1975 7.5hp Mercury Thunderbolt - Problems: Reverse lock, shift handle, & shift shaft

[mabolt]

1975 7.5hp Mercury Thunderbolt - Problems: Reverse lock, shift handle, & shift shaft

Purchased this motor for $150 which had not been started in a few years. After dropping the Lower Unit, I noticed the impeller needed to be replaced. Got a new part, replaced impeller, and that's where everything went south! After attaching the lower unit, I have the following problems...

1. Shift handle, in "vertical" position, is in Forward. Move about 45 degrees to the left, i'm in neutral, and all the way down (horizontal), you got reverse. From all the videos/forums, i'm aware that vertical should be Neutral, and about 45 degrees to the right is Forward, and opposite direction from Neutral, you have Reverse. I cannot move the shift handle from it's vertical state to the right, but in vertical, I know it's in FWD because the propeller has resistance when spinning left/right..

2. I am almost certain the Reverse Lock mechanism was either broken and/or disengaged by the previous owner. The link rod, or "hook" was completely outside of the cam. My thinking is that he did this to bypass the reverse lock cam in order to be able to move from FWD to Neutral and Reverse. I have reinserted the link rod and it does not easily engage/disengage the reverse lock when moving from Fwd, to Neutral, to Reverse. Once I move the shift handle to reverse, the reverse lock mechanism does not want to disengage.

3. Last...and i'm not sure if this is/not correct...the "blind" spline on the shift shaft, when in Neutral, should be facing forward, correct? I feel like this has some how moved or jumped. When I move the shift shaft with a pair of vise grips/padding on shaft, the "blind" spline is off center in Neutral (props spins freely left/right). If I rotate the shift shaft CCW, it jumps into FWD. If I return back to Neutral position and turn the shift shaft CW, i'm in Reverse. Is this correct or, as I mentioned, should the flat part of the shift shaft "blind" spline be facing directly toward drive shaft?

I feel this could ALL be somehow related to the fact that the Reverse Lock (cam/link rod) is not working properly. Ordered a Mercury Service Manual for 1975-78 models for $20 off Amazon (what a bargain) and should be arriving in a few deals. Hopefully this helps.

Thoughts???

 

[flyingscott]

You probably have an earlier lower unit on your motor. That means it shifts opposite of a standard lower unit. That is why the reverse/neutral does not work with that lower. Or your motor is older than you think what is your serial #.

 

[mabolt]

Thanks for the response flyingscott. My serial number on the "upper" unit reads 4254443. When I look this up, I know the "upper" unit is a 1975 according to various websites (like this one), BUT, you could be right, the "lower" unit could be from an earlier model Merc. I also noticed the "bushing assembly" for the shift shaft does not match up with the one in the parts list for a 1975 model...

https://www.marineengine.com/newpart...MER23-30676A+1

Just removed the lower unit and see Part# 1630 4916 (with "B" and "2" stamped on other side). I have included some more shift shaft pix of where the "blind" spline is facing. Also noticed the screw on "bushing assembly" is different...should have a square head but this could simply have been replaced.

So...I also agree that the previous owner could have disabled the "reverse lock" mechanism for that reason you mentioned (older LU). I noticed the "hook" from the link rod was completely removed from the cam system before I replaced the impeller. I put the link rod "hook" back in the cam, and since then, I can't shift out of forward easily (or at all at times) without manually getting in there with a flat head screwdriver and turning that cam. Going to remove the "hook" again and see if I can shift into all gears. Thanks again.

 

[mabolt]

MORE PIX...I also think the previous owner messed with the horizontal and vertical shifting (planetary?)gears (I believe referred to as GEAR, SECTOR - SHIFT CONTROL SHAFT (HORIZONTAL) - AND GEAR, SECTOR - SHIFT SHAFT). Hopefully my service manual will show up today with some insight into that area of the engine and what it should look like. I'm also leaning towards removing/replacing the Reverse Lock mechanism. I ordered the reverse lock cam and retainer parts and they just came in. When I have the lower unit removed, I can move the Shift Lever to (what I think is) FWD, NEUTRAL, REV. But when I put the LU back on, knowing that I have both the Shift Lever in FWD and the Shift Shaft also in the FWD position, I cannot move the Shift Lever to change gears. I wonder if the Reverse Lock mechanism has been the culprit all along.