MPEM diode replacement - 1996 GTX

[Ckrawiec]

A pointer to the MPEM diode replacement instructions would be appreciated. I keep having a 12V low problem (I think) with the ski....everything else checks out that I can determine (stator output is good), but I'm not definitively convinced that I'm getting 14.4 V dc back to the battery (consistently) when it runs.....sometimes I hook up the meter to the battery with ski running and I get a nice voltage greater than 12.6 to 14.4...but other times its all over the place (jumping erratically).

The ski will be hard to start (some times) but it always wakes up on a button push. My buzzer is shot (I know, I will fix it as soon as I can...ski is out of the water at the moment but 3 hours away). Attaching the DESS key and hitting the S/S button always gets her cranking/turning over but sometimes it'll catch and start, sometimes not.

As it is trying to start, I will get a 12V low message...but once it does start the 12V low message then goes away along with the yellow warning light.

I have a love/hate relationship with this ski.....I want her to be reliable again, because when she does run, she is a beast on the water....strong, stable, fast.....but there is always that nagging feeling that you just don't want to shut her off mid-water as she may not re-start (she always has but still a little concerning).

I don't know if the MPEM diode is bad just yet...but I'd like the info in my back pocket IF I need to go there.....I know I've seen/read about this on the forum before.

 

[GGuillot]

I don't know if the MPEM diode is bad just yet...but I'd like the info in my back pocket IF I need to go there.....I know I've seen/read about this on the forum before.

I don't think it's a diode issue in the MPEM. A bad MPEM diode means the check valve function of the current is causing a short, in turn blowing a fuse. The 12 volt low is coming from the drop in voltage upon starting, it'll dip low like below 11 volts then still start. Then it'll start charging back up and the light will go out as you describe.

This past weekend I experienced this same symptoms on the GSX. After riding 3 days prior I needed to charge the battery before going back out, before I could easily go a couple weeks with it sitting and it wasn't an issue to start it. Then I rode for awhile and had to do a nearly 2 hour tow at 3-4K rpms for a fellow seadoo buddy. When I docked and went to start up the battery was nearly dead and couldn't turn the engine over. Back at the house, 12.3 volts and when starting dropping below 10 down to 9 and 8 volts when holding the start button.

The service manual says over 14 volts at the battery when running, right now I'm getting a low 13 volts and it probably fades downward while running when it (rectifier) heats up. This weekend I'm going to do more tests at idle, but I think it's a rectifier issue.

 

[etemplet]

Check your plug connections. Are you getting a no-start with the engine turning over or just plain "nothing"?

 

[Ckrawiec]

I don't think it's a diode issue in the MPEM. A bad MPEM diode means the check valve function of the current is causing a short, in turn blowing a fuse. - I have no / nor have I blown any fuses at any time...so this is a bit encouraging. The 12 volt low is coming from the drop in voltage upon starting [YES, agreed], it'll dip low like below 11 volts then still start [ In my case, sometimes YES, sometimes NO....but I always get pretty good intial cracking /turn over of the starter]. Then it'll start charging back up and the light will go out as you describe [In my case as stated earlier...my running battery voltage is ERRATIC at best...sometimes stable, sometimes not - a bit frustrating for sure...but regardless, I really like this ski nevertheless].

This past weekend I experienced this same symptoms on the GSX. After riding 3 days prior I needed to charge the battery before going back out, before I could easily go a couple weeks with it sitting and it wasn't an issue to start it. Then I rode for awhile and had to do a nearly 2 hour tow at 3-4K rpms for a fellow seadoo buddy. When I docked and went to start up the battery was nearly dead and couldn't turn the engine over. Back at the house, 12.3 volts and when starting dropping below 10 down to 9 and 8 volts when holding the start button.

The service manual says over 14 volts at the battery when running, right now I'm getting a low 13 volts and it probably fades downward while running when it (rectifier) heats up. This weekend I'm going to do more tests at idle, but I think it's a rectifier issue.

I too am also leaning on the rectifier as a strong culprit here (the MPEM diode was just something rattling around in my background brain).....I have changed it (rectifier)out twice already (so I am still searching for the holy grail of a rectifier that works)...in fact I have another NEW one in my car waiting to go in. I've cleaned, replaced and chased every "other" wire and connection through the ski and like I said....when it starts and runs, she runs great......power and speed up the yingyang. I'll get her sorted out soon enough but other priorities have taken all my time lately....new 500 SF deck on the cottage.

FYI...parts replaced to date (this season):
Battery (I will put a brand new one in just for fun - load tested first, etc.)
All new battery and starter cables (fabricated myself, new ends and crimps)
New Starter
New rectifier(s) - 2 so far
New ignition coil
New solenoid
New spark wires and plugs
All wiring and connections cleaned and tested for continuity throughout the ski, everything was tidied up, bundled and zip tied to the pads
New gas gauge and fuel lines throughout, new air/water separator (should also replace the fuel selector switch - it's on the list, didn't have one during the fuel line replace operation...dummy me
New oil lines throughout and completely removed, cleaned and re-installed the oiling system (tank, etc)
Rebuilt carbs with MIKUNI kits, all throttle and choke and oil cables adjusted for synchronization
New seat covers (old ones were missing foam and ratty)
New foot pads (BlackTip self stick black diamond pattern pads) from back to front (looks 100x better now than a raw white deck)

 

[Ckrawiec]

Check your plug connections. Are you getting a no-start with the engine turning over or just plain "nothing"?

Starter will crank well and sometimes it'll catch and start, sometimes not....battery voltage will drop during cranking and then starting voltage will fade away if it does not start after a crank or two.....
An order of business will be dropping a new (load tested) battery in there to see if that'll perk things up......then I still think I'll be chasing the rectifier (again).....
I also keep having this feeling like the carbs are dry (not getting fuel when she starts cranking).....so maybe another go around with the carbs will be in order as well.....if she starts immediately with a little pre-mix down the throats (before I do any battery work)....that might point me directly back into the carbs (again). She just needs a little more love and patience.....I think I'm close but not quite close enough just yet!

 

[etemplet]

Are you getting fire? If you are not, I'd suggest after checking all else that you open up the front mag cover and have a look in there.

 

[Ckrawiec]

Are you getting fire? If you are not, I'd suggest after checking all else that you open up the front mag cover and have a look in there.

Yes, good spark on the plugs.
--> front mag cover < -- do you mean the cover on the fuel pump on the MAG side carb? I did once have a stupid issue where I couldn't get the ski started....UNTIL i realized that when I re-installed the carbs I forgot to attach the vacuum line from the engine to the carb (pulse)....I was DUH! I've been in there already, plastic check valves and gaskets are all in place, and squeeky clean.....but I'll be double/triple checking again (for S & Gs)

 

[etemplet]

If you are getting good spark I wouldn't even open this up. I bought a ski that would only start every now and then. Most of the time "no fire" I bought it in this condition. The Mag cover is the forward most part of the engine and you have to removed the front motor mount to remove the cover. As I said... I wouldn't check this if your getting fire to the plugs consistently. This is the internal side of the cover. You don't need to do this which is a good thing.

 

[Ckrawiec]

Ahhh, got ya.....you meant the magneto cover. Understood now.....I have tested the stator output voltage (without opening it up - 3 yellow wires from the stator output plug to the rectifier) and she is putting out the proper AC voltage...the ski is also getting the proper pulses to fire the plugs (as she does start and run well - when she does).

I've pretty much ruled out "mechanical" issues.....more electrical and/or fuel issues at this point [I BELIEVE].