25hp (1980 / 81) blowing switch boxes

[Nobody]

Short story - replaced a (working) 30 yr/old factory switch box at the start of the season with a Mallory switch box correct for the serial number of my engine - I did this for the peace of mind of not having to worry about it this season.

Twenty minutes after leaving the dock the motor stops - no spark, long ride back to the dock and replace the new box with the old box, spark returns and we motor on for several hours.

Fast forward a few weeks and the warranty replacement for the Mallory box has been installed and spark has been verified - twenty minutes to the T after leaving the dock the motor stops again, no spark, slow ride back, replace the box with the old factory box and we motor out for a few more hours.

Thus far I have done the resistance checks on the stator, trigger and coils per the service manual (all within spec) however I have not performed any tests with a DVA / meter or scope. (On the agenda for next week)

Of interest too is that I occasionally see arching from the back of the old factory switch box to the engine at idle, makes a unique snapping sound when it happens.

Questions - 1) Are the Mallory boxes rubbish? (cheapest of the offerings)

2) Could there be some part of my ignition system that is causing this and by coincidence the factory box has a higher tolerance for it?

3) Why do I insist on fixing stuff that isn't broken?

Ideas?

 

[CDI Tech Support]

Re: 25hp (1980 / 81) blowing switch boxes

Of interest too is that I occasionally see arching from the back of the old factory switch box to the engine at idle, makes a unique snapping sound when it happens.

Sounds like one of the connections on the switch box is arching to engine ground.

Does this switch box have stud terminal connections, like this?

http://www.cdielectronics.com/AppGuide.aspx?Criteria=114-7452A+3

Or does it have all wires, like this?

http://www.cdielectronics.com/AppGuide.aspx?Criteria=114-7452K+1

If it's the stud terminal switch box, it very well could be one of the stud terminals arching to ground, which is obviously bad news. In this case, make sure all wires attached to the switch box have rubber boots (in good condition) placed over their terminal connections prior to attaching them to the stud terminals. Any rubber boots that look deteriorated or have cracks, should be replaced.

If it's the all-wires switch box (or the stud terminal switch box's rubber boots are in good shape), remove the switch box & look for any signs of metal sticking out of the potting encapsulation. Then thoroughly clean the switch box's mounting tabs (for dirt and/or corrosion) with mild soap & warm water, then dry thoroughly, then re-mount.

Three main issues are notorious for blowing switch boxes (on all cylinders) quickly:

1. Poor switch box ground connection

2. 2+ Volts DC on the Stop wire from the harness with the key switch On and/or Off

3. Prolonged 400+ Volts DVA from the stator at any RPM

Are the Mallory boxes rubbish? (cheapest of the offerings)

Manufacturers "S" & "M"'s products are made in Germany or Mexico & offer no certified Tech Support.

CDI Electronics components are made in the U.S. & offer a 2-year warranty plus the best certified Tech Support in the industry.

 

[Nobody]

Re: 25hp (1980 / 81) blowing switch boxes

....

Thanks for the pointers.

The boxes I've used thus far have the screw terminals and not the wires, the old factory box has a ground tab that comes out under one of the screws the new boxes have a separate wire that has to be installed with the screw. The rubber boots are present and still in decent shape.

This is a tiller control motor with no external key switch so nothing coming back on the stop wire when I checked.

I'm going to attempt DVA readings this evening, since both of the other boxes failed at low speed (never made it out the no-wake zones) I'm thinking that if there is a high voltage condition on the stator it should be easy to find in the garage.

 

[CDI Tech Support]

Re: 25hp (1980 / 81) blowing switch boxes

1. Poor switch box ground connection

i.e. Make sure you check resistance from your switch box's ground point to the negative post on the battery. If using a quality digital meter, it should not measure more than about 0.3 ohms. If it does, run a new ground wire & attach it to the switch box's ground point & run it to the negative post on the battery.

 

[sschefer]

Re: 25hp (1980 / 81) blowing switch boxes

CDI Tech, is the over 400+ DVA reading a check for a bad coil or coil ground?

 

[CDI Tech Support]

Re: 25hp (1980 / 81) blowing switch boxes

CDI Tech, is the over 400+ DVA reading a check for a bad coil or coil ground?

Prolonged 400+ Volts DVA from the stator at any RPM

 

[sschefer]

Re: 25hp (1980 / 81) blowing switch boxes

Prolonged 400+ Volts DVA from the stator at any RPM

So then is it a myth that excessive rise in the voltage required to cause the coil to fire can damage the switchbox. Just curious since its one that's been floating around for a while.

 

[CDI Tech Support]

Re: 25hp (1980 / 81) blowing switch boxes

So then is it a myth that excessive rise in the voltage required to cause the coil to fire can damage the switchbox. Just curious since its one that's been floating around for a while.

