here rather than start a new thread, hopefully not too much of a hijack-

My water pump decided to crack in half, so I replaced it. While doing so I replaced as much of the coolant as I could easily drain out, about 2.5 gallons.
I will add- all of this took place while I was in the middle of an 0411 ECM swap, not sure if it's related or not, but figured I'd include it.

My temp gauge has been off from "actual" by a bit since I bought the truck, when verified via ECM readings from ODB blue tooth dongle and cell phone app.
Now I get wild swings on the temp gauge - to the point of triggering the "check gauges" light... however the ECM reading is between 190-205 when the gauge reads way high. It comes down pretty fast....

Is it possible there are bubbles or something triggering a high reading on the gauge while the computer things all as well? Or is it just a bad sender/gauge related issue? And why the change now, is there some wiring ran through the 0411 swap that could have been impacted? Or unrelated and just happens to be a coincidence?

Would be nice to get some insight, I would like it if the darn dash gauge was somewhat useful, rather than have to rely on using an app to know if i'm overheating all the time-
 

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does the fact that this part moves the temperature gauge mean that I do have the right part, even though it does not read as high on the gauge as I was expecting,
If the gauge moves fairly smoothly and gradually from "cold" to "warm/hot" as the engine temp gradually increases, you've got the correct style of part--a sending unit not a switch.

If the gauge goes from "cold" to "suddenly" warm/hot, you've got a switch. NORMALLY a switch would go from "cold" to "overheating--far end of scale". But if there's enough resistance in the wire harness, it might only go to some mid-point.

So there's the difference: Does the gauge needle creep up in synch with the increasing engine temp? Or does it suddenly jump from cold to warm/hot?

First Guess: You have the correct unit. But it's possible that they sold you a sending unit for some other application that has a differently-calibrated gauge. Or potentially a defective sending unit. Maybe someone put the wrong sending unit into the correct-part-numbered box.

As I've said before--the most likely (but not guaranteed) reason for the gauge not indicating hot enough, is excess resistance in the wire harness.

My temp gauge has been off from "actual" by a bit since I bought the truck, when verified via ECM readings from ODB blue tooth dongle and cell phone app.
Now I get wild swings on the temp gauge - to the point of triggering the "check gauges" light... however the ECM reading is between 190-205 when the gauge reads way high. It comes down pretty fast....

Is it possible there are bubbles or something triggering a high reading on the gauge while the computer things all as well? Or is it just a bad sender/gauge related issue? And why the change now, is there some wiring ran through the 0411 swap that could have been impacted? Or unrelated and just happens to be a coincidence?

Would be nice to get some insight, I would like it if the darn dash gauge was somewhat useful, rather than have to rely on using an app to know if i'm overheating all the time-
WHAT VEHICLE???

When the gauge goes too high, but the sensor for the computer shows reasonable temperatures, my first thought is that you've got defective insulation on the temp gauge wire, and that bare spot is intermittently grounding. When it grounds, the gauge goes up, and when it moves away so that it doesn't ground, the gauge reads "normal".

With gasoline V-8 or V-6 in a GMT400, it's almost impossible for "air bubbles" to cause problems. As soon as the thermostat opens, they're transferred to the radiator where they show up as "low coolant level". I don't know about Diesels in GMT400s.

OTHER applications may trap air in the cooling system somewhere. I've got Luminas that have cooling system bleeder screws to eliminate air pockets that won't purge on their own.
 
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