Have a 2002 Grand Caravan with 140K miles, original owner. Recently a strange thing started, where the transmission has a bucking feel when car is in gear and not moving (such as waiting at a light). Here's the weird part; this only happens when the climate control power switch is turned on. If I turn the power switch off the bucking goes away. Doesn't happen in park and doesn't happen when the car is underway. Never saw anything like this. Is it vacuum related? Any ideas would be appreciated.

 

Sounds to me like the A/C compressor cycling. It will cycle if you have either the A/C on or the defrost on (not the rear window defrost).

 

No vacuum in the Caravan HVAC system. All electronic.

 

The load of the A/C compressor cycling on will cause a small surge sometimes. I had this issue with my van, it started first when sitting still in Drive, then it started bucking at 40-60km/hr when accelerating gently then it threw a CEL for a bad EGR valve. Replacing the valve made my problem go away. I happened to have the scanner connected when it threw the code and it first had a pending EGR code then about 10 minutes into the drive the light came on and it set the code and took a freeze frame. Perhaps check for pending codes and see if you are also experiencing an EGR problem?

 

Thanks to all who responded. Turns out, after spending way too many days in the shop to diagnose, it was the alternator. Once that was replaced the symptoms disappeared and car runs great.

 

Second erratic tranny caused by an alternator I've read about today. On the other one the odo was increasing with the key off too. Odd.

 

Awesome that they properly diagnosed and repaired this right. These vans are stupidly sensitive to voltage issues.

I had a weak battery cause my transmission to drop into limp mode. New battery and problem disappeared.

 

You should have seen the problems in the early days. The spark plug wires would send an RFI signal thru the main wire harness going to the transmission cotrol module. That was from 1989 thru 1993. Then 1994 and 1995 was the knock sensor on the engine block that casue a transmission buck.