I've noticed on my 4.9 that there seems to be a coolant temp senser on the corner of the trunk side head.
It's kind of above and between the #8 spark plug and exhaust port. (Trunk side, nearest the tranny.)
Nothing was plugged into it. The normal coolant temp sensor on the intake manifold is the one that is being used.
In addition, this sensor appears to be "dead". Open circuit. No resistance reading at all.

Neither the Deville or the Allante manuals mention a sensor in this location.
I'm doing an Allante intake swap, so it would be really useful if I could install a working sensor there.

So what is it? Is this a useable location for a sensor? Even if it's just a gauge sensor, that would work for me.
(Of course, I suppose I could unscrew it and see if coolant comes out.)

Anyone?

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Raydar
88 4.9 Formula IMSA Fasback..........................88 3.4 coupe -soon to be something other than red

Read Nealz Nuze! Praise the Lowered!

[This message has been edited by Raydar (edited 09-20-2009).]

It a switch that runs an idiot light for cylinder head temperature.

This is from Mickey_Moose 4.9 wiring thread https://www.fiero.nl/forum/Forum2/HTML/098096.html

"NOTE: Some 4.9's have a temp switch in the side of the head:
1) if you car does NOT have a temp gage and just an idiot light you can leave the stock 4.9 sensor in the thermostat housing - wire this to the ECM, the one in the head gets wired to the idiot temp light on the dash.
2) if you have a temp gage in your car you change out the sensor to the above info and wire, the head temp switch can be thrown out (or used for an additional light that you have added to the dash)."

head temperature, not coolant - switch closes when hot (overheat)

How many wires does it have? Can you post a pic? I've been trying to see what they look like for a while. I'd like to hook one up in my own car.

I found that a head temp sensor from a 4.1 caddy will screw into the 4.9 head and is a single wire grounding switch. This is perfect for the fiero install.

if you are talking about this sender:




They call it "Engine Metal Temperature Switch" in my Haynes. I will use it for the "Temp Idiot Light".

Great info. Thanks!
The "switch" in my 4.9 head looks just like the one pictured above, which also looks like the coolant temp sender on the intake manifold.
Same style, same shape. Same connector. Two wires, etc., etc.
Does it go into the water jacket? Or is it just screwed into a blind hole on the head?

[This message has been edited by Raydar (edited 09-21-2009).]

It is in a blind hole. That is why I dont think I will able to use it for something else than a¸n idiot light.

Okay. Great info.
Thanks guys!

Hmmm. Two wire.

Well thats not going to work. Only need a single wire grounding switch to run the idiot light. I'm going to go with the one from the 4.1. I'll post the part number if anyone is interested.

According to Haynes, 1 wire is a ground the other for a Bulb.

I dont see why it can not be used with the Fiero temp lite.

I didnt even know that I had one on my 4.9 till I read this thread. I will wire it in while I am building my harness. I will ground the second wire.

This is a great place to spend time

[This message has been edited by josef644 (edited 09-24-2009).]

PFF is a great place to spend time.

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"I'm not driving too fast, . . . I'm just flying too low."

4.9 ongoing project

quote Originally posted by falcon_ca: According to Haynes, 1 wire is a ground the other for a Bulb. I dont see why it can not be used with the Fiero temp lite.

Why a ground wire? Doesn't the sensor ground through the head already? I had just figured it was for something else. No reason it wont work either way. Of course its in the Haynes manual. I couldn't find it in the FSM.

quote Originally posted by Fieroseverywhere: Why a ground wire? Doesn't the sensor ground through the head already? I had just figured it was for something else. No reason it wont work either way. Of course its in the Haynes manual. I couldn't find it in the FSM.

I would think the lead is hot from the bulb to the back. When the switch closes it offers the grounded lead to the circut and the 'Hot' lite would come on.

quote Originally posted by Fieroseverywhere: Why a ground wire? Doesn't the sensor ground through the head already? I had just figured it was for something else. No reason it wont work either way. Of course its in the Haynes manual. I couldn't find it in the FSM.

Depends how the switch was built - maybe GM reasoned that it needed some sort of isolation from the head or something...

quote Originally posted by Mickey_Moose: Depends how the switch was built - maybe GM reasoned that it needed some sort of isolation from the head or something...

Makes sense. Seems odd though considering the early 4.1's only had a single wire switch. Maybe they figured something out along the way. I can't think of any reason why it would actually need a seperate ground.

quote Originally posted by josef644: I would think the lead is hot from the bulb to the back. When the switch closes it offers the grounded lead to the circut and the 'Hot' lite would come on.

I have already located an extra connector that fit the 'Hot' head temp switch. I am gonna wire it up with one side grounded, and the other to the 'Hot' lite feed. we'll see how it works out after start up

quote Originally posted by josef644: I would think the lead is hot from the bulb to the back. When the switch closes it offers the grounded lead to the circut and the 'Hot' lite would come on.

According to Haynes diagram, you are right. The wire is "dead" until the switch gives it ground. Other wire of the bulb is hot.

I found on a Cadillac forum that the switch closed at 380 degrees Farenheit (160 Celcius) which is very high for the stock Fiero engine temp gauge, but seems not to be so high for a 4.9 Cadillac V8.

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"I'm not driving too fast, . . . I'm just flying too low."

4.9 ongoing project

quote Originally posted by falcon_ca: I found on a Cadillac forum that the switch closed at 380 degrees Farenheit (160 Celcius) which is very high for the stock Fiero engine temp gauge, but seems not to be so high for a 4.9 Cadillac V8.

Keep in mind that the switch is bolted into metal, about an inch from the exhaust port. It never touches the coolant.
I would suspect that it gets much hotter than the coolant.