Mmdc

[Spimbo]

I have a 2004 Maristar and am having the following issues, which all showed up at teh same time?
1) SPEEDO INOP
2) CHECK ENGINE LIGHT COMES ON, AND AUDIBLE ALARMS SOUNDS FOR 2 MINUTES.
3) RADIO INOP

My votgage gage is only reading 10.5v, even thou i just replaced the batter. I understand that the system in teh boat will shut down the radio if and when the voltage guage reads below 11.5v. This is to ensure you hear the low battery alarm that sounds. I am thinking the issue is the MMDC, but wanted to get your thoughts? And any way , short of replacing it, to test the MMDC?

[ricford]

Use a multimeter and check the voltage across the battery. When boat is off, it should read about 12V. With the engine running it should be around 13.5V or so. If it's reading low when running I would guess alternator.

[ricford]

Truth is, I only clicked this thread to find out what an MMDC was. I guess I still don't know.

[Spimbo]

Multiple Module Data Computer - MMDC.

[ricford]

Ahh. I'd still start with the alternator. You can remove it and have it tested at a motor rewinding shop.

[Spimbo]

Yep, did that, in fact i changed the alternator, but no joy...

[ricford]

The computer would be the last thing I would change, because of the cost. Next I would remove and clean all the connections from the battery to the alternator, and starter. Also do the ground at the battery and the block. Then check your voltage with a multimeter. Don't trust the voltmeter on the dash.

[bturner2]

MDC should have no effect on voltage and typically when you have a problem it's with gauges not working at all. I'm thinking grounding which tends to be the number one culprit. Start at the block and clean every connection, check every connector for condition to see if you have corrosion or a loose connection. Work your way from the engine (the ground source) to the battery then to the dash. Not fun but it's typically where you'll find the problem.

[Spimbo]

I have check the ground on the block to the battery. I will check to the dash. i agree, it just does not seem like the MDC to me.

Thanks!

[bturner2]

As you're probably aware you're basically looking for where the voltage drop is occurring. If you have 12 volts at the battery and 10.5 at the terminal block under the dash somewhere between the two points you're dropping 1.5 volts. Running temporary wires for ground and hot (one at a time) and checking voltages for change will tell you which of the connections is having issues.

Like wise eliminating circuits and checking voltages will help in determining if you have a short that is drawing down the voltage. I've heard that bad diodes in the alternator can cause some weird electrical issues but it sounds like you've already eliminated that.