The Importance of Interlockers

David runs a pressure test on a "D" type overdrive

David runs a pressure test on a "D" type overdrive

You’re looking at a simple but effective basic pressure test of a “D” type overdrive fitted to an early MGB 3-synchro transmission being performed in our driveline shop directly underneath where you saw Butch was working on an “A” type overdrive last week.    Your correspondant is runnning up the TX in 4th gear with an electric drill mounted in the bench vice to counter torque reaction, and coupled thru a piece of 5/8″ heater hose secured with four NAPA hose clamps.

You can make a quick check of the overdrive hydraulics on your own work bench in a similar fashion by running the same test and simply checking that, once engaged, hydraulic pressure prevents the TX output shaft from turning backwards until the pressure is released by cutting power to the solenoid.

3synchro & 4synchro interlockers

3synchro & 4synchro interlock mechanisms

Now the reason that we were in this gearbox to begin with, was because of an interlocker problem.  It didn’t have one.  Our client brought it to us  and asked us to take a look at it and determine why it was stuck in gear.  We popped off the side cover, saw a nice looking gear set, worked it thru the gears manually and told him “nothing wrong” and sent him on his way.  Well, we were the ones who were wrong.  The interlocker was M.I.A. !

Illustrated above are from left to right, are a 4synchro interlock mechanism, modifed by us to suit the 3synchro O/D application, the standard 3synchro interlock arm, and the 4synchro (std & O/D) unit.  The purpose of the interlocker is to prevent the simultaneous selection of more than one gear.

modified interlocker installed

Modified Interlocker Installed. Click to Enlarge

Here’s the modified 4-synchro interlocker installed in the overdrive adaptor housing.  It’s purpose is to prevent the peg on the remote control rod from engaging more than one selector at a time, which with the benefit of 20/20 hindsight, was exactly what was happening.  Our Bad, missing this obvious problem the first time !  What we didn’t miss was the fact that the similar appearing 4synchro interlocker lacked sufficient clearance to do the job, but after a little massaging on the Bridgeport, all was well

farm-style breakfast

A Farm-Style Breakfast

Attention 4-wheeling fans: Owing to recent weather conditions, we’ve been providing additional rations to the 29 heaad of cattle pastured behind the shop.  Seth Holton’s crew hauls over these 1000 pound round bales which we transfer into the back of our our farm truck for delivery on the other side of the mud bog created by the recent heavy rains.

charging back thru the mud wallow

Return Passage. Click to Enlarge for Flying Mud

It’s the getting back that’s the challenge.  Here’s your correspondent charging back up the muddy barnyard hill with chains on.  To borrow from someone else’s classic advertising slogan: “You Can Do It In A GMC !

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