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Twin Merc 7.4 MPI Electrical Issue?

J

Jackthebear

New member
Vessel: 1998 Formula 330 SS with twin 7.4 MPI's with Bravo III's and about 360 hours on each. Boat has 1 battery selector switch with Off, 1, both, and 2 selector options.

Issue: Boat starts and idles great. When I give it throttle in gear, both engines accelerate but quickly lose RPM while simulataneously the voltage meter jumps from 13.8 up past 16 volts.

Troubleshoot: I ran the boat in 1, 2, and both options of the selector switch chosen and the issue remained the same. I ran the boat with no accessory switches on and the issue remained the same. I checked for a loose ground connection at the batteries (2) and at the battery bus bar. I checked the back of the selector switch for a loose connection. I check the 12V panel board for a loose connection. Everything appear to be tight and not corroded.

Question: Has anyone else had a similar issue and/or does anyone have any suggestions or ideas as to what could be causing this issue and the best way to remedy the situation.

Thanks in advance for the help.

Jack
 
Boat has 1 battery selector switch with Off, 1, both, and 2 selector options.

Theres the first problem ,you should have a battery switch for each engine
An alt will do that and more if not connected to a battery and if the batteries are 1 on each side of the engine compartment add a battery cable BETWEEN both engines manifold to manifold. Would not be surprised to find the charging systems connected together also
 
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Wow you guys are right on this stuff. Thanks for all the replies.

Sandkicker - There are 2 batteries. Do you have an idea where the block ground would be on the engine? Please don't say underneath. lol.

Thanks Bondo.

BT Doctur - Not sure what you mean. I only have 2 batteries and they are both on the port side. Are you suggesting adding a bonding cable block to block between the engines?
 
You have to separate the batteries from each other and dedicate them to each engine. You will need another battery switch like you have now, Follow the cables from the starter , they should go to the "C" common lug of the battery SWITCHES, #1 position of each switch goes to the + battery, Position 2 gets a jumper between them ,2 goes to 2 If you ignition systems have been combined under the dash at the pos and neg buss bars you need to split them. You can test what you have by hooking up ! battery and see what works and what dosent , what cranks, what dosent, With 1 battery connected, do the gauges on both engines come alive, crank, etc. Then disconnect that batery cable and test the other one. Yes , a battery cable jumper block to block , or manifold to manifold Are you USA?
 
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Do you have an idea where the block ground would be on the engine? Please don't say underneath. lol.
Click to expand...

Ayuh,..... Just follow the ground cable from the batteries ground(-) post,......

Usually it goes to a stud/ bolt on the flywheel cover,....
 
All early 330ss are wired like this... Leave it the way it is.... Chances are you have a bad ground or dirty positive connections. An alternator will produce 40 volts if it is not connected to anything.... I would start at the battery switch.. make sure it is good...
 
Lousy rainy day here in Jersey today and tomorrow. I will check the block grounds on Tuesday. Thanks Bondo.

Here's the thing...I would assume the boat left the factory with this wiring arangement and worked just fine, so what changed? I talked to a mechanical where my boat was stored over the winter and he said he had a similar problem with another boat where one of the alternators went bad which caused the other alternator to over work, but in that case the boat shut down completely. Perhaps I should have the alternators bench tested if the grounds are intact.

Lastly, would it do any good to put a voltage meter across both engines to see if there is a difference in potential? I am more of an AC guy, DC? not so much.

Thanks
 
Chris looks like I was typing my last post when you replied. I tried to private message you but your inbox is full and it wouldn't let me send the message until you clear some of your existing messages.
 
Yes Bayville, Glen Cove section. Good luck with the repower. I may call you later in the week depending on how I make out. I know you're probably busy as heck this time of year.
 
Kinda local for me too, I can`t see how the factory would go with 1 switch for 2 motors having 2 independent charging systems when the whole purpose of twins is the reliability of each system to stand alone .
Alt run wild from losing the ground back to the battery or an internal regulator failure.
 
