1988 Mercury 90hp Switchbox voltage readings

[Schnidly]

Dear Forum, Im hoping some of you can give me your interpretations of these switchbox readings. These I took with a DVA meter I recently purchased. I set the meter on the 400 Volt setting for the DVA and this is what I got with the motor running at around 1000rpm on my onboard tachometer. All these readings are on the switchbox studs. From the stator: blue to engine ground was 280volts, red to engine ground was 90volts. The next readings are for each of the three coils, the wires going to the coils to the grounds on the switchbox: green wire to ground: 210volts, green/white wire to engine ground: 200volts, and green/red wire to engine ground: 200volts. The next readings are from the trigger to the switchbox: brown to white/black: 7volts, white to white/black: 8volts, and purple to white/black: 9volts, brown to engine ground: 2volts, white to engine ground: 3volts, and purple to engine ground: 3volts. I'm leaning towards maybe a failing trigger? I have intermittent spark on the green/red wire feeding from the switchbox to the coil at times. PLEASE ADVISE. The motor is a 1988 90hp Mercury outboard with a serial number of 0B302485. 3 cylinder. Forgot to mention I already replaced the stator with a CDI stator.

 

[oldman570]

Re: 1988 Mercury 90hp Switchbox voltage readings

Sounds as tho the switchbox could be going bad as they are usually are the problem with those motors. I've had to replace mine on a 70hp twice already. Not sure about your readings right now as I do not have my service manual with me.JMO
Oldman570

 

[CharlieB]

Re: 1988 Mercury 90hp Switchbox voltage readings

It is either the switchbox or the trigger.

You need more testing.

Trigger wires flex and can break inside the insulation causing intermittant misfiring when the wire ends fail to connect.

Retest the triggers. Disconnect the throttle cable, remove the spark plugs and ground all the plug wires to save your battery and ease the load on the starter.

As you test each trigger, move the throttle/timing lever from idle to WOT and back while watching for any drop in trigger output. Wiggle the wires. Retest each trigger in the same manner.

With NO RETURN on electrical parts and at the price of parts today, you want to be double sure that you can PROVE any fault before ordering parts.

 

[Schnidly]

Re: 1988 Mercury 90hp Switchbox voltage readings

Looking at CDI's readings mine all seem to be within specs. My next question is the wires coming off of the trigger and going to the switchbox are those three wires from the trigger one for each coil pack? if so my problem might be the trigger wire going to the green/red wire to the coil pack might be broke??? Im not sure how the wires from the trigger correlate to the coils?

 

[boatguya1]

Re: 1988 Mercury 90hp Switchbox voltage readings

Dear Forum, Im hoping some of you can give me your interpretations of these switchbox readings. These I took with a DVA meter I recently purchased. I set the meter on the 400 Volt setting for the DVA and this is what I got with the motor running at around 1000rpm on my onboard tachometer. All these readings are on the switchbox studs. From the stator: blue to engine ground was 280volts, red to engine ground was 90volts. The next readings are for each of the three coils, the wires going to the coils to the grounds on the switchbox: green wire to ground: 210volts, green/white wire to engine ground: 200volts, and green/red wire to engine ground: 200volts. The next readings are from the trigger to the switchbox: brown to white/black: 7volts, white to white/black: 8volts, and purple to white/black: 9volts, brown to engine ground: 2volts, white to engine ground: 3volts, and purple to engine ground: 3volts. I'm leaning towards maybe a failing trigger? I have intermittent spark on the green/red wire feeding from the switchbox to the coil at times. PLEASE ADVISE. The motor is a 1988 90hp Mercury outboard with a serial number of 0B302485. 3 cylinder. Forgot to mention I already replaced the stator with a CDI stator.

Do you have intermittant spark on that cylinder or do you have intermittant 200v on the green/red wire? If it is spark I would swap that coil with another one and see if the problem moves. You can do the same with the trigger wires, swap the problem cyl trigger wire with another, do non running spark test (cranking), if intermittant spark follows moved trigger wire you have a trigger problem. Does your spark box have terminals with no wires on them? Some of these motors used a 4 cyl spark box and only used 3 circuits in them leaving a 4th unused ign circuit you could try.

 

[Schnidly]

Re: 1988 Mercury 90hp Switchbox voltage readings

I had intermittent spark on the spark tester on the cylinder and I also had no voltage readings at the coil and no voltage readings at the terminal on the switchbox, this was intermittent as well. So intermittently there was nothing from the switchbox terminal all the way to the spark plug, but then I'd have voltage, 200 volts which is normal to that cylinder all the way from the switchbox to the spark plug.

 

[CharlieB]

Re: 1988 Mercury 90hp Switchbox voltage readings

Re-read my post #3

A broken yet abutted trigger wire can cause this.

PROVE the fault before you order any NON-returnable electrical part.

IT isn't often that a switchbox fails intermittently, it IS more likely that the trigger wiring fails.

Test, test, and test the triggers again to be CERTAIN.

 

[Schnidly]

Re: 1988 Mercury 90hp Switchbox voltage readings

OK Charlie, thank you, if I connect an ohm meter to each trigger wire while flexing and bending the wire, ever so gently, I should not get an open reading at all, the most I should read is 1400ohms and no less than 800 ohms in my manual. This on each wire from the trigger. I will also start the motor and check each DVA output on the switchbox where the trigger wires are connected and flex the trigger wires while watching the DVA output. Does this sound like the best method to approach this since I will be running these tests and checks myself?

 

[CharlieB]

Re: 1988 Mercury 90hp Switchbox voltage readings

Sounds like a plan.

 

[Schnidly]

Re: 1988 Mercury 90hp Switchbox voltage readings

I just have one more question Charlie before I post the results of my testing. Does each trigger wire, white, brown and purple correspond to each wire going to the 3 coil packs for my motor? I mean, the trigger wires on the switchbox lugs, do they line up on the switchbox with each output from the trigger to the coil?

 

[CharlieB]

Re: 1988 Mercury 90hp Switchbox voltage readings

Yes, three triggers, each triggers a separate 'section' of the switchbox to fire each cyl.

So one bad trigger wire will only affect the one cyl.

As a test, you can switch two triggers but you also have to switch the coil leads on the switchbox. So instead of the switchbox being 1, 2, 3, you could have it 1, 3, 2, the switchbox doesn't know the firing order.

By switching you can determine if it is the one section of the switchbox or a bad trigger.

You should see a trigger problem with careful DVA testing while moving trigger wiring.