Trinten's SBC/F23 build - The work has begun! (Page 76/76)
fieroguru AUG 25, 06:54 AM

quote
Originally posted by Trinten:
Okay, so his sensor might have worked because - if I understand things right, it doesn't need the ground except to kick on the warning light?



Both the sensor wire and the switch ground through the body of the sensor. So no ground wire is needed. The 2 wire sensor has 2 signal wires and 0 ground wires.


quote
Originally posted by Trinten:
The FAST temp sensor we have in the head is kind of a pain to get to, and having a third point to see temp / temp differences at various points is not a bad thing.
If we go with the Camaro unit, which pin is the correct one for the gauge? (thank you for the link to the right part!!)



Pin C is the analog signal wire for the temp gauge.



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Originally posted by Trinten:
For the speedometer, I have the FAST software set to do an output at 4k PPM for the gauge. What is the function of the circuit?



It changes the type of the output signal to what the Fiero speedo needs to see. The ECM will create the 4000 ppm output, but the shape of the signwave will likely be incorrect and not work with the fiero speedo. This buffer circuit changes the shape of the output signal so it works with the fiero speedo. Every OBD2 GM ecm needs this circuit as well as the Haltech Rebel LS, so odds are your FAST unit will need it too.

[This message has been edited by fieroguru (edited 08-25-2025).]

Trinten AUG 25, 08:19 PM
Got it!

So I have learned a disturbing amount on how coolant sensors work this week. For those that don't know.

1. There is the "GM Curve", where 0 ohms is hot, and 90 ohms is cold. Odd, but okay.

2. There are sensors made specifically for "short sweep" - which is the 90ish degree arm sweep of our coolant gauges (compared to 270 degrees, the other most common). One must make sure their sensor is for the right 'sweep'.

3. This one was a dead-stop... oh hell that make sense moment... even if you buy a sensor with the right 'curve' and 'sweep', the sensitivity might differ based on the max temp your gauge shows! So a sensor meant for a gauge that goes up to 300 (for example) will not show a correct value when feeding a gauge that goes up to 220!

It's wild stuff. Unfortunately it seems the LS1 sensor that FIeroGuru recommended has a thread of M12-1.5, and the opening currently on my water manifold is 3/8" NPT. This is likely why Mike grabbed a GN sensor, because the stock Fiero sensor is 5/8"-18 UNF (and yes, I did buy, and find, my stock coolant sensor and PICO connector).

I don't recall how much space is on the water manifold on each side of the existing port to know if we can drill/tap it open to take the LS1 coolant sensor, so in the meantime I'm trying to find another sensor that checks all of the 3 points above. It is not as easy as it you'd think.

I did find out that the speedo output sender is using a "Square wave" signal. So if that is the wrong type, I will need to build that buffer, as Red Devil shows it on back order.