ZF IRM 220 A-1 problem

[Poach]

I have a ZF (Hurth) IRM 220 A-1. On 2 occasions, when dawdling for an hour or so, (in and out of forward and reverse gear) while waiting to lock through, my port gear stops engaging in reverse (it still goes into forward). On the first occasion I took the boat to the yard. The following day, they tried to duplicate the problem without success. More recently while returning from a trip and again dawdling for about an hour, it quit engaging in reverse. I motored to my marina on both engines which takes about 1/2 hour. Just before turning into my fairway, I gave the port reverse another try--and it engaged. I have since tested it in the slip on one occasion and it is engaging. The gear does not make any unusual noises, and in general goes into and out of both forward and reverse gears with a gentle "thunk". I have pressure gauges, and when engaged in either direction, there is a momentary drop in pressure and then pressure resumes. During the 2 episodes, the pressure did not drop when I shifted into reverse. The manual briefly only gives trouble shooting for the gear not engaging at all. Thoughts?

 

[C lectric]

During the 2 episodes, the pressure did not drop when I shifted into reverse.
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Almost sounds to me like a problem in the shifter not moving the spool. Have you gone down and looked at the cable attachment and shift lever? Maybe something is loose, shifter actuation arm, cable clevis, cable sheath, loose brackets, broken fasteners. Look for wear.

Check also at the shift lever mechanism at the helm.

The fact that the pressure normally dips when it works and then does not dip when it does not work indicates the spool is not moving so the relative ports are not being opened.

The dip is the result of the clutch pressure plate moving which needs lower pressure to move untill actual clutch lockup occurs. At that time the pressure will come back up.

Check that there is no wear or something loose and periodically moving out of place.

 

[twistedtree]

And is the shift control on the gear mechanically actuated, or electrically actuated. ZF offers both. It might be mechanical linkage as C lectric suggests, or it might be an electrical problem. Depends on the gear configuration.