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

[mabolt]

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.

 

[flyingscott]

Mechanically I do not think anything is wrong. What I think is happening is somebody along the line changed the lower unit to an earlier lower unit. The earlier lower units shifted the opposite of a standard lower just something Mercury did. So forward would be backwards and reverse would be forward on the shifter. I also believe that is why the neutral/reverse lock is disabled because it will not work correctly with that lower unit. Or your motor is earlier than you think it is give us a 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. Going to look now to see if their is a part# and or ID stamped on the lower unit, but if not, would I know? 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/newparts/part_details.php?pnum=MER23-30676A+1

Going to remove lower unit again and post a picture of the current bushing assembly. I recall that even the screw is not the same...should have a square head.

So...I also agree that the previous owner could have disabled the "reverse lock" mechanism for that reason you mentioned. I noticed the "hook" 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.

 

[mabolt]

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.

 

[racerone]

Shift sequence appears to be correct.-----The plastic shift lever mounts on a square shaft so just rotating that by 90 degrees might not help.-------So from far away looks like somebody messed with the gear quadrant under the carburetor too !

 

[timguy]

Would love to have my hands on this one to help you. Gonna be tough to do long distance. Compared the lower unit to what I have around here and it looks correct. Almost every old Merc I get in here has reverse lock problems. Plastic parts get "messed up". New parts are available, of course, but you have to shop around carefully. Your service manual will be of great help, good move, good price. Tim's Outboard in Hackensack MN can help with new and used parts, seal kits......look like a good idea here, right? Ask for Mike or Dan, tell 'em Tim sent ya. 218 682 2331. They ship fast and cheaply. I've spent thousands of dollars with them. In business with clean Minnesota parts now 51 years. Tim is still around, but retired.

 

[mabolt]

Thanks for the response timguy, glad to know my lower unit is in fact the correct model for my motor. I hear ya, I'm leaning towards removing the Reverse Lock mechanism and seeing what's happening (have parts in hand). I'll be sure to contact Tim's Outboard in the future for parts if need be...thanks!

Agreed racerone. 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.

 

[timguy]

When you originally dropped the lower unit, did the shift shaft pull part way up, enough to disengage the shift dog down in the gear housing/transmission? Unless something strange happened, it should go back into the dog with proper orientation anyway. Merc played around with different things here, however....as Mr. Scott mentioned. Parts may have been shuffled around. I have personally (am guilty of)....done this myself. Lets put it this way: I have quite a few spare parts around and my customers are in a hurry sometimes.

 

[mabolt]

I could have timguy. This motor had not run for quite a few years, so it was a bear to detach the LU when I changed the impeller (but I took my time, rubber mallet, etc.). But I could have done just that. I do feel my shift shaft is off, maybe for that reason. From a few videos i've seen ( donyboy73 on YouTube when I searched for impeller replacement), the "blind" spline should be facing towards the drive shaft when in Neutral. That is not the case for me. If you look at my positions from the picture above, it's off. If I actually did move the shift shaft, would I have start by removing the propeller and all that follows, to move it back to it's proper location?

 

[timguy]

Gonna have to check to make proper orientation when manual comes in. I have however, on some models, changed orientation from lifting shaft out of shift dog and turning. Your dog may be keyed, however......but how else could it be off? Maybe it moved when you did the pump work.....especially as you already mentioned it was fine shifting before your water pump work.