Did I fry the switch box?

mfh6980 · Dec 23, 2010

I was in a hurry the other morning trying to get ready for a bowfishing trip with my son and hooked my motors (1979 Mercury 500 Thunderbolt 50HP) negative battery cable to the positive terminal and my negative accessory wires (including my closed kill switch) to the negative battery terminal. I heard a pop at the motor (guessing the switch box) and now the engine will not even crank. Even the choke switch does not click when pressed? Would a blown switch box prevent the motor from cranking and the choke from engaging? Just need confirmation or diagnostic steps before I order a new switch box or take it to the shop.

Thanks

Mike

CharlieB · Dec 23, 2010

Re: Did I fry the switch box?

More probable that you smoked the rectifier in the charging system as well as the main fuse that feeds the ignition switch, located on the motor near the positive battery cable connection.

Disconnect the rectifier, replace the fuse, then check for spark, if all's well, then replace the rectifier.

boat1010 · Dec 23, 2010

Re: Did I fry the switch box?

Hopefully like CharlieB said it was just the fuse not the rectifier. Check the fuse first and go from there.

cribber · Dec 23, 2010

Re: Did I fry the switch box?

Check your fuses... you likely just blew the main fuse if you did'nt try to start it up and the ignition was off when you had the polarity reversed. Worst case is what CharlieB stated.

Chris1956 · Dec 23, 2010

Re: Did I fry the switch box?

Those old Mercs did not have OEM fuses. The switchboxes were pretty tough, so you have a good chance it is OK. The rectifier is very likely blown.......

mfh6980 · Dec 23, 2010

Re: Did I fry the switch box?

No fuse that I can find or that I am aware of. I disconnected rectifier and tested. Still won't crank at all not even a click at the solenoid. I jumped from the yellow starting wire to the positive on the solenoid and the engine will crank. I think per what I have read verified the solenoid is still good. Is there a fuse in the key switch/throttle assembly? I know I heard a pop at the engine but turning the key seems to be doing nothing. I have attached the wiring diagram for my engine. Really appreciate the ideas so far.

Thanks

Mike

merc850 · Dec 23, 2010

Re: Did I fry the switch box?

It sounds like the harness connectors are not making good contact, separate them and check for corrosion and check your battery terminals too.

Faztbullet · Dec 23, 2010

Re: Did I fry the switch box?

The ADI systems have fuses, it should be at top of switchbox left bolt under a loom bracket. Below is link to wiring harness...fuse is #19

http://www.mercruiserparts.com/Show...ng&inbr=275&bnbr=50&bdesc=ELECTRIC+COMPONENTS

mfh6980 · Dec 24, 2010

Re: Did I fry the switch box?

Looks like the key switch is fried. I had my wife hold the switch in the "Run Start" position and checked for a complete circuit with my ohm meter. Nothing. I should see a closed circuit per the diagram, right? Might explain why the choke is not working as well since the key and the choke share the white wire which is not used on the engine harness. I know the engine harness does not have a fuse as I had to rebuild it when I first got the boat back in June. Most of the wires were corroded and crumbling.

Anything else I should check before ordering a new key switch and choke switch?

Thanks a ton for the suggestions and ideas.

Merry Christmas to all of you! I have learned a lot reading over the past few months. Hopefully I will be able to help some new guy one day as well.

Mike

1fishbone · Dec 24, 2010

Re: Did I fry the switch box?

You should ohm out the wiring...(I think you mentioned they were replaced)
Also check the back of the ignition switch with your ohm meter.
Check the switch for voltage too.

mfh6980 · Dec 24, 2010

Re: Did I fry the switch box?

Good point. It did not occur to me that the short could be between the switch and the end of the wiring harness. Will check that out. Looking at the key switches on iboats I don?t see a separate key switch and choke switch. They are all combined. Any recommendations? I was thinking of ordering this one: http://www.iboats.com/Mallory-Switc...8542398--**********.575501476--view_id.365854

I will confirm my wires are good and check the switch at the terminals as well.

Thanks

Mike

mfh6980 · Dec 24, 2010

Re: Did I fry the switch box?

OK. The wiring harness is shot. As I tried to test the key switch insulation was falling apart as well as the wires just breaking. I cut everything loose from the key switch which is tested fine but looks like it is 30 years old (which it is) so I will replace it. I started cutting back the wire to find good wire but I don?t think there is any. Just wire skeletons/remains. The harness is $230 (http://www.iboats.com/Wiring-Harnes...8542398--**********.243802016--view_id.174724) I already rebuilt the engine side wiring harness and at that time considered just building it from scratch. Any gotchas to doing that? Looks straight forward. Just home runs for each wire to the switch as follows:

Red ? Solenoid to key position A
Black ? Solenoid to key position D
Orange ? Switch box to key position E
Yellow ? Solenoid to key position B
White ? switched power from key position F used by the choke and other keyed accessories
Gray ? Choke Solenoid to whatever that is on the new push to choke key switch
Brown ? unused as I don?t have a tachometer

Thanks again

Mike

mfh6980 · Dec 29, 2010

Re: Did I fry the switch box?

