rebuild wiring harness Mercury 850

[edyson]

Hi - I have a 1974 Mercury 850. I recently had the entire block overhauled (new pistons, rings, bearings, etc.). I also bought a new CDI switchbox, rectifier, plugs and plug wires. The boat fired right up and idled for about 8 hours with no problems at all. I dropped it in the water last week and ran it for about 30 minutes. Everything ran great. I went to idle and then turned the engine off via the key as I would normally do. After that, the engine would not fire again, but turned over just fine. I am showing extremely weak spark now, and sometimes none at all. Does anyone know what could have happened? I hate to go and drop another $200 on a new switchbox, especially if A) that is not the problem or B) the problem that caused the first box to fail threatens the new box - whereby I'll be back in this same position. The wiring harness, both interior and exterior are in visibly bad shape. The insulation is cracking off in many places. I checked the voltage on the white wire out of the switchbox on cranking and I only get 6 volts, and this is even after I wired the box (white/red wire) directly to the battery (thereby avoiding the wiring harness completely). Any ideas?

Question - Can the wiring harnesses be "rebuilt" using regular hardware store wire? How do I get the plug ends off to access the connection points? Does anyone think the harness may not be the culprit here? Could the box get fried by a bad wiring harness? With new wires, will the voltage come back up or will I have to replace more parts?

 

[Chris1956]

Re: rebuild wiring harness Mercury 850

You may easily build a new motor wiring harness from wire, butt joints, shrink tube and ring terminals. You will need to save a 1/2 piece of wire on the quick disconnect plug and crimp and solder the butt joints to splice the new wire on. Use the shrink tube to insulate. Make sure the connector is in good shape, or you have wasted your time. Same procedure for the stator and dissy wires.

Alternatively, you can buy a new harness with a plug for $100 at Surplusunlimited.com

BTW - The white wire and red wire on the switchbox must have +12VDC for the ign to fire properly. I think your switchbox is OK, but I would not subject it to low voltage for too long.

 

[edyson]

Re: rebuild wiring harness Mercury 850

Hi Chris1956 - After much troubleshooting, we finally decided to buy and try out a new switchbox. As soon as we dropped it in, we noticed the steady high-pitched "whine" that comes from a functional switchbox (our "old" box made a "whine", though it varied in pitch and seemed to crackle some). As soon as we tested the white/red for voltage, we got 12+ and the engine then fired right up. Now a question remains - was the "old" (and I say old, even though it was purchased new and installed only 3 weeks prior) manufactured poorly and simply failed without an external influence, or did something within my Mercury engine cause it to fail? Is it possible that the very sad condition of my wiring harnesses may have caused a short and made the switchbox fail? I ask this because we found several areas along the internal harness wiring that was exposed. We have since taped up as much as we could, but I am still concerned that we may burn out another box before we have time to replace both harnesses.

-E

 

[ifeeltheneed4speed]

Re: rebuild wiring harness Mercury 850

I know you have probably figured this out by now. "Any" loose or bad connections in your engine can cause part failure/stalling. I have rebuilt several Mercury 850 engines. Every one of them had bad wiring in the harness plug. Just keep cutting the wire until you reach a good connection point. I use liquid electrical tape instead of shrink tube. If you have any bad wires, they should be replaced even if your not having problems. It sucks to drive 1 hour to a lake just to find out the engine wont start. Been there. Mine will have full spark sometimes and other times little to no spark. Be sure to check wires and points under your flywheel. My newest 850 has been a nightmare!!!!!!!!! Ran good in a bucket. Get it to the lake, runs for maybe 20 minutes full throrrle and then, cough, cough stall........... No restart..Weak spark. Good luck!

 

[mfh6980]

Re: rebuild wiring harness Mercury 850

edyson,

What did you do with your wiring harness? I am looking at the same issue. I rebuilt the engine harness already but now I am finding the external harness to be junk. Wires will just break off clean when bent... I am thinking about redoing the whole thing and eliminating the plugs all together. I am pouring money into this engine it seems and have not enjoyed time on the lake yet. Mine is a Mercury 500 50HP 77 or so model.

Thanks


Mike

 

[SeaKaye12]

Re: rebuild wiring harness Mercury 850

I guess you realize that this is a very old thread?

As long as you use proper gauge marine wire and know how to splice it correctly, there is no reason why you can't "build" a harness. It just takes time.

One problem I had though was that in the case of my stator and trigger wiring; bad original wiring led clear up into those two devices; leaving no "good" wire to splice onto. I did the best I could and the repair lasted a couple of years, but eventually I ended up replacing a good trigger and stator only because the wires leading to it were toast.

I've never had to rebuild the external harness...but from the looks of it I can't imagine it being very difficult; you're working out in the open. Just do one wire at a time.

Good luck; if you have more questions it might be better to start a new thread.

Chuck