Most switch box's capacitors are rated for 400V. If a stator is producing 400+V DVA for prolonged periods, it can cause a switch box to fail. It all depends on the capacitor's tolerance i.e. how long can it tolerate prolonged 400+V DVA before failing and/or how high the stator's DVA gets.

When a stator is producing 400+V DVA for prolonged periods, that means its resistance has decreased...in Layman's terms...many of its windings on the charge coil(s) are losing their protective varnish coating & are shorting together (usually due to heat & vibration). If this is the case, it's time to replace the stator.

We've had mechanics verify 400-500V DVA on a few handfuls of cases over the years. In a couple of cases, we've had mechanics verify 600-800V DVA from a stator. Obviously, a switch box won't last very long, but again, those were just a couple of cases.

However, keep in mind, prolonged 400+V DVA is rather uncommon...not rare...just uncommon.

The line (or myth) that we hear quite a bit is "The stator blew my switch box." They can, but it is not common as there are several components in a switch box that can simply fail, having nothing to do with prolonged 400+V DVA from the stator.

We only suggest checking for 400+V DVA from the stator IF a switch box fails quickly. However, the more common culprits in this instance would be:

1. 2+ Volts DC on the Stop wire from the harness with the key switch On and/or Off

2. a poor ground point for the switch box

 

[Nobody]

Re: 25hp (1980 / 81) blowing switch boxes

I would rate 20 minutes as a fairly quick failure, might have been quicker had I been at speed.

It makes sense how one box can have higher rated components in it and thus be capable of surviving in situations of high DVA from the stator so I'm very curious to see what comes of DVA testing.

In terms of a poor ground - in a pull start, tiller control, no charging system (basic) motor setup I see that the stator has a connection to ground as do the coils, if the box has a good ground which of the others having a bad ground could cause box failure issues? Or am I thinking this through incorrectly?

 

[CDI Tech Support]

Re: 25hp (1980 / 81) blowing switch boxes

In terms of a poor ground - in a pull start, tiller control, no charging system (basic) motor setup I see that the stator has a connection to ground as do the coils, if the box has a good ground which of the others having a bad ground could cause box failure issues? Or am I thinking this through incorrectly?

If the stator has a poor ground, it most likely will just fail to produce AC voltage or eventually destroy itself if the problem persists. The switch box, in theory, would just not function if there is no stator ground connection or if it's poor. So a poor stator ground should not destroy a switch box.

If an ignition coil has a poor ground, if it causes the switch box to fail, typically, it will just take out the switch box's cylinder it's attached to.

If a switch box has a poor ground, it can certainly fail on all cylinders.

Ground is THE most important connection in any electronic circuit since that's where voltage empties to. That's why it's essential to clean all ground connections (preferably with a steel bristle brush) prior to any component install.

 

[Nobody]

Re: 25hp (1980 / 81) blowing switch boxes

...

Thanks again for the excellent information as well as the helpful links in your signature line.

I noticed this evening that my stator has a black / green wire and not a black / yellow wire as indicated by the writing on the switchbox, the wiring diagram in the service manual and the info on the CDI website, it does however have a black white but based on my latest discoveries I'm wondering if someone replaced the stator at some point with a poor cross reference or just hillbillied it together.

Being of frugal nature I attempted to assemble my own DVA for use with a DMM which led to questionable readings so I broke out the oscilloscope which was mentioned a diagnostic tool in a different thread related to the same subject.

My oscilloscope knowledge has waned over years of disuse so hopefully someone can provide some insight based on the following snap shots - based on the info I would say that stator is either incorrect for my application or fried. (Oddly the resistance checks are correct per the Merc manual and CDI website info.

I recently used this unit / probe to test a DC motor controller so I'm confident that the voltage scaling is reading correctly based on my observations from that test.

Black / Green wire to ground connected to the switchbox

Black / Green wire to ground disconnected from the switchbox

Black / White wire to ground connected to the switchbox

 

[CDI Tech Support]

Re: 25hp (1980 / 81) blowing switch boxes

Are those measurements taken at cranking?

The 2nd picture shows that the low-speed coil of the stator is measuring beyond 400V DVA. Even the scope's estimate has it at 508V, which is questionable, but the graph is still too high.

If those measurements are taken at cranking, replace the stator.

Our replacement for that stator is part# 174-6617K1 List price is $148.00. We sell direct or you can purchase through iboats.

 

[Nobody]

Re: 25hp (1980 / 81) blowing switch boxes

Yup cranking speed - new stator already on order...

 

[Nobody]

Re: 25hp (1980 / 81) blowing switch boxes

Replace the stator (CDI) as well as the ignition box, (CDI) showing about 188V on the scope now.

Hoping for a good trip next week if this wind goes away.

Thanks for all the help.