My truck only overheats when the a/c is on - 1998 GMC Jimmy

A bad tensioner can cause an engine to overheat in the sense that when a bad tensioner breaks, it will cause a loss of turning movement to all the accessories of the engine including the water pump, and when the water pump stops turning, the flow of coolant will stop and the engine will start to overheat.

For car running hot and overheating issues, I suggest to you go through this help links. Click this link below and go through the troubleshooting procedure to take care of the problems: ----Car runs hot? http://howtobyme.blogspot.com/2011/11/car-runs-hot-in-traffic.html
-----------All types of car overheating problems and troubleshooting?

http://technoanswers.blogspot.com/2011/11/all-types-of-car-overheating-problems.html
Car freeze overnight and started overheating while driving?http://technoanswers.blogspot.com/2012/01/car-freeze-overnight-and-started.html-----------Engine Overheats and heater blowing cold?

http://howtobyme.blogspot.com/2011/11/engine-overheats-and-heater-blowing.html

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Car overheats and Air blows hot at idle?
http://howtobyme.blogspot.com/2011/11/car-overheats-and-air-blows-hot-at-idle.html
--------overheating? http://schematicsdiagram.blogspot.com/2011/12/buick-century-overheating.html
----------Car engine Overheats and transmission starts slipping?http://schematicsdiagram.blogspot.com/2011/12/car-engine-overheats-and-transmission.html
----------This details will help.Thanks.

For car running hot and overheating issues, I suggest to you go through this help links.

Click this link below and go through the troubleshooting procedure to take care of the problems: ----

Car runs hot?

http://howtobyme.blogspot.com/2011/11/car-runs-hot-in-traffic.html
-----------

All types of car overheating problems and troubleshooting?

http://technoanswers.blogspot.com/2011/11/all-types-of-car-overheating-problems.html
---------

Car freeze overnight and started overheating while driving?

http://technoanswers.blogspot.com/2012/01/car-freeze-overnight-and-started.html

-----------

Engine Overheats and heater blowing cold?

http://howtobyme.blogspot.com/2011/11/engine-overheats-and-heater-blowing.html

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Car overheats and Air blows hot at idle?
http://howtobyme.blogspot.com/2011/11/car-overheats-and-air-blows-hot-at-idle.html
--------

overheating?

http://schematicsdiagram.blogspot.com/2011/12/buick-century-overheating.html
----------

Car engine Overheats and transmission starts slipping?

http://schematicsdiagram.blogspot.com/2011/12/car-engine-overheats-and-transmission.html
----------This details will help.Thanks.

For car running hot and overheating issues, I suggest to you go through this help links. Click this link below and go through the troubleshooting procedure to take care of the problems: ----

Engine Overheats and heater blowing cold? http://howtobyme.blogspot.com/2011/11/engine-overheats-and-heater-blowing.html

----------

Car runs hot? http://howtobyme.blogspot.com/2011/11/car-runs-hot-in-traffic.html
-----------All types of car overheating problems and troubleshooting?

http://technoanswers.blogspot.com/2011/11/all-types-of-car-overheating-problems.html
Car freeze overnight and started overheating while driving?http://technoanswers.blogspot.com/2012/01/car-freeze-overnight-and-started.html-----------Engine Overheats and heater blowing cold?

http://howtobyme.blogspot.com/2011/11/engine-overheats-and-heater-blowing.html

---------------------------

Car overheats and Air blows hot at idle?
http://howtobyme.blogspot.com/2011/11/car-overheats-and-air-blows-hot-at-idle.html
--------overheating? http://schematicsdiagram.blogspot.com/2011/12/buick-century-overheating.html
----------Car engine Overheats and transmission starts slipping?http://schematicsdiagram.blogspot.com/2011/12/car-engine-overheats-and-transmission.html
----------This details will help.Thanks.

Does it slowly overheat while driving down the highway..If so the radiator is probably clogged..It can be partially clogged and cause the car to overheat,,but usually takes a little while...If it overheats in traffic..(stop and go ),,but does fine going down the road,,then it make sure your fan is coming on..

It could definitely be the thermostat. It is a cheap part to replace. If the car overheats while running down the road (not stop and go traffic), then it is almost certainly the thermostat.

Is the coolant level low.

note if it overheats on the freeway or in city driving. If it overheats soon after you leave the house then in has to be the thermostat. If it overheats at city driving where there are lots of stop and go driving or while you are parked with the motor running, then check the radiator fan function properly. If it takes a while to overheat and tend to do it when driving on the freeway, it is a clogged/dirty radiator. If it looses coolant due to a leak, then it will also overheat. The least likely cause which don't even pertain to newer car is a retarted timing. A retarted timing causes a car to loose noticable loose of power accompanied by overheating. A retartded timing is usually common in older car where timing is adjustable by turning the distributor. Your car is exempt from this.

no the computers only job is too turn on the fans. check for enough antifreeze check for leaks. these cars are known for leaks. if the water pump is broke internally it will overheat with in minutes, or if the thermostat is stuck it will overheat too.

GET COOLING FANS CHECKED SOUNDS LIKE A DODGEY FAN OR FAN SWITCH