I pulled the engine and wiring harness and put new wire in from the old key switch and the old choke switch. Could not get a new key switch delivered until next week and could not find one in town. In any case got everything put back together tonight and the engine cranks again as well as the choke working. Looks like the mistake I made just fried the skeletal remains of the wire in my harnesses. Tomorrow I will hook up water to it and see if it will fire up. Will post the results so the thread will be complete.
Thanks

Mike

mfh6980 · Dec 30, 2010

Re: Did I fry the switch box?

Tried to start the motor today. Cranks great but won?t start. I disconnected the kill wire and disconnected the rectifier as well. Nothing. I don?t think I am getting any spark to the plugs. Will test that tomorrow but looks like I am going to have to order a new switch box.

Thanks

Mike

mfh6980 · Jan 2, 2011

Re: Did I fry the switch box?

Hooked up a timing light to each of the plug wires and I have nothing. I do not have a DVA. Any final checks I should make before I order a new switch box?

Thanks

Mike

TheDerby · Jan 3, 2011

Re: Did I fry the switch box?

I did the same thing. Have a post on it too. "Mistakes Were Made"

I checked out the stator and found open circuit on Blue-Blue/White wires using DVM.

I too am wondering what else could be wrong while I wait for new stator to ship.

Wiring integrity is solid. I did fry the rectifier and replaced with RadioShack version. Still no Spark.

Wondering why no spark on all cylinders instead of just two.

Might want to go through the rest of your electrical components as well. Maybe we can solve this together.

mfh6980 · Jan 3, 2011

Re: Did I fry the switch box?

I don't have a DVA. I tried to ohm it out and am getting no resistance/open. Should we crank the engine when we take ohm readings? I will take a look at your post and see if I can offer any advice.

Thanks

Mike

TheDerby · Jan 4, 2011

Re: Did I fry the switch box?

No you do not need to crank for the resistance test. Pull wires off switchbox and test. Should get resistance matching the CDI charts. I would give you a number but fear our motors are different.

Check out CDIElectronics website. They have a pretty extensive troubleshooting guide for most outboards.

I don't have a DVA either but they are easy to make. Like I stated earlier with the open circuit not sure its necessary. If all the resistance readings checked out you would need to proceed with the DVA.

If you need to make one search "DVA*Diagram" or "DVA*Schematic" on all forums.

My new stator will be here on Thursday. I'll let you know what happens.

circle hook · Jan 4, 2011

Re: Did I fry the switch box?

my advise would also be 1) check the stator with your ohm meter, 2) check your trigger with your ohm meter, if they both check I would think the problem tooks like the switch box, how many cylinders do you have, on my 135HP I had (2) switch boxs and (1) went bad by switching them I was able to tell if it was a box or not. also make sure the mercury switch is ok I dont know if your engine has one or not and also check the kill wire if your engine has one of them that shorts the engine out too. good luck Bob

mfh6980 · Jan 4, 2011

Re: Did I fry the switch box?

4 cylinder single switch box. I have done the following per the CDI Electronics Mercury 4 cylinder, single switch box troubleshooting guide.

I have no spark on any cylinder.
1. Disconnected orange stop wire and retest. No go.
2. Disconnected yellow wires from the rectifier and retest. No go.
3. Check cranking RPM. Excellent. New starter. Cranks great.
4. Inspect and clean all engine and ignition ground connections. Check.
5. Verify the correct flywheel is installed. Check. The motor was running fine before I went color blind in the first thread above.
6. Check stator resistance and DVA output. I do not have a DVA meter but following are what CDI says the Ohm readings should be, what I got with all wires connected to the switch and what I got with the wires disconnected from the switch box:

Wires to check >> CDI || wires connected to pack || wires disconnected from pack
Blue to Blue/White >> 2200-2400 || 34 || 1517
Red to Red/White >> 45-55 || 35 || 49
Purple to White >> 800-1400 || 711 || 796
Brown to White/Black (Black) >> 800-1400 || 724 || 813

Based on the readings above do I have problems outside of the switchbox? As an FYI this engine got the full work over in September. The shop installed a new stator, trigger, switch box and a new starter. It was running great so any problem is related to my mistake.

Thanks

Mike

Did I fry the switch box?

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