850 Switch box died!

[hereto]

Hi all,
Sure could use a bit of help. I have a 1976 Merc 850 with ADI ignition. This guy had been running GREAT up to a couple of days ago. My out of gear Idle is set at 800 RPM, when I would put it in gear it would stumble and die. Would restart, it would idle great then die when put in gear. I tested the stator, trigger, and v-reg per my manual, all checked good. Borrowed a switch box from a friend, hooked it up and my problem was gone, runs great again.
Now, I have returned the switch box I borrowed and feel confident I can make the repair on the switch box, with the help from someone more knowledgeable with the electronics on this guy than myself. I just need to know which part failed and exactly where to start the dig.

Thanks so much in advance,
hereto

 

[redjmp]

Re: 850 Switch box died!

Having just baked and dug one out myself, I can tell you there is no way to get the urethane out without applying enough heat to destroy the delicate components. There are way more components in that circuit than you might think and without an ocilliscope and signal injector, you wont stand a chance...
I just wanted to see what the circuit actually was, in order to copy it, not repair it.

 

[chum1]

Re: 850 Switch box died!

That is just one item you may have to bite the bullet on, there was probably some old crusty wires that shorted the switchbox so take a close look at the trigger/stator also, remember, these things were made when the new transistor bicentennial AM radios were popular.

 

[hereto]

Re: 850 Switch box died!

All wiring is new along with both the trigger and stator. The problem has been isolated to the switch box. I am just trying to find someone knowledgeable of the electronics of this switch box and could tell me which part I need to replace and where it is located.

 

[Chris1956]

Re: 850 Switch box died!

Those switchboxes are potted with epoxy. The schematic is probably still not in the public domain. I would think you will need to abandon the idea of repairing the switchbox as impractical.

 

[Faztbullet]

Re: 850 Switch box died!

Sounds like a blocking diode bad..

 

[hereto]

Re: 850 Switch box died!

Thanks for the schematic Faztbullet, checked diodes and all seem to be ok.
Haven't had any positive replys so far, but I still think the repair won't be a problem if I can find out which component died. Made a trial dig, the material is not an epoxy and won't be a problem. Thanks again for the schematic.