95 Lumina 3.1 bad temperature gauge?

Does anyone know if the coolant temperature sending unit on a 95 Lumina is used for both the dash gauge and the computer (to control fans), or just the computer?

I replaced the head gaskets, it has a new radiator, thermostat, coolant temp sending unit. When I first started the car up after this, after 3 minutes the temp gauge was starting go near red, but the fan didn't kick on and the thermostat didn't open. I thought this was strange until I got an infra-red thermometer and checked the thermostat housing temp... it was not even near overheating. I let the car warm up more (gauge in the red by this time), and sure enough the thermostat opened and the IR thermometer was reading about 190degF.

Before the rebuild the gauge was always seemed funky, but not enough to suspect it. Should I assume the new sending unit is bad, or look at replacing the instrument cluster? I was thinking if it was the sending unit reading high, surely the fan would have come on? I'm pretty sure the fan works although I haven't tested it since the rebuild. The only other thought I had was that the gauge on the dash may be controlled by a different sending unit (not the one in the thermostat housing), and if so I don't know where it is.

This was my first ever repair work (never even did an oil change before this), and I was really impressed that it started first time. You can do a lot with a Haynes manual, some borrowed tools and a great resource like this site.

Any help appreciated.

The exact same scenario happened to me! My headgasket was also broken after severe overheating. Everything is fixed now except the temp gauge. It shows around 250F I believe the gauge is wrong since my car runs fine, I actually took it to a shop and have it confirmed that the car is not running hot. They said it's running normal and not hot enough for the fan to turn on, I assume it to be around 200F since the fan kicks on at 212F. I thought of getting an aftermarket 2" temp gauge but since you have to drill another hole for the temp sender and the lack of space in the instrument cluster I got another idea. The shop told me that fixing the gauge would cost around $300, so I'm thinking of getting a cluster from the junkyard and to solder the temp gauge from the junkyard gauge into my cluster if possible.

A junkyard cluster seemed to be the best choice for me too. I was thinking of just replacing the whole thing. I read in 60degreeV6.com that W body cars (of which the Lumina is one) usually have a separate sender for the temp gauge. I suspect that the Lumina doesn't because I couldn't find one, but I was hoping someone could confirm it. I don't want to be drilling holes in stuff either. It wracked my nerves taking the engine apart and putting it back together, and I could do without the extra stress!

I was hoping I could find an electric gauge that just happened to be calibrated to the same resistance range as the GM sender, but I haven't had any luck yet.

Thanks for your reply.

Jim

I've checked my Haynes manual and there is another temp sensor but for the automatic transmission. I really need to check the wiring diagram again to see where the gauge is connected to.

A small note of interest on junkyard clusters, I bought a 2000 Lumina cluster with the tach on it (because I wanted one) and everything works perfectly except the temp guage is slightly below what the original cluster read. So it may just be something that varies car to car, or maybe the PCM. Or who knows :p

There are two temp sending units on the 3.1. One is right by the thermostat in front, the other sticks out of the corner of the rear head under the EGR valve. The guage sender has a long green wire coming out of it which is pretty easy to get at. An ohmmeter to ground should tell you if it is reading the right resistence for the temp or not. I can't remember the specs, but below approx. 150 ohms is way to high. The hotter the engine gets, the lower the resistence goes. I have seen these senders go bad from sitting dry, a crust forms on them.

Great stuff, at least I can go looking for it now. Thanks for your reply.

Great stuff, at least I can go looking for it now. Thanks for your reply.

Any resolution to this issue?

Kinda. I put it down to it not being fully bled of air when I first started it. The bleeding thing is really a pain and does take longer than it seems it should. The thermostat housing wasn't getting hot because it simply wasn't open, so in actual fact it probably was overheating for a little while, but once bled of air (using alternating bleed screws) everything went back to normal, and I haven't had a problem with it since (18 months). Well, apart from an exploding water pump, but that was easy to diagnose and replace since it was in pieces when I popped the hood.

Jim

Kinda. I put it down to it not being fully bled of air when I first started it. The bleeding thing is really a pain and does take longer than it seems it should. The thermostat housing wasn't getting hot because it simply wasn't open, so in actual fact it probably was overheating for a little while, but once bled of air (using alternating bleed screws) everything went back to normal, and I haven't had a problem with it since (18 months). Well, apart from an exploding water pump, but that was easy to diagnose and replace since it was in pieces when I popped the hood.

Jim

Wow, so the bleeding fixed it, eh? I'll have to do that next - I have somewhat-similar symptoms on my '96 3100 to what you have, and others have had and I'm betting it's that or the coolant sensor(s).

Was there an appreciable coolant loss in the bleeding process? I don't want to make a mess...

Bummer about the water pump! The 3100s have yet to cease to amaze me. :P

has anyone heard of a bad water pump somehow creating air pockets - or working intermittently?

i know this is late but,i have a 92. 3.1 .same temp problems,changed everything that had to do with high temp.except one.temp guage.i replaced it with a mechanical guage i bought at advanced auto parts.screwed in the sensor were the original was.temp stays at 195 deg on new guage.i plan to mount the new guage on bottom left side of dash just as soon as possible.for now it stays under the hood.

The only way a bad water pump can allow air into the system is through the weephole, or a hole in the casting. The weephole will almost always leak outside and if bad enough to allow air into the system, will start making a mess under the hood. A water pump is directly driven by the serp belt, so it cannot work intermittently.

hmmmmmmmmmmm

this is making me think...
I've just gotten the waterpump and thermostat replaced in mine, and as expected, it's a 3.1

mech says that the signal isint processing. The relay to kick on the fans is operational, but the signal to trigger the relay itself isint working.
Total cost so far: about 200 bucks. I dont want it to get any more expensive than this...

A little trick for helping to bleed the cooling system. When you install the t-stat, drill a 1/8" hole in the flange of the t-stat. IF your stat mounts sideways, make sure you install it with the hole at 12 o'clock. If it mounts flat, it doesn't matter. This hole will give the air a way to escape when the t-stat is closed, making bleeding much easier. It will have no effect once the t-stat opens.