Temp gauge quandary (Page 1/2)
steve308 APR 16, 09:00 AM
My temp gauge sender finally rotted away and yesterday I replaed it with the gauge sensor TS168 and replacement plug S649. I cut and spliced the wires one at a time to hopefully insure I didn't reverse them. Gauge now does not respond and I have a ever so slighty glowing idiot light on in the instrument cluster. So, did I reverse the wires therefore causing the idiot light to confirm I am truely an idiot? Thanks in advance.
olejoedad APR 16, 10:29 AM
You are using the wrong sending unit - the unit you are using is for the ECU temperature input.

You need the gage sending unit (which uses the rectangular plug), which has two separate circuits - one for the gage and one for the HOT lamp.
steve308 APR 16, 12:39 PM


This is what came out and was replaced with the identical unit and a fresh pig tail.

Sorry for the massive picture.

[This message has been edited by steve308 (edited 04-16-2023).]

theogre APR 16, 02:11 PM
1 pin has some Ω to the case threaded into the engine. That's the gauge pin. (Doesn't matter actual Ω measured.)
other has nothing because thermal switch that's Open. that's the light pin.

GM FSM and other docs show a wiring problem and cause gauge slamming. GM never fix this. Not even a TSB covering this except maybe saying is normal.... Not. Isn't a "Safety" problems so didn't involved NHTSA recall.

So a Big problem is are several ways to "fix" gauge slamming starting the engine and can't trust GM doc's.
1. only make a change inside of car to the dash cutting/changing wire to I-switch on S-column.
2. switch wires on dash plug and at Sender.
3. switch wires on dash plug and C500.

to test light/gauge... ground the engine end of the wires.
Light wire to ground = Light On
Gauge to Ground = "Pegged" cold. To Nothing then gauge peg to above high i think.

[This message has been edited by theogre (edited 04-16-2023).]

donuteater306 APR 16, 10:33 PM
My guess is a bad sending unit. Simple test to see if it's your wiring: (tho I highly doubt it) unplug it and use two test wires to cross the connections.
ArthurPeale APR 17, 09:15 AM

quote
Originally posted by donuteater306:

My guess is a bad sending unit. Simple test to see if it's your wiring: (tho I highly doubt it)



ALL Fieros came from the factory with incorrect wiring. Even if this specific issue isn't what's causing the problem as described, correcting the wiring issue can only help.
olejoedad APR 17, 10:38 AM
That is the correct sensor.

The dim glow on the HOT lamp is likely due to the wires being reversed, as is the gage not responding.

The HOT lamp side would be an open circuit connected to the gage, and the variable resistance circuit is allowing enough of a ground to cause your HOT lamp to glow.

[This message has been edited by olejoedad (edited 04-17-2023).]

Vintage-Nut APR 17, 10:47 AM
I'm refurbishing my '88 GT and the incorrect GM Coolant Temperature Gauge wiring was on my 'fix list'!

I didn't save the link of the infamous "pegging" gauge problem/fix, but I saved a 'word' copy......

From the "Coolant Temperature Gauge Fix - TROUBLESHOOTING" section:

"If your temp gauge doesn't read correctly or the temp warning light doesn't function when you turn the ignition switch to the "Bulb Test" position, the first step is to test the sensor wiring."

"Pull the plug off the sensor and ground either of the two sensor wires to the engine (when the ignition switch is ON). This should cause either the temp warning light to come on or the temp gauge to peg. If this does not happen, then something is amiss in the wiring between the sensor and instrument panel."

"If you get the correct reaction when grounding the sensor wires at the engine but not when the wires are connected to the sensor, either the wiring in the sensor plug is bad or the sensor itself is bad."

"With good wiring and good battery voltage the gauge should peg when the sensor wire is grounded."

And yes, the Standard Ignition TS-168 sensor is also GM 25036809 sensor (coolant temperature gauge and warning switch) which is on the cylinder head near cylinder #5 just below the ignition coil on the V6.

I swapped the two wires on the sensor and the two wires at the instrument cluster so the 'Bulb Test' position applies 12 volts to the Coolant Temperature Warning Light and the Coolant Temperature Gauge works normally without "pegging" when the engine is started which is Awesome!

If you don't find the link in the internet, maybe I can send a copy if you want it,
VN
Vintage-Nut APR 17, 11:50 AM
P.S. - I forgot to mention,

Sensor HS125CM-12V relies on electrical contact with the engine as 'ground'.

IF you used Teflon tape around the sensor threads, perhaps your problem is “No Ground” with the engine…
VN
steve308 APR 17, 09:53 PM
Thanks to all for the suggestions. I hope to trouble shoot tomorrow afternoon and will post an up date as to what I find.