Venture overheating

I have a Chevy Venture that has been overheating for a couple of months now.

Normally it happens sitting in traffic - temp gauge touches into the red. Once we get going it cools off (naturally).

I noticed that if the AC is on, the vehicle doesn't get as hot.

There are 3 relays that control the fans. When the vehicle is started, both fans turn on at slow speed for about a minute, then shut off. If I let the engine idle, the fans will turn on again when the gauge reaches about the 3/4 mark (on low speed).

Just for laughs, I jumpered past relay 10 in the "Fuse Relay Center" under the hood, and the fans switched on at high speed - sounds like an airplane taking off! I suspect that a relay or two are faulty ... Has anyone else seen this problem? I don't have a wiring diagram for the vehicle, so troubleshooting is difficult. There are 3 relays marked COOL FAN ... 9=COOL FAN, 10=COOL FAN 2, 12=COOL FAN 1. The fuses are okay.

Another note: the cooling system is full with Dex-Cool, a little gunk but nothing bad. Temperature runs around the 1/4 mark normally and sometimes gets up to 1/2 way when it's working correctly.

Any ideas? Is there somewhere I can download a wiring diagram for just the radiator fans???

Thanks for any help,
Bruce C. Headley

pick up a 16 dollar haynes manual, it will have the wiring diagram and a test procedure for the relays. If you have any gunk in your radiator at all, i would suspect that first. If your coolant passage ways are clogged or partially blocked, it would cause this sort of behavior. Get the coolant system cleaned and flushed professionally, and if you ever have to "top off" your coolant, suspect an intake gasket leak and get it looked at ASAP!! coolant will eat away at the aluminum parts in the engine and eventually lead to failure or broken camshaft. Get it looked at right quick!
There is a chance it could be a relay, but overheating (especially if its in the red on the gauge) is VERY bad in an aluminum block engine. These parts warp and cause damage to gaskets and other internal parts. I would avoid driving it all together until you get this resolved. Also, just for giggles, with the engine running and the hood open (make sure you are at normal operating temp, or hotter) feel the upper radiator hose. If its cold, your thermostat is stuck closed and needs to be replaced. If its hot (as it should be) suspect something else as mentioned above.
Good luck!

Change your temperature sensor. 100% that is the problem just did the same thing. The part is about $20.00 canadian. Grab a roll of teflawn tape as well. The whole procedure takes about 15 minutes. The sensor is located near the top right of the block. Removing the air filter connection helps you get at the sensor. Good Luck...

I am guessing you did not follow the Haynes manual procedure then for replacing the temperature sensor ? They recommend draining before removing.

Did you just do this when the engine was cold ? unscrew and screw in the new one ?

You can just remove the old sensor and throw the new one in, leaking just a bit of fluid. No problem.

don't go crazy with the teflon tape, as that sensor needs to be grounded to the block. Too much tape will cause it to not function. you don't need to drain the cooling system. Have teh new sensor ready to go, and quickly pull the old one out and put the new one in. You shouldn't lose much more than a cup or so of coolant.

With A/C on the fan comes on automatically, cooling the system. Change the sensor. Worked for me.
The new sensor comes with sealant pre-installed on threads from the dealer. No tape needed.