Bad MPEM - Impact to Other Systems?

[AndyC]

Hey Everyone,

I'm trying to breathe some life back into a 1995 Speedster that was in pretty rough shape when we started, and have gotten the engines at least reasonably functional, so now it's on to electrical. Based on everything that I have read here, I am pretty sure that I have the diode issue with the MPEM (blowing one of the 5A supply fuses) and plan to tackle that over the winter. But the boat has a handful of other electrical issues, and I am trying to determine if they would correlate with the failed MPEM, or if I should be trouble-shooting them individually. Basically, I don't want to waste any time trying to fix problems that will be corrected when I fix the MPEM (or have it fixed).

The 5A supply fuse that is blown is the vertical one on the starboard side, and the electrical items that aren't working are:

Starboard tach - haven't tried switching the wires to the port tach yet to see if it's a gauge issue.
Bilge blower - fuse is fine when boat is in the on position, blows as soon as the switch is turned on
Bilge Pump - fuse is fine, but does nothing when you turn on the switch. No voltage on connector when switch is on.

All of the switches tested out fine. Does anyone know if any of these are dependent on a functional MPEM? Or do I have a bad MPEM and three other problems?

 

[Waterluvr]

Hi Andy, your bilge blower isn't related to a malfunctioning 5 amp diode but that should be easy enough to run down with a test light and wire extension to get a good ground from the engine bay. It's possible the tach signal or bilge pump is related with an mpem failure / blown fuse but a quick swap of the tach or unplugging the auto and manual connections to the pump will confirm that with a slight chance it could be an issue in the pump harness.

I have been trying my hand at repairing these MPEM's and have quickly come to the conclusion that for the diode replacement it's far safer and less likely to causing any further damage attempting to remove excess potting by removing only the potting surrounding the top of the diode and cutting the legs and soldering the new one to the remains of the previous one on top of the board. A pencil flame torch to heat the potting and small razor hobby knife will get you in there quickly.

Seal it off with black RTV and it's an easy fix if you ever need to get back in there, that's a Motorola MR2535 diode btw.

 

[AndyC]

Thanks for the info! I'll swap the tach connections, and like you said, that should be pretty easy to determine whether it is the gauge or the signal. As for the blower and pump, at least now I know that I should go ahead and trouble-shoot them without MPEM dependency.

But I guess then I have a (probably dumb) follow on question, which is: Both engines ran well today, not perfect, but nothing I think a little tuning couldn't fix, so if all of these electrical problems are not dependent on the MPEM, what does that fuse actually provide supply to? If it impacts the tach, then that would be worth tearing into the module for, but if not, I'm not sure I even know what I am missing out on by leaving the fuse open. What symptoms do people typically see when they have the MPEM 5A fuse blowing issue?

Thanks again for the info!
Andy

 

[Waterluvr]

Does that fuse blow the instant you power up the MPEM or has it been intermittent? I'm going to send you some wiring diagrams and a few things to check.

 

[AndyC]

definitely not intermittent. The fuse was missing when I first opened the MPEM cover, so I went to put a new fuse in, and it sparked off of the contacts before I could even try and seat the fuse. I bailed at that point, figuring that was the reason that fuse was originally absent, and the cover missing. (not big on making sparks in the engine compartment) I am not 100% sure, but I don't think the key was on when I did that, so it might be blowing before the MPEM powers up if that helps.

Not sure if it is relevant, but the previous owner showed that the engines would start with both positive and negative terminals tied to a single battery.

 

[Waterluvr]

Perform the continuity test for the starboard engine stator and then leave that unplugged, then unplug the bilge pump's manually switched connection and automatic sensor both are located just above the pump and replace that fuse and see what happens.

 

[Sportster-2001-951C-Stock]

The avalanche diode failure causes the 5A MPEM power fuse in the MPEM to blow, so it's not clear that's the fuse you are experiencing difficulties with since this particular 5A fuse in the MPEM when blown will cause the MPEM not to work, thus the boat wont run.

Sounds like your fuse that's blowing is powering the bilge blower or bilge water pump.

I agree, the tach is a separate issue. Either the gauge is bad (often this happens if water gets inside and corrodes the movement) or the tach signal to the gauge is being disrupted.

 

[Waterluvr]

I'd like to see what happens when he unplugs both the connectors on the bilge harness and then takes a continuity reading on the demand side of the fuse location and see if there's a direct short that remains to ground.

 

[hotrodssnova]

Can you post a picture of your mpem and indicate which fuse is blowing?

 

[AndyC]

Can you post a picture of your mpem and indicate which fuse is blowing?

I think I have attached a pic with the fuse that is blowing circled in red. I did some more testing today, and it looks like the blower and bilge pump are unrelated and probably just both bad parts.

I measured from one side of the fuse holder to ground today, and there was hardly any resistance. Also tried the same with both of the wiring harnesses from the magnetos unplugged and it was still showing ground. Still pretty stumped on why it might be shorting out.

 

[AndyC]

Update and a question

I finally got a chance to dig back into this today, and it looks like everyone was right about the bilge blower and bilge pump. I replaced both parts, and they are both now working.

Bilge blower was seized so it was blowing the fuse.

Bilge pump was getting 12V just not working.

As far as the tach issue goes, I swapped the tach connections, and the starboard gauge is working when fed from the port connections, so I am not having a gauge problem, but something with the signal.

My question is: Is there a way to troubleshoot the tach signal without dependence on the MPEM? The diagrams for the '96 look like the tach signal is fed from the rectifier, which Waterluvr was already suspicious of, so maybe I get off light on this one?

Alternatively, is there a way to isolate and troubleshoot just the rectifier? I've got a fairly fancy multi-meter, so I should be able to hit the test points if I just know what I'm looking for.

Thanks as always everyone!

 

[Waterluvr]

Rectifiers will lie to you when testing them, they are really difficult to diagnose sometimes from a definitive point of pass/fail. You can take them out of the loop and make an educated decision though. Basically you can test for function by starting with a fully charged battery and just leave the three AC inputs connected and separate the connector for the red DC charge wire and see what happens.

Your operating everyting at that point from clean battery voltage with no chance of DCV fluctuation from a bad rectifier or even worse adding dirty AC into the mix from a failing unit.

 

[Sportster-2001-951C-Stock]

I always look for something more than 13.5vDC and less than 14.4vDC, typically 13.8vDC is a reasonable voltage for replacing the battery drain from cranking. A healthy fully charged 12v battery should have at least 12.5v across it many days to a few months, between uses.

I'm not sure where the tach signal comes from on your boat, I think the MPEM is the source. Check your harness for 12v gauge power (purple) and ground as well, one of these could be missing?

 

[AndyC]

Thanks, guys. I'll do some more troubleshooting this weekend and let you know what I find. I'm starting to get a real gut feeling about the rectifier, so hopefully I can come up with enough to make an educated decision.

 

[ueww40]

I happen to have the proper diode MR2535L, if you need one. $5.00 incl. mailing.