75 Formula Shift Issue

[805HD]

If you want to see if the switch is bad, better to use an old bulb type of test light. That will allow adequate current to flow thru the switch and also eliminate the 'digital' effects from most DMM's.

I used a led pod light (all I had) wired from a 12v battery through the switch. Closing the switch kills the light, with the switch open the light comes back on.

With the switch closed (arm depressed) the light will flicker if I tap the switch.

I think I need to verify down stream electrical at this point. The switch itself is suspect to me but the electrical (ground and wire to coil negative) need to be ruled out.

If problems persist after that I can turn my attention back to shift adjustment.

 

[kghost]

Wire to Coil - is not ground.
The wire from interupter switch that goes to the coil gets shorted to ground when switch closes.

So if you suspect the wire from switch to coil all you need to do is, while running engine in water or on trailer is depress switch and if engine dies, wire would be good in my opinion.

The switch is bad in my opinion. Depressing it by hand does not represent the actual depressing by the V notch.

Now, if you bend the lever of the switch about 1/8 " from roller, so roller sits a little deeper into the V notch it may make it work better. Small bending not a lot. If it improves but not 100%, bend a little more..
There was a service bulliten way back when for what I describe.

If asjustment of the arm/lever does nit belp the bite the bullet and find a new switch.

 

[805HD]

The switch has a life span. You may be depressing it far more then the shift bracket V does.

A picture would help.

Also those switches are very hard to come by and if found sell for far more that the switch itself is worth. Its worth is only with Mercruiser shift brackets.

Part Number is, and all years that used this type of switch the part number is the same.

39670A28 - Switch Some prices are anywhere from $50 -$100+​

Most locations other than Ebay will Be NLA. In the '90's they changed the switch type/design and bracket.

So if you have a "Boat Junk Yard" anywhere near you, That would be a place to find a good used one.


View attachment 37258

Heres a before and during shift screen shot from a video I took during a forward shift. I ordered a “new switch” and am going to verify wiring as soon as the boats back at the house where I can look into it.

Again, the boat shifts fine. It’s coming out of gear that’s the problem so everything is pointing to the interrupter switch and or it’s wiring.

 

[kghost]

Send me your phone number

 

[kghost]

Spoke on the phone.
His switch is an original from his vintage of boat, it is a single layer arm. Newer versions became two layer as in the picture I posted above.

Issue is pointing towards intermitten switch activation internally of the switch.

I advised him to make sure the arm is very flat, slight bend near roller as in my picture and confirm whem V notch moves switch arm it actually depresses the plastic plunger that the arm makes contact with at switch.

He will update when he completes his testing

 

[805HD]

UPDATE***

Today I jumped back in the boat on the trailer, Four things were discovered..

One, the interrupter switch was never even wired to the coil negative. 51 years is a lot of time for someone to do some backdoor wiring for some reason (not even sure if that’s possible) but remember, it was working initially.

Two, the original ground wire that feeds the switch was coming through what’s left of the original engine harness (I have most of this bypassed as the condition of it was unknown during the “rebuild”). The wire itself was very stiff and hard to bend. Definitely compromised.

Three, some how the terminal block was grounding itself out through the shift plate. Not sure how this was happening as one side of the block is encased in plastic. But somehow had continuity between the two.

Four, I had a damn mouse and its babies living in the boat and although there were no chewed wires pertaining to the switch I did see some evidence of wires chewed else where.

Fix was to re-wire everything and bypassed the terminal block with heat shrink butt connectors. Started the boat, depressed the switch at idle and it immediately stumbled. Looks promising.

Lots of odd variables here, I’m not even gonna try and make sense of it.

Mouse has been evicted, will verify shift adjustment and put her on the water!

 

[kghost]

"""""Three, some how the terminal block was grounding itself out through the shift plate. Not sure how this was happening as one side of the block is encased in plastic. But somehow had continuity between the two.""""

On the Terminal block on shift bracket, The ground wire screw actually screws into the shift Bracket. The wire that goes to your coil screws into other terminal on block but makes no contact with bracket or should not make any contact. On the back side of shift bracket (not visible) there is another stand alone ground wire attached to bracket I believe also connect to the Ground terminal block location, They may have attached it to the Wroung terminal location.

 

[805HD]

"""""Three, some how the terminal block was grounding itself out through the shift plate. Not sure how this was happening as one side of the block is encased in plastic. But somehow had continuity between the two.""""

On the Terminal block on shift bracket, The ground wire screw actually screws into the shift Bracket. The wire that goes to your coil screws into other terminal on block but makes no contact with bracket or should not make any contact. On the back side of shift bracket (not visible) there is another stand alone ground wire attached to bracket I believe also connect to the Ground terminal block location, They may have attached it to the Wroung terminal location.

I did have the coil wire on the wrong side initially, when I got continuity on the meter I noticed and switch it to the other side of the block that is isolated and it was still grounding out somehow.

Regardless, the problem has been solved and I’m feeling confident about it.

Is what has me scratching my head is how in the hell was this thing shifting out of gear without anything but the distributor and tach hooked to coil - ?!

Unless it was being done another way through the harness. It is possible Stewart little chewed up the right wire at the right time. 1 in a million, but possible.