Automotive Forums  Where Car Enthusiasts Go to Discuss Cars Online

Hello,
I have a 2003 Montana that is starting to overheat.
I notice that the cooling fans are not coming on.
If I jump out the relays I can get the fans to turn on.
The engine temperature guage is about half way on the dash display.
If I am on the freeway, the guage sits at about 1/4 (just above the 40C mark)

Is it possible that the temp sensor is bad causing the PCM not to turn on the relays?
I tried swaping the two fan relays, but no success.

If the guage is working, wouldn't that indicate that the sensor is OK?

Any ideas would be much appriciated.

Douglas  

the ecm controls  the cooling fan relay...

it looks at the engine coolant temp sensor...   most gm's turn the fans on between 215F and 238F.. 101C and 114C depending on the programming..

i would look with a scan tool at the live data stream   (ECT) .. see if it matches the measurement done with a hand held temp gun measured next to the sensor...

it could also be a thermostat stuck open...  broken..   removed...   or it was a fail safe thermostat and it latched open during an overheat..


i do have one warning.. jumping the relay connector terminals can sometimes burn out the ecm... or at least that circuit in the ecm..  the transistor that controls the relays is usually rated at one amp...  

with better scan tools.. you can even get the cooling fans to cycle on for testing..


i am not sure about the montana.. but most earlier gm's had a separate sensor for dash board gauge operation..   but your montana is multiplexed...


infrared temp guns and pens start at about 20 bucks...  price them around..  they get real expensive... but you only need one for cooling system testing.. 240F / 275F max or so...


you can tell if the thermostat is open...  on a cold start..    take the radiator cap off.. within the first minute or so... see if the coolant is circulating through the radiator... it should take about 2 minutes before the coolant starts flowing through the radiator if the thermostat is working properly..   it stays closed until the temp comes up.. then it barely opens letting the coolant switch from the engine to the radiator.. then it closes... letting the coolant in the engine pick up heat... while the coolant in the radiator stops so the fans can take the heat out of it..  

Thank you so much for your detailed response!

I ran the van tonight with the rad cap off for an hour, and the fans eventually did come on. I really thought they would have come on sooner.

The top rad hose does get hot, but visually the circulation doesn't look like my old 97 GMC Sierra, (Why I let my wife talk me into a trade lol ).

The coolant does flow a ***

My problem started when we lost heat and I realized that the rad was almost empty, I filled it up and checked it carefully for the next week and it was fine, the rad cap was nearly impossible to get off, I had to use a set of plyers and carefully remove it.

I must have a small leak, cause I think I smell coolant a little bit when it is running, I was so sure that I hit the jackpot when I noticed the fan not running.

Hopefully it's not the intake or the heads, cause with the engine how it sits, I don't think you can replace the gaskets without removing the engine. I did a S-10 once in the middle of winter, my first real fix it job, and I was very lucky that I was able to complete it.

Do you think the top rad hose would get hot even if the water pump was toast or do you think this is just a leak issue since I have loss significant fluid after a couple of weeks?

Thank you again.  

i did not intend for you to run it for an hour with the cap off..
only to see if the coolant circulated cold.. that would indicate the thermostat is broken or stuck open...

the ecm won't turn on the cooling fans till the coolant reaches 215F...   the coolant circulating through the system at full speed.. it will take forever to warm up...  


(other things that a stuck open or removed thermostat will ***  the motor will stay at fast idle a long time.. usually about 1,000 rpms,  the heater won't work well.. as there is no pressure to force the coolant the long way through the heater hose...)

you need to find the leak..    i see it takes a conventional style radiator cap...

many of the autozones and some of the other chain stores have radiator pressure testers in their loan a tool section behind the counter...  this is where you pay for the special loan a  tool and return it for a full refund within a few days)

fix the leak... fix the leak... change the thermostat...  192F version..  the last digit of the part number will be a 9...