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Pulsing Voltage To A/C Compressor, 02 VW Jetta


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A 2002 VW Jetta arrived at our shop the other day, the customer complaint…no air conditioning. The system was fully charged so we started our diagnostic tests and found that there was a pulsing voltage to the a/c clutch. The fan control module would pulse a 12 volt signal every few seconds.

 

After a little research we learned that a faulty compressor clutch could be the fault. We ohmed the clutch coil and found it open. We replaced the compressor clutch and now the FCM sends a constant 12 volt signal and all is ok.

 

Maybe others have seen this, but it stumped me and my lead tech at first. Apparently, if there is a problem with the clutch coil, the voltage will pulse from the FCM.

 

It’s a good thing we did a little research to find how the system works before shot-gunning the problem.

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A 2002 VW Jetta arrived at our shop the other day, the customer complaint…no air conditioning. The system was fully charged so we started our diagnostic tests and found that there was a pulsing voltage to the a/c clutch. The fan control module would pulse a 12 volt signal every few seconds.

 

After a little research we learned that a faulty compressor clutch could be the fault. We ohmed the clutch coil and found it open. We replaced the compressor clutch and now the FCM sends a constant 12 volt signal and all is ok.

 

Maybe others have seen this, but it stumped me and my lead tech at first. Apparently, if there is a problem with the clutch coil, the voltage will pulse from the FCM.

 

It's a good thing we did a little research to find how the system works before shot-gunning the problem.

 

 

I've run across that one before, but, I have one problem I've found with the newer beetles that will throw you a curve ball... If you get a Beetle with A/C problems... do this little test first.

With the car idling and A/C on... and you get no cold air... BUT... if you bring the engine rpms up to say... 2 grand or so ... if now you have perfect cold air the problem is not the A/C ... it's the charging system. The feedback current from the alternator is too low at idle for the PCM to recognize that the charging system is working. However, it is high enough to keep the charge light off. The PCM reacts by assuming the charging system is shot and starts shutting down accy systems to avoid overloading itself.

To make matters worse... everytime I ran across this problem I have never been able to see a voltage change at the battery with a meter. But, believe me... it's the alternator. The last one I did was probably the worst one. Everything looked great, but I changed the alternator anyway. As soon as I did... the A/C kicked on... blowing cold air just like you would expect.

Hope this helps... Gonzo

Edited by Gonzo
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I've run across that one before, but, I have one problem I've found with the newer beetles that will throw you a curve ball... If you get a Beetle with A/C problems... do this little test first.

With the car idling and A/C on... and you get no cold air... BUT... if you bring the engine rpms up to say... 2 grand or so ... if now you have perfect cold air the problem is not the A/C ... it's the charging system. The feedback current from the alternator is too low at idle for the PCM to recognize that the charging system is working. However, it is high enough to keep the charge light off. The PCM reacts by assuming the charging system is shot and starts shutting down accy systems to avoid overloading itself.

To make matters worse... everytime I ran across this problem I have never been able to see a voltage change at the battery with a meter. But, believe me... it's the alternator. The last one I did was probably the worst one. Everything looked great, but I changed the alternator anyway. As soon as I did... the A/C kicked on... blowing cold air just like you would expect.

Hope this helps... Gonzo

 

That's a great tip, Gonzo. Maybe we should post more of our technical war stories. Afterall, if we can help each other to become more profitable in the shop, it helps our business.

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That's a great tip, Gonzo. Maybe we should post more of our technical war stories. Afterall, if we can help each other to become more profitable in the shop, it helps our business.

 

let's do...

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  • 8 months later...

I've run across that one before, but, I have one problem I've found with the newer beetles that will throw you a curve ball... If you get a Beetle with A/C problems... do this little test first.

With the car idling and A/C on... and you get no cold air... BUT... if you bring the engine rpms up to say... 2 grand or so ... if now you have perfect cold air the problem is not the A/C ... it's the charging system. The feedback current from the alternator is too low at idle for the PCM to recognize that the charging system is working. However, it is high enough to keep the charge light off. The PCM reacts by assuming the charging system is shot and starts shutting down accy systems to avoid overloading itself.

To make matters worse... everytime I ran across this problem I have never been able to see a voltage change at the battery with a meter. But, believe me... it's the alternator. The last one I did was probably the worst one. Everything looked great, but I changed the alternator anyway. As soon as I did... the A/C kicked on... blowing cold air just like you would expect.

Hope this helps... Gonzo

 

On these cars check voltage output in ECU measuring blocks. I've seen a number of odd problems caused by voltage issues on VW/Audi cars and Benz's. A new regulator solves the problem. I often have it on the top of my list to check voltage and alternator output and ripple when diagnosing drive-ability or anything that stumps me at first. I do forget sometimes. Had a Chrysler in here the other day with erratic idle and instrument cluster quirks, luckily stumbled on the negative terminal being loose, a quick clean and tighten solved all the problems.

 

Haven't stumbled upon this A/C problem yet but its that time of year now so I'll keep it in mind.

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A 2002 VW Jetta arrived at our shop the other day, the customer complaint…no air conditioning. The system was fully charged so we started our diagnostic tests and found that there was a pulsing voltage to the a/c clutch. The fan control module would pulse a 12 volt signal every few seconds.

 

After a little research we learned that a faulty compressor clutch could be the fault. We ohmed the clutch coil and found it open. We replaced the compressor clutch and now the FCM sends a constant 12 volt signal and all is ok.

 

Maybe others have seen this, but it stumped me and my lead tech at first. Apparently, if there is a problem with the clutch coil, the voltage will pulse from the FCM.

 

It’s a good thing we did a little research to find how the system works before shot-gunning the problem.

 

 

Sounds they are using a current detecting transistor in the output from the module, either that or they are monitoring the feedback diode for current load and if the system sees a out of spec condition it will refuse to power the circut.

 

I'd have to check the diagram but this AC clutch was not relay drive ?

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