[sidedraw]

#4 coil no fire all others OK,installed new coil still no fire.swapped switch boxes now no fire on #3 but do have on #4.so installed the new switch box which goes to 135 and the good old one to 246.But now old one completely bad. Is there something else that can short the switch box out?I am going to have to buy a new switch box but do not want it to go bad also. Book I have had me ohm the trigger with a reading of 1100 to 1400 OK and stator between two yellow wires of less than 1ohm wires to ground with no continuity.Merc V-200,1990,oc217777 Thanks

[j_martin]

The switchboxes are tied together with a bias circuit. (the jumper between them)

Before I bought any of those expensive, unreturnable parts, I'd do the DVA testing on the ignition system. (Stator, trigger, coil signals)

Then......
Put the old switchboxes in the original position, and verify the problem.
Then remove the bias jumper.

If #4 comes to life, replace the switchbox on 1-3-5
If it doesn't, replace both switchboxes.

Hope it helps
John

[sidedraw]

Never have used or own a DVA tester..I will check it out..Thanks

[j_martin]

Originally Posted by sidedraw

Never have used or own a DVA tester..I will check it out..Thanks

Systematic diagnostics helps contain parts costs for sure.

John

[sidedraw]

OK have a manual that tells which wires I need to hook up too and a DVA meter..put all original parts back in their place but I need some guidance because there is not a lot of help from the manual... are all test done with the wires you hook up to disconnected from it's component ??? and do you just turn over engine without starting it to get the readings

[j_martin]

I don't have any idea which manual you have, but generally if it doesn't tell you to disconnect a wire, you leave it in place. That way the reading can reflect the health of both the driving force and the load.

Don't forget the safety stuff. It's sheer hell to have an engine fire with the prop against your shin.

hope it helps
John

[sidedraw]

Question...
180V to blue wire and 30V to red wire inner switch box

Have no reading on blue/white or red/white wires on outer switch box.

disconnected all wires and swapped the position of the 2 switch boxes and hooked back up...now have 180v on blue/white and 30 on red/white but nothing on blue or red...

Swapped out one switch box with a new one and now have readings on both switch boxes.

Question is.. Why am I thinking the DVA reading at the ends of the red and blue wires would be what the stator is producing and nothing to do with the switch box... Kind of like you would check for an 110v at the end of the wires hooked up to a receptacle off a generator. Why would changing out the switch box make a difference?

[j_martin]

The stator is current limited by design. If the output is shorted, it won't smoke or anything, it just won't be there.

If the bug moves with the switchbox, it's the switchbox that's bad.

These switchboxes have a terribly complicated integrator circuit for bias compensation, and they're tied together for averaging. They can fail in a way that the bias crossover from one will take the other switchbox out.

When troubleshooting switchbox problems, you should remove the bias jumper between them. If the bug moves or goes away, the bias circuit in the other switchbox is defective.

At any rate, when you find this sort of trouble, both switchboxes need to be replaced as the bias circuit has been compromised.

Used switchboxes in pairs can be found. Just check them out carefully when you install them. Bad switchboxes can and do burn down powerheads.

The part numbers of both switchboxes need to be the same. Folks that have played a lot with them find little difference between "high performance" and standard switchboxes, except the price.

hope it helps
John

[Faztbullet]

To check the bia's circuit, disconnect the White/Black jumper between the switch boxes and check the resistance from each boxes White/Black terminal and engine ground. You should read 14-15,000 ohms on stock switch boxes. MAKE SURE THE READING IS THE SAME ON BOTH SWITCH BOXES! Any problem on either bias circuit and BOTH switch boxes must be replaced as a set as the bad circuit will damage the new box.(Per Mercury manual)
Racing boxes will read in the 9,200-9,800 ohms range.

[CharlieB]

While the factory manual is correct in recommending that you replace BOTH boxes when either develops a Bias problem. The CDI brand boxes are better built units and you need only replace the one box with a bad Bias, saving you a bunch of money without risking further problems.

Also note, Mark the bad Bias box clearly 'BAD Bias' and shelf it, it will work just fine on many of the three cyl motors, you could save someone else some money and also recover a part of your new box costs.