Lower shift cable and shift interrupt switch nightmare on my '67 Merc 153cu in 4cyl

[redgoat]

Last summer I replaced a shift interrupter switch (39671A3) on my 153 cu.in. 120 hp 4cyl Mercruiser. I considered my self lucky to find this NLA part at a local dealership. This seemed to solve my forward to neutral binding up problem. My perennial problem for many years has been engine stalling when hot when I would shift from neutral to forward or neutral to reverse. The engine would die. It would usually start right up again but would die when put into gear. I tried increasing the idle speed on my carb to see if that would help get it into gear before it stalled. No luck. For what its worth, I have thoroughly rebuilt the carb, new fuel pump and fuel filter, new plug wires and plugs, new coil, double gasket on carb base, adjusted needle valves, and set timing. I am aware of heat soak so I run my blower constantly to remove hot air from engine compartment. This Mercruiser engine is 45 years old but has been meticulously maintained. Now my problem is more evident when I run engine on muffs at home after sitting for a couple of days. After starting I will put gear into reverse only to have the engine promptly die. A second try may result in the engine not dying. Forward is OK. Days later I did the same routine with someone moving the shifter to reverse while I watched the A Bracket and shift interrupt switch. It did move and everyone knows that the switch is not supposed to do this. The switch is for killing the engine briefly when moving from forward to neutral or reverse to neutral. Clearly the plastic roller was out of the V notch and ready to kill the engine. I have taken this boat to TWO repair shops and one said he could not find anything wrong short of a lake test and suggested I had bad gas. Another facility (authorized Sea Ray Dealer) said he could not find anything wrong either and that the shift cable was on specs. I feel that the problem must be something other than the shift interrupt switch because it has no problem killing my engine. My lower shift cable was replaced 5 years ago and the boat has seen minimal use in that seasonal time period. In reading some older threads ACHRIS stated that when a cable is sticky it can trip the switch and kill the engine and that is likely the first sign of a bad cable. Another mentioned something about interrupter springs being too weak. Also, someone cautioned against lubricating the cable as being a definite no no. Maybe mine was lubricated 5 years ago. This problem has to be solved as I am thoroughly frustrated and tired of being towed back to the dock with disappointed guests. The photo is my old style shift interrupt switch. What should I do next?

 

[81 Checkmate]

Re: Lower shift cable and shift interrupt switch nightmare on my '67 Merc 153cu in 4c

Question.... How did it shift 5 yrs ago when the shift cable was new?

Was the shifting issue the cause for the new cable?

 

[Pete104]

Re: Lower shift cable and shift interrupt switch nightmare on my '67 Merc 153cu in 4c

Replace the cable.
Sounds like you've been nursing this thing along way too long! And who says it was replaced properly whenever it was?

 

[generator12]

Re: Lower shift cable and shift interrupt switch nightmare on my '67 Merc 153cu in 4c

"I feel that the problem must be something other than the shift interrupt switch because it has no problem killing my engine."

If you see the problem on muffs, why not disconnect the switch to verify whether it's actually what's killing the engine?

 

[Eric 3.7LX Alpha 1]

Re: Lower shift cable and shift interrupt switch nightmare on my '67 Merc 153cu in 4c

Shift cable time!

 

[redgoat]

Re: Lower shift cable and shift interrupt switch nightmare on my '67 Merc 153cu in 4c

Five years ago, as best I can recall, the lower shift cable was replaced because I was having some problems with my shifter control box. The lower shift cable and related parts were replaced and the shifter worked much better. However, on the first or second time out on the water I experienced the stalling problems described above. At that time, 5 years ago, I had never heard of a shift interruption switch and likewise had no idea that the lower shift cable is closely related to its proper operation. Had I known of this connection between the two I would have returned to the dealership and complained. I just figured that the problem that I experienced before was something else and I would just have to live with it or continue to try to diagnose the problem.

 

[81 Checkmate]

Re: Lower shift cable and shift interrupt switch nightmare on my '67 Merc 153cu in 4c

Im still thinking lower shift cable. Draging or binding and killing the motor to long.

When you shift it needs to be swift shifts.....do not do it slow.... shift then clunk and your in gear.

Could be way out of adjustment and its letting the interrupt to stay open/closed to long and killing the motor. It just kills it for split second to make the shift.

Could of been when they replaced it they didnt adjust it right becouse its an older boat....They have not seen/worked on a 1967

You have a 67 yr boat could be alot different. Get a manual and check the adjustment.

All i have...Good luck!

 

[Don S]

Re: Lower shift cable and shift interrupt switch nightmare on my '67 Merc 153cu in 4c

Sounds like an improperly installed, after market, old style shift cable was installed. Time to put a new style OEM (now teflon coated core wire) in and get it adjusted properly.

This is all assuming the drive itself is ok, and the shift shaft in the bellhousing is not binding.

 

[redgoat]

Re: Lower shift cable and shift interrupt switch nightmare on my '67 Merc 153cu in 4c

The idea of disconnecting the switch whenever the engine is killed when putting the shift lever into forward or reverse sounds like a sure way of determining if the shift interrupter is being activated by the lower shift cable. When an engine kill occurs when going into gear I think I will try this. Can I disconnect the wire at the coil that leads from the shift interrupter switch? This would be easier to get to than a wire at the interrupter switch. I think this would confirm once and for all that the lower shift cable is activating my shift interrupter switch whenever the boat is put INTO forward or reverse.

 

[Don S]

Re: Lower shift cable and shift interrupt switch nightmare on my '67 Merc 153cu in 4c

If the engine dies when you shift into or out of gear, then the shift interrupt switch is doing what it's suppose to do. All it does is ground out the igntion so the engine will not run. But it's only supposed to happen quickly. It should not hold the shift interrupt engaged, and it shouldn't engage the shift interrupt switch at all going into gear. Read this thread. http://forums.iboats.com/showthread.php?p=1930449#post1930449

 

[fishrdan]

Re: Lower shift cable and shift interrupt switch nightmare on my '67 Merc 153cu in 4c

DonS is right, the switch is doing what it's supposed to, the problem is elsewhere (as he suggests)

The idea of disconnecting the switch whenever the engine is killed when putting the shift lever into forward or reverse sounds like a.................

.................good way to limp back to the dock when the shift interrupt switch keeps killing the engine. While this will get you back to the dock, the drive will be stuck in gear (forward or reverse) and you will have to turn off the engine to get it out of gear. It's called "limp home mode", not "run around the lake looking for something to run into because the drive won't come out of gear mode".

 

[achris]

Re: Lower shift cable and shift interrupt switch nightmare on my '67 Merc 153cu in 4c

Sounds like Don's suggestion of an old style cable is on the money. The fact that it has had little use is NOT a good thing. Those things like to be used and move regularly, if not, they get sticky...

When you pull the drive off you can check the shift mechanism in the drive. But I suspect a new style, correctly installed, shift cable should clear up the problem. You have to pull the drive to replace the cable anyway, so why not make that the first diagnostic move....

Chris....