revdarkwing

2001 S 500
107K Miles
Symptom, normally reads between 80 and 100 depending upon outside temp and driving conditions. when not at freeway speed, can move from the 100 to beyond 120 within a minute. Once I pick up speed, it often begins to go back down to normal area almost as quickly. At the same time I notice my ambient temp goes from 90 to 130 in less than a minute. I just discovered that the ambient temp sensor was out of the grill and laying on the splash shield so will be replacing that tomorrow. The two gauges do seem to move at or near the same time.
The car does not blow through coolant like it might with a head gasket problem and runs and sounds fine even when near the red zone.
I am about to order a new sensor but am worried it might be something else.. like a bubble or something else. Any ideas? I think the sensor is bad but have to trust it for safety sake.
Thanks

tusabes

Do you get a coolant warning message in the center cluster display ?

usually it’s a bad fan controller causing these types of temperature jumps but try the sensors first

revdarkwing

I only got that coolant too hot warning when the temp started to go into the red so I pulled over and turned off the engine and waited about 20 minutes till it cooled down then continued on my way. It was not low on fluid. It was the heat warning.
Last year, I replaced the radiator, and the fan ($1400) as well as the control unit. Before I started on my way, I started the engine and the fan went on immediately. It does not run when the engine is off but the ignition is on however.

wallyp

"...can move from the 100 to beyond 120 within a minute...."
"At the same time I notice my ambient temp goes from 90 to 130 in less than a minute."

That would make me suspect an electrical or electronic fault that affected both gauges. The overheat warning is troublesome, though.

"I started the engine and the fan went on immediately."

I think that operation is driven by the HVAC (air conditioning) system.

revdarkwing

Well, I checked for a head gasket leak testing for exhaust gas in coolant.. THANK GOD.. that was negative. Ran it on idle and waited for the fan to turn on.. With AC on, it's always on.. and with AC off, it was off too until about 95 degrees... so I think I'm good with the fan and the control.
while doing the test, it stayed at about 95 with fan running but went n o higher. So, now I know I've got no blown head gasket.. new radiator and control unit, new fan. Not sure where to go next other than the possibility that for some reason the ambient temperature and the coolant temperature sensors are connected in some way. I can see how the ambient might jump up given it was laying on the splash guard and wind currents could have blown hot air from the radiator onto it... So, where to look now?
PS: I really appreciate the help.

redaotrosh

I was facing high temperature(97C) before so my old radiator was clogged and the fan is faulty so i have replaced them and the temp cool down between 80 to 89C even driving hill and traffic, so if you have replaced them as you said, you need to check the coolant sensor and the thermostat if it is functioning properly.