Bt Doctur said:
Kinda local for me too, I can`t see how the factory would go with 1 switch for 2 motors having 2 independent charging systems when the whole purpose of twins is the reliability of each system to stand alone .
Alt run wild from losing the ground back to the battery or an internal regulator failure.
Click to expand...

That is what they did.. This is a coastal inland water boat..... Small cabin day cruiser.... Not an off shore fishing boat.
 
Still dosent give them the right to remove the twin engine reliability by combining electrical systems. If there are 2 independent systems you can detect a problem in one or the other, being dead in the water with combined systems and no clue as to what engine did what is stupid
 
This is just me, but that 1990's vintage electrical system isn't doing you any favors. In fairness to Formula, they set it up so that it would work. That doesn't mean it holds up under the scrutiny of today's knowledge base.
I urge you to either hire a qualified marine electrician or do the research yourself that will update the wiring to a true three bank system. These boats need a start bank, a "house" bank and an accessory bank. The two alternators allow you to charge all three banks simultaneously while maintaining full charge on the start bank so that you can always be sure the engines will start. the house bank runs your plumbing and electrical and the accessory bank is for your stereo, air conditioning, etc that you can "do without" in an emergency.
In addition, the manual 1,2, BOTH, OFF switch is deleted in favor of automatic switching. A nice battery display helps you keep tabs on it all.
You want to be sure that when you are busy hosting the Swedish Bikini Tanning Team that you don't find yourself dead in the water (but would it be so bad if you did?)
 
No I get it. Today's technology is better than yesteryears. As this boat is new to me and this will be my first season with it, my priority will be to get it working correctly and enjoy it for a while. Then I will start to upgrade systems if the boat proves to meet all my "Swetish Swimsuit Model" needs. Eventually I would like to change out the throttle/shift controls, add a GPS, rework the DC power system as mentioned here, add a windlass, and tighten up the steering.

But first, you know, the models.

Thanks all for the help. I will update this thread when progress is made finding the "current" (electrical pun) problem.
 
Update.

I was able to replicate the condition dockside. With both engines running at idle there doesn't seem to be a problem. When I bump the RPM's up to around 1800 and wait 5 minutes or so, both engines start to lose RPM's and then gain them back, lose them again, etc. At the same time, the voltage meter jumps up past 16 volts then back under 14, etc.. in time with the RPM's. I was able to determine that I could get the port engine to do this without the starboard engine running but not able to get the starboard engine to do it when the port engine was not running. So I think I have isolated the anomaly to the port engine. Wait.....Anomaly. Anomaly? I think I just found the new name of the boat. But I digress.

My next move is to have the port engine alternator bench tested for a 10 minute duration. Hopefully that will identify a faulty internal regulator. Then I'll have it rebuilt.

Oh, by the way, both engine block grounds are secured tightly.
 
A voltage drop measurement would give a better indication of the condition of the ground leads.....just my 2₵
 
Jackthebear said:
Update.

I was able to replicate the condition dockside. With both engines running at idle there doesn't seem to be a problem. When I bump the RPM's up to around 1800 and wait 5 minutes or so, both engines start to lose RPM's and then gain them back, lose them again, etc. At the same time, the voltage meter jumps up past 16 volts then back under 14, etc.. in time with the RPM's. I was able to determine that I could get the port engine to do this without the starboard engine running but not able to get the starboard engine to do it when the port engine was not running. So I think I have isolated the anomaly to the port engine. Wait.....Anomaly. Anomaly? I think I just found the new name of the boat. But I digress.

My next move is to have the port engine alternator bench tested for a 10 minute duration. Hopefully that will identify a faulty internal regulator. Then I'll have it rebuilt.

Oh, by the way, both engine block grounds are secured tightly.
Click to expand...

One engine is the problem, the other engine is following ecm commands.... The engines are possibly linked together so when one engine goes down RPM are reduced on both... Which MPI do you have? L29 or the Lingenfelter set up?
 
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