Temp sender GM: AC-257M

**Lynn** · #1 06-25-2016, 04:01 AM

Came across a temp sender, single peg (not a spade type) stamped AC-257M.

Can't find any info. Gauge or light?

**SS69chevelle** · #2 06-25-2016, 04:56 AM

Not sure if it is the same but I have an original 1969 sending unit with AC-248MX and it is a single post connector, pretty sure it is for lights vs gauges. hope that helps, take care Andy

**Lynn** · #3 06-25-2016, 01:35 PM

It has the single peg and even has the connector with it. I don't believe these are being reproduced.

I also don't know if the vehicle had lights only or if there were gauges.

Been a long time (1972) since I owned a 69 Z that had gauges, and I don't remember if it also had a light.

Did the 69 and early 70 cars that came with gauges, also have lights? Did they use two different senders?

**SS69chevelle** · #4 06-25-2016, 04:50 PM

I know the oil pressure unit is different for gauges vs lights, not sure about the sending unit for water temp, if i recall correctly the gauge unit is retained with a nut vs a plug-spade or pole, im sure someone with more info will help out. Also i checked my parts manual it shows one for exc gauges but it isnt clear on the descriptions. Im sure there are two based on the manual. Take care

**Lynn** · #5 06-25-2016, 07:31 PM

Problem with the parts books are the GM number isn't the same as the AC number stamped on the part.

I am thinking this is for a light. It even has the correct connector terminal. I looked forever finding one of those for my 69 Z a few years back.

**SS69chevelle** · #6 06-26-2016, 04:58 AM

Understand, my manual gives the part number and identifies some of the stamped numbers but none like yours or even the one I have. Mine looks the same as that one except for the stamped number. If you find any definitive info I'd like to know the result, it is interesting to me. perhaps someone may have an old AC manual that has that number? take care and good luck

m22mike · #7 06-26-2016, 12:22 PM

The single pin or spade brass AC units are not senders, they are a simple NO switch. The # stamped on the is probaly the trip point where the switch closes and puts the ground on the idiot light. I would guess the internal contact is a bi metallic design.
The threaded post units are varistors, and send a variable resistance to a gage.
If you have VOM play with them. If you want to check the switch, heat the base in light oil.

Mike

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