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Hi, My girlfriend has a 2003 Beetle with the 2.0L and its recently started having a problem where the coolant fans will not kick on. if the car sits at idle or is driven in stop and go traffic it typically begins to overheat (blinking red temp light and boiling coolant). whoever previously worked on the car added green universal coolant instead of G-12 but I cant see it mattering too much.
I tested the coolant fans through the fan switch plug and both the high and low fan speeds work. I went to my nearest pick-n-pull and snagged a Fan Control Module which seemed to be in very nice condition and popped it in. no luck. ive replaced the fan switch, coolant temperature sensor, and the thermostat with working parts all with no luck.

Im not sure if the problems are related, but the AC compressor will not kick on. ive tried replacing it but when refrigerant is added, the low fans kick on and still no compressor. from what ive read, the fuse box on the battery cover is vulnerable to corrosion and melting but everything looks good. ive also checked fuses and they look good too. Are these problems related? Id like to get these issues straightened out. Any help would be appreciated! Thanks
 

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I don't have any specific info but you might want to check out this thread, for service manual testing procedures:

 

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5/23/10 <3
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Also, if the system wasn't flushed out before adding the universal coolant, there could be some sludge which can clog up some of the cooling system and cause problems.
 

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whoever previously worked on the car added green universal coolant instead of G-12
I think that could be the problem. Most green coolants are not compatible with the VW coolant and create sludge in the cooling system when mixed. The temperature switch that turns the fans on is located in the radiator where it senses the temperature of the coolant that has passed through the radiator and is about to return to the engine. If there is not a high enough flow rate through the radiator the hot coolant never gets to the temperature switch to turn the fans on. There are a few reasons this could happen. One is a radiator that is clogged with sludge. The others are a failed coolant pump which would be a definite possibility if yours has never been replaced. Also, an air pocket in the engine. There needs to be flow of coolant through the line that goes to the top of the coolant bottle for air to be able to escape the cooling system as it is filled.
 
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