Overtemperature inline resistor

[18Fiatsandcounting]

On some Fiats, there are two temperature sending units in the cylinder head, one for the gauge itself and one that is just a grounding switch that sends the gauge full scale if the temperature goes over a certain set point (230 or 250 oF if I recall). In the wiring for the switch sensor, there is a blue inline fuse holder, and there should be a special resistor there. My inline fuse holder in my '71 spider has nothing in it (but of course).

Using my resistor substitution box, 33 ohms seems to be about right for this resistor and sends the gauge softly to the top of full scale when the sensor wire (green/black) is grounded. Two questions: 1) does 33 ohms sound about right? 2) does anyone know where I can even get such a resistor? I could make one up from my resistor stash, but original might be kinda cool if I could even find one.

Thanks!
-Bryan

[18Fiatsandcounting]

I'm still interested in your thoughts, but in the meantime, being kinda impatient, here's what I did: I took a scrap (blown) standard plastic Fiat fuse that still had the copper end caps on it, and soldered a 33 ohm resistor across them. I stuck this puppy in the blue inline fuse holder and yes sir Bob, the temperature gauge softly rises to full scale when the green/black wire to the overtemperature switch on the cylinder head is shorted to ground. I guess the only real way to test this whole setup is to let the engine really get overheated, which I'm not willing to do. So, for now, consider this as another item that's fixed.

But, if anyone knows of a source for the actual resistor component, I'd love to hear it. I've not found any vendors that even mention it, let alone sell a replacement.

-Bryan

[manoa matt]

I will look, I might have one. I know I have two of the holders, saw them the other day, did not open up and look inside though.

Edit: Update, each holder contained a resistor. One for you, a spare for me. Send me an email matt.scarton@gmail.com

[TeeCee]

Does the 82 have this resistor, I see what appears to be one on the schematic but cant locate it.

[18Fiatsandcounting]

TeeCee, I'm gonna guess that the locations for this resistor are different on the later model years, but if it's any help, on my '71 spider, this inline resistor is in a sky blue fuse holder (about 3 inches long) down in the driver's footwell against the firewall, near where the steering column goes through the firewall. It is just tucked in amongst the other wiring in that general area.

-Bryan

[SanJoseSteve]

A perfect example of "Curiosity Killed the Cat" was when I saw that fuse holder in my 72 and thought; "That's a funny looking fuse holder.....I wonder whats in it?" I opened it up and took the wire wound fuse out. It looked to me like it -had- been encapsulated in some kind of wax or glue to hold it all together when new. I took it out and all the copper wire immediately sprung off like a fishing reel full of cheap line.

I had to carefully wind it all back on. I was telling the guys at work about it on Monday and they thought it no big deal to just measure the resistance and wind me a new one (I worked for implantable defibrillator manufacturer at the time.) on our coil winding machine. I said since it was working, I never wanted to open that fuse holder again. Now I wish I had taken them up on it and made a dozen or so. Hahahaha!

[TeeCee]

You guys are awesome! I now have a working original temp gauge.
I disconnected the over temp sensor, replaced the gauge temp sensor and wahala

[Kenneth]

Hi spider-friends,
my 1974 EU spider is missing the in-line resistor - but the blue "holder" is there.
Do any of you know what size (Ohm) it has and possibly more technical information?
A picture would also be nice

[18Fiatsandcounting]

Kenneth, it's a wire wound resistor of somewhere around 30 ohms (going by memory). I don't think the resistance needs to be exact, and it's just there so that the gauge needle doesn't get damaged as it "pegs" when the overtemp switch closes. A 33 ohm resistor is going to be the closest commonly available value.

The only reason that I can think that Fiat used a wirewound resistor is heat dissipation. My recollection is that the gauge draws about 0.2 amps at full scale, which would be 2.4 watts. That's a lot for a regular carbon resistor, but power resistors are commonly available to handle 5 watts or more.

By the way, the temperature sensor (usually towards the rear of the cylinder head) has a green wire that leads to the gauge, and the overtemp switch (usually towards the front of the cylinder head) has a green/black wire that leads to the resistor, and then the other end of resistor connects up with the green wire from the temp sensor to the gauge.

-Bryan

[Kenneth]

Hi Bryan,

thanks for the help, for quick and good answer - you are a star
I'll order a resistor that is at least 5 watts and builds into the system.

BR

Kenneth

[18Fiatsandcounting]

5 watts will be plenty sufficient, although I may have lied a bit. After I posted my reply, I went into ye olde Fiat shop and set up a circuit with a spare temp gauge. 33 ohms works just fine, but the power across the resistor was less. So, a one watt resistor would probably work just fine, as will 5 watts.

-Bryan