I wonder, if the crank pulley was left on the Malibu engine, and the A/C and the alt pulley are from the HHR, could it be that a smaller belt is required? Do you have the original HHR crank pulley to measure the diameter or the circumference to see if it matches the Malibu pulley?
I hope I got that correct.

 

I did a little research a found aftermarket crank pulleys are the same from Malibu to hhr but if you look up a Delphi a/c compressor the Malibu is cs20063 and the hhr is cs10077.

I wonder if the a/c clutch is different?

So my question would be did you reuse the hhr a/c compressor for the install?

 

What about alternators? I have a feeling the Malibu would be different.

 

Good point. I looked that up and there are two different part numbers for hhr and Malibu

 

So then the alt pulley may be the culprit!

 

I'll Check on that, thanx guys

 

Let us know !

 

Well fellas it turns out the ac compressor was missing 2 bolts, and it was starting to lean to the side. I guess the mechanic who did the engine swap didn't put all the bolts on. Just started running it today, hopefully this was the cause of the belt coming off, thanx for the help though guys

 

Glad to here you got it figured out. Sound like an easy repair.