I recently went through this with my 1991 420SEL, which has the 722.3 transmission. Over the previous year or so, the time to engage reverse had grown longer, until it was taking 4 seconds or more. Then, it started to not engage in reverse at all, until you blipped the gas a bunch of times (problem was worse when the transmission was warmed up or hot). Miles on the car was right around 170,000.

As stated elsewhere, the problem is with the B3 reverse clutch. Most of the time, it is that the reverse clutch plates are worn out. Unfortunately, this requires removal of the tranny, and opening it up. This of course leads to the pros and cons of whether to do a total rebuild, versus just replacing/repairing the B3 clutch plates. (Or, alternatively, dropping a rebuilt tranny in.)

My transmission shop told me that the rest of the transmission looked to be in pretty good shape, so he was comfortable with either approach. The cost difference was $1300 for just repairing the B3 clutch (as well as replacing seals, etc., while the tranny was out), versus $2600 for a total rebuild. Keep in mind that this is in the inflated-price confines of the SF bay area - I think both prices would be a fair amount lower in other locales. It was also only a 3 month warranty on just the B3 repair, versus 1 year for the total rebuild. But, I went with just the B3 repair, since 1) I'm not going to keep the car for more than a year, and 2) the B3 problem is a known weak point of this transmission, and 3) the transmission shop advised that wear on other parts of the transmission seemed pretty low. (If I knew that I wanted to keep the car for multiple years, I probably would have gone for the rebuild.)

It has been about a month and 1500 miles or so since the repair, and the tranny is working perfectly since, including rapid engagement of reverse.

The one thing that frustrated me a little is that earlier versions of the 722 transmission (722.0 - 722.2) have a B3 band, instead of a B3 clutch, which can be adjusted externally, without removing the transmission. On one of these older transmissions, the fix for lack of reverse gear engagement can be as easy as a 30 minute adjustment.

Good luck,

Bob Schleicher
1991 420SEL, 172,000 miles