Possible short 7.4 Mercruiser

[brygervais]

Hoping SOMEONE has seen this. One 1990 7.4 engine I've determined has a short somewhere. Not sure how or why, but one engine is not affected and the other is. I thought it was fuel related so I emptied the tank, replaced the fuel pickup, the fuel pump fuel line, water separator and fuel filter. Still having issues. Sometimes if both engines are running it's ok, but alone 80 percent there is issues. So while the engine was running I discovered if I shut the battery switch off it ran nice and smooth off the new alternator (replaced that also, I don't believe previous one was correct). Turn the battery switch on won't rev over 1000rpm and just choughs and chokes. Shut battery off again and it runs smooth. Sounds like a short somewhere??? Any ideas???

 

[dubs283]

turning off the battery switch with the engine running is not good for the alternator, but i guess for troubleshooting purposes it allows you to see that is does have an effect.

i would start by removing and cleaning the battery cable terminals at both ends, i.e. the battery side and starter/engine ground side. my guess is you have an issue with the ground (-) cable. if this remedies the situation i would just replace all the battery cables

what is the age/condition of the battery?

 

[alldodge]

Use a voltmeter and measure the batteries when running.

 

[brygervais]

turning off the battery switch with the engine running is not good for the alternator, but i guess for troubleshooting purposes it allows you to see that is does have an effect.

i would start by removing and cleaning the battery cable terminals at both ends, i.e. the battery side and starter/engine ground side. my guess is you have an issue with the ground (-) cable. if this remedies the situation i would just replace all the battery cables

what is the age/condition of the battery?

1 battery is brand new this year the other battery is 2019. 1 battery died over the winter, I felt which might of been because I left the mercathode thinggy connected?? I will check the cables, at the battery they look good and when I installed the (1) new battery I cleaned, inspected and re-installed with dielectric grease at the connections. I recently purchased this last Sept. and feel this may of been an on going issue. I'm finding various things changed and when I ask its "oh I just didn't like the look of it"
I'll check the connections at the starter (which isn't easy on one motor, the one having the issue). Also recently changed plug wires because I thought I heard an aching, and like everything else I'm not sure when they were last done.

 

[brygervais]

Use a voltmeter and measure the batteries when running.

Yes, I recently did that. While running it goes up to about 14.1. One battery is new this year and one is 2019. I'm going to check all connections from battery and starter

 

[alldodge]

Do both alts charge both bats at the same time or are they separated where each ALt charges only one bat? Should be each charge only one

Most twin motors will be setup either
one start and one house or two start and one house bat. One ALT will be used to change the house bat

Is this a new to you boat?

 

[brygervais]

Do both alts charge both bats at the same time or are they separated where each ALt charges only one bat? Should be each charge only one

Most twin motors will be setup either
one start and one house or two start and one house bat. One ALT will be used to change the house bat

Is this a new to you boat?

I BELIEVE whatever I have the battery switch on both motors will charge. I have 2 batteries, so if the switch is on one only one draws and charges, if on 2 only 2 draws and charges, if on all