One night I noticed the stop engine temp light barely on. The car hadn't run all day. I started it it ran fine shut it off and the light stayed on. I pulled the fuse and left it until I had time to mess with it. I put the fuse in, the light was off. Ran the car it was ok, no light. Several times I ran the car and then one day while letting it idle in the driveway the light came on full brightness. I wound up changing the thermostat, figured I've never changed it might be a good idea. The light comes on after a few minutes. I don't see any leaks coming from the water pump. It appears to have good circulation inside. My next step is going to be a temp gage to see exactly the engine temp. I feel like its a gremlin somewhere and don't want to just change parts. The engine is a 368 carbed motor, in a 84 commercial chassis, Hearse last year they put the motor in a caddy.
If it is the set-up that I have seen there is a temp switch that is on the drivers side head , way back close to the firewall. It has only one wire going to it . It is close to the last spark plug. I would just pull the wire off and if you feel you got to have a new one I think rock auto had them a while back.
FIrst off, it sounds like the sending unit is wired to the battery side of the car's
electrics… or, some back feeding…. either way, it shouldn't be that way.
Secondly, in 40 years of old car adventures I have regularly been freaked out by
bad gauges, bad sending units, bad wiring to the those sending units, bad grounds,
I highly recommend that anyone into old cars BUY and USE a good quality infra-red
thermometer with the laser pointer. About $50 these days.
They allow you take the actual temperature of any engine part. I have recently verified that my
temp gauge in my 49 Willys is off by about 20-25 degrees on a hot day after the car is thoroughly
warmed up. So, it reads 210-220 when the motor is actually at 190 ish. I even take the radiator cap off and measure the
surface temp of the coolant, & down the top and sides of the radiator, the intake manifold,
thermostat housing, cylinder head, etc.
I take the gizmo with me on every old car outing …. it's a great comfort when
all you have to rely on is an iffy gauge reading & you can't take your eyes off it at stop lights!
my 2 cents.