97 5.7 Vortec Temp/Coolant Sensor/sending Unit Clarifications...?

[Rick Werner]

Hey guys! First post here (Be gentle... it's going to be long ) ...

I've been doing some searching and reading on here and I just cant seem to find the exact info I'm looking for...

SO,.. We have a 97 Vortec 5.7L (350) w/4l60e trans along with the complete ECU and harness(s) swapped into our 98 S10. (See avatar) We did not do the swap and this thing has all sorts of issues that I'm attempting to address. One of which is getting the Electric fan set up sorted out and wired up correctly.

My specific question is this: There are two Coolant/temp type sensors on this engine currently. I would like to know what the original and specific function and operation of each is?

1. One is at the front of the intake manifold next to the thermostat housing, and it is a two prong type connection.
2. The other is screwed into the drivers side (Left Front) Head between the 1st and 2nd exhaust port and has a one wire connection.

From my limited understanding, the one at the front of the intake communicates with the ECU and allows it to make adjustment to various things based on the temperature that the sensor is sending it. I'm not 100% sure on the one in the head as I originally thought that maybe that was for the Temp gauge in the dash cluster, but wouldn't that be supplied to the dash gauge by the ECU via the same sensor in the front of the intake?

Either way at the end of the day I'm looking to get this electric fan set up wired up "correctly" by using a high/Low On/Off temp sending unit that is say 210 on and 195 off(? - I've heard these engines need to get up to the 195 running/operating temp?) I would like to use a sensor/switch that is directly in the coolant flow and I'm leaning towards purchaseing one of those thermostat housings that has the threaded port in the top of it (?)

Also, I believe the current E. Fan we have (No idea what brand or specs or anything..) is wired into the AC switch. Not sure if they're using that as their Switched 12V source or what.

I am very knowledgeable when it comes to wiring thngs up via a relay and correct voltage sources and amperage ratings... along with everythying being fused and protected. .. Just need to get the correct plan of attack and implement it

Thanks in advance!
Rick

 

[HotWheelsBurban]

Yes the one in the intake manifold under the thermostat housing is the one that tells the PCM how to set and change the mixture. Having intermittent issues with this on my 99 5.7.
The one in the head runs the gauge.

 

[88monteSS]

The sensor on the intake is for the pcm, the one in the head feeds the gauge as you thought. It wasn't until the gmt800s that the pcm fed the temp info to the cluster.

It would require a tune, but that pcm does have the ability to run an electric fan as there was an optional auxiliary fan available. I'm not sure how much of a range for temp settings is available for it in the stock PCM though, I converted mine to an 0411 before playing with the fan settings.

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[mtl111]

I have a 99' 2 door Tahoe, the temperature guage sending unit is in the passenger side head on mine. I have a 16" Flex a Lite fan running off of a Hypertech fan switch that I mounted in the Driver's side head. Since you said yours is in the drivers side , check the passenger side for a round plug that has a square cut out in it. Mount your fan switch there. It requires a special 3/8" socket that has the correct sized square end to remove the plug. Can't remember what Snap On calls theirs or the exact size but don't try to remove it without the correct tool or you'll round the inside square of that plug out.

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[Rick Werner]

I have a 99' 2 door Tahoe, the temperature guage sending unit is in the passenger side head on mine. I have a 16" Flex a Lite fan running off of a Hypertech fan switch that I mounted in the Driver's side head. Since you said yours is in the drivers side , check the passenger side for a round plug that has a square cut out in it. Mount your fan switch there. It requires a special 3/8" socket that has the correct sized square end to remove the plug. Can't remember what Snap On calls theirs or the exact size but don't try to remove it without the correct tool or you'll round the inside square of that plug out.

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I think that would definitely be the "easy button", but I think the consensus is that the temp in the threaded head port to the water jacket would be much hotter than in the upper intake and would cause the fans to come on sooner and run longer potentially not letting the engine get up to operating temperature? ... Which it seems weird to me why they would have the Gauge sending unit in one of those ports unless it is calilbrated to account for the hotter temp it is reading? .... Or I could just be overthinking this whole thing

Thanks for all of the input thus far as it confirms that I'm somewhat on the right track

 

[mtl111]

The Hypertech fan switch I used is actually for older Camaro's and Firebirds, in the cylinder head is where GM originally had those fan switchs mounted. Its the same thread pitch as our 5.7 Vortech's. I have mine set up to kick on at around 210 and it also kicks on and stays running when the AC compressor is on. Trucks runs cool, AC blows colder air since you have air moving across the condenser even when sitting still. Cooling fan switch's should be in the cylinder head, not the intake.

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[east302]

I think that would definitely be the "easy button", but I think the consensus is that the temp in the threaded head port to the water jacket would be much hotter than in the upper intake and would cause the fans to come on sooner and run longer potentially not letting the engine get up to operating temperature? ... Which it seems weird to me why they would have the Gauge sending unit in one of those ports unless it is calilbrated to account for the hotter temp it is reading? .... Or I could just be overthinking this whole thing

Thanks for all of the input thus far as it confirms that I'm somewhat on the right track

Any problem there was remedied by using a dash gauge that displayed only vague levels of accuracy.

Resistance values...

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[Erik the Awful]

Your AC switch is likely wired to your fan so that it comes on whenever your AC is on. In other words you should have two power sources, the AC button and the temperature switch you're trying to install.

 

[Rick Werner]

Thanks again guys! Some good info here to chew on and get a good game plan moving forward! We will be pulling it in the garage next week and starting on it. Need to change out the intake gaskets and probably going to change the water pump as well... then dive into the wiring and get that all sorted out and straightened up!