coolant temp sensor questions 3500-7730-fiero

[3100 MPFI]

In my 1990 STE, it read the same as the head one. As for my 3100 in my 1988 Cutlass Supreme, it was a custom build (and I never got to run the engine before I did the work too it), but after almost 5 years, it was still consistent.

In my 1990 3.1 MPFI, there was no variance. When I first did the swap, the car was overheating, and tests were showing that the original sensor was bad. I didn't have cash at the time, but I had a spare 3 wires sensor, so I wired it in. The car still read overheat.

To make a long story short (too late) after I fixed 2 WARPED HEADS, AND BLOW HEADGASKETS, the gauge then read normal.

[Superdave]

To those that went from the temp sender in the head for the gauge to the 3 wire, did it read differently? And if so does it read cooler or hotter than it used to?

Much cooler, it actually reads propper.

with the 3500 sensor mine would read 240-260 while in traffic, with the 3 wire from my old 3400 it would read around 180-190 in the same conditions.

I have one of every coolant temp sensor for these engines sitting in the garage, maybe i'll go take a multimeter to them later.

[merlot566jka]

ive been looking at this backwards from everyones setup, mines in a fiero.... the rear head to me is your front head...

that being said, my set up should work as long as i add a cts on the hot side of the coolant (trans side)

[IsaacHayes]

Dave, if it reads cooler after I do the 3-wire I'm in trouble. It reads below 1/4 mark when up to temp normally... hahahaha But I get what you're saying, since you used the 3500 sender and that one isn't compatible it seems with the older ECM and it reads high. My question was regarding STOCK single wire in head vs 3 wire in thermostat housing. I will go to the 3-wire when I swap my throttle bodies at the same time. My gauge sender doesn't register anything half of the time now.

I still can't figure out where this loose 3 wire sender came from on the workbench. It's not from any of my engines. And the only other car's it could have been from is an 88 subaru, a 95 nissan maxima, or a 86 2.5l grand am, but I looked that up and it uses a 2 wire.... I will assume it was a bad one and replaced and use the spare one that came in my 97 3400 manifold.... I will have an ALDL cable by then and can scan to make sure it's reading proper too and isn't sending a wrong signal to the ECM as you never know with old parts.


merlot: I would not hook up the 3500's temp sender since Dave has pointed out it reads higher than it should and that will cause tuning issues if you hook it to the ECM. Since it reads different I bet that is why they changed the thread size so people didn't mix them up. I would leave it in the head unplugged (or take it out and plug it) and then run a 3 wire in the thermostat housing just drill and tap a spot like the 3400 and 3100s have there.

[merlot566jka]

yah well im an idiot and didnt look at the theards on the 3500 sensor and bought a bunch of hardware parts to ri it up...only to discover the threads are different. could i tap the head or is that a no-no? im going to use the two sensors that i had on the last motor, instaed of trying a different one. i just need a place for both sensors. i got a tap that fits both the old sensors, its a 3/8 18npt tap. its pipe theads, in case anyone was wondering.

would a 't' fitting where the 3400s had thier coolant temp sensor work? on both sides of the t i would have both of my cts', would coolant flow into my t fitting?

[IsaacHayes]

Whoa, thats silly to run 2 sensors where the 3400 one is. Just get the 3 wire and put it there. If you're taping for a T there might as well screw in the 3-wire sensor and do it right so it's in the coolant. The connector is the same as the TPS and we already showed what pins go where. Just drill and tap there and you're done. No need to make it over complicated

[3100 MPFI]

I went and dug up some pics for you. This might help you some.

There 2 pics are the 3 wire sensor installed in my 1990 Grand Prix STE with the 3.1 MPFI. It is kinda hard to see, but in the first pic, that brass thing is the sensor, and in the second pic, the GREEN, YELLOW, and BLACK wires that are taped up, those are the wires for it. All I did was cut the green wire from the rear head, re run it, repin the connector, and then solder in the one green wire.




This is the stock 2 wire sensor compared to the 3 wire sensor (which is still in the lower intake manifold). The intake manifold is the stock 3100 SFI manifold from my 1996 Grand Prix SE. I did the intake manifold swap and had this one lying around still. The 2 wire sensor was the one I removed from the 3.1 MPFI. You can see that the threads are the same.




There is no need to "find something simple and complicate it". Leave that to the politicians.

[merlot566jka]

i used to work with nukes....i nuke everything. i tapped the head and am about to tap the coolant neck area, so both sensors have a happy home, almost exactly like the orig 2.8 that was in the fiero.

and for what its worth, my guage sender cts has 2 wires. it different from all others ive seen on the 660's it was only used on gt fieros. the rest had a 1 wire for the guage. thats why ive decided to use two sensors. i think ive got it now, and i thank everyone for thier responses

[IsaacHayes]

Cool as long as you don't use the 3500 sensor I think you'll be fine.