You wouldn’t think a bad radiator cap would cause transmission problems but in this case it did. Follow this Real Fix to see how a bad cap caused hard shifts on this Buick Rendezvous.
Vehicle
2005 Buick Rendezvous CX 3.4L
Title
Transmission Shifts Hard, Replaced Radiator Pressure Cap
Complaint
The customer states that the transmission shifts hard.
Cause
Confirmed the customer’s complaint and found the transmission shifted hard. Observed the engine coolant temperature gauge and found the engine coolant temperature exceeded allowable specifications. Performed a visual inspection of the engine coolant condition, but found no obvious faults. Performed an inspection of the engine coolant and found the engine coolant was low. Topped off the engine coolant and connected a cooling system pressure tester to the engine cooling system. Pressurized the engine cooling system and found the engine cooling system held the applied pressure. Removed the radiator pressure cap for inspection and found no faults. Attached a pressure tester to the radiator pressure cap and tested the cap by applying the specified amount of pressure. The radiator pressure cap leaked and did not retain the applied pressure, which verified the cap was defective. The elevated engine coolant temperature affected the shift pressure schedule which resulted in the hard shifting.
Correction
Replaced the radiator pressure cap and verified the vehicle operated properly. The customer’s concern did not return.
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