Experiencing problems with Quantaray QDC900 WA flash with Rebel XTi

[STBickerton]

HI, I just bought a used Quantaray QDC 900WA digital zoom flash for my Rebel XTi. I am experiencing troubles with it. First, if I use the "P" (Program) mode on the camera, and the "TTL" mode on the flash, the pictures are coming out extremely dark--as if the flash did not work--however, I see the flash so I am assuming that the flash is triggering before the image is captured (if it were a film camera I would say before the curtain opens). If I move the switch on the flash to the "second curtain" flash, I will get a picture, but it is underexposed. The "E-TTL" light on the flash does not illuminate in either case.

The shutter speed that is automatically set under the Program mode is 1/60, and the aperture is 4.5. The flash indicator in the viewfinder indicator panel indicates flash prior to taking the picture, so the camera is recognizing the flash.

If I go to "M" (Manual) mode in the flash, the shutter speed slows down to 1/200, and the aperture is 22 (flashing). I get a picture in the "M" mode, but it is slightly underexposed.

In the "Multi" mode on the flash, I get the same results as the "M" mode.

In the "Model" mode, the flash does not work.

In the "Remote" mode, I get the same results as M and Multi.

If I go full manual exposure on the camera, I can get good exposure, but only in the second curtain setting with the "TTL" mode, or either curtain setting with the "M" or "Multi" modes on the flash.

Am I doing something wrong here, or is this a bad flash? Or could it be a problem with my camera? Please help, because I was asked to shoot a wedding on January 1st, and I am getting married myself in February (with someone who will use this camera to take pictures).

Thanks in advance for your help and advice.