Mercury Switch Box

[TRRANGER]

Is there a way to test switch boxes on a mercury 175 carbureted engine

[fishinwrench]

Yes, but you'll need a voltmeter with a peak-reading attachment (voltage is pulsed).

There is no "bench test" for switch boxes, only an output test. (150-180+v)

What is the situation....No fire at all? Or No fire to some cylinders?

[TRRANGER]

I think I'm losing cylinder at least 1 boat usually runs 60 miles hour and 5900 rpm easily but only got 47 mile an hour and a little under 5000 at wide open throttle very hard to plane off

[fishinwrench]

Have you pulled your plugs to compare how they all look? See if one plug is cleaner/dryer/or wetter than all the others ?

Either way you need to have that checked out. If one of the carbs is plugged you'll score a cylinder....if you haven't already.

[TTJJ]

nm

[fishinwrench]

Yeah, there is bias voltage between the switch boxes (white/blk wire). It isn't very common for the bias voltage to fail but it CAN happen. When it does it is usually the result of a past stator failure. So if you have had a stator failure in the past and are now having switch box failure then replacing BOTH switch boxes is probably the best thing to do if you want to avoid that ever happening.

There is a simple DVA test for the bias circuit though.

Do have a good shop perform the necessary output tests though, before throwing a bunch of expensive parts at it.