I am called to Mr. D's shop to look at a 1997 Dodge Stratus 2.4 liter with an overheat condition at an idle in traffic conditions. The car has 182,374 miles on it so the sky is the limit. Given the fact that it only seems to overheat at a standstill makes me really want to look at the cooling fans and the cooling fan circuitry. I hook up my scanner and check for codes. No codes are present. The reason I check for codes is for circuit integrity. If there was something open or grounded in the control circuitry it should set a code for that particular circuit. In this case we have low speed and high speed fan circuits. These fan circuit codes don't always trigger the MIL lamp and I never take anything for granted. Next thing is I command the fans on low speed and high speed with my scanner. I hear the fans come on so I know something is happening. With that being said. I start the car and select from my scanner the Engine Coolant Temperature Sensor pid along with Time From Start, Low Speed Fan Command, and High Speed Fan Command. When I start the car the temp is around 85F. I use time from start all the time for coolant related issues. It is very useful for looking for thermostat issues, etc. As I let the car warm up I graph my pids. I am also listening to the banter at Mr. D's shop. It is one of those fun shops. You never know when you are going to get sprayed with carb spray and then lit on fire. Or, if you drop an attached caliper from the control arm while doing a brake job Mr. D will yell out "should I order a brake hose now?" Meanwhile, he is all the way across the shop at his desk and you ask yourself how did he know? Yeah, it keeps you on your toes but at the same time it keeps things light. Shop life at it's best.
Alright back to the overheat. About 12 minutes later the temp is creeping up to low speed fan turn on temperature which is about 221F. I see the low speed fan command flip to "on" on my scanner and I hear the fans come on. The temperature however instead of dropping stays right around turn on temperature. I look at my fans and only the drivers side fan was on. The passenger fan is dead. Huh? I know from experience that these fans run at the same time. I smack the offending fan with a backend of a screwdriver and it comes to life. Ok, we are on to something. I am still worried about the quick time to get up to operating temperature.
It was a little quick for me. I shut off the vehicle and check the coolant level. Well a gallon of coolant later and we are filled and I am back monitoring. Now, both fans are coming on and the temperature is behaving. Was I seeing things with that fan not coming on? At this point I can look at the fan motor's integrity through my low amp probe. I have two pretty much identical motors so I can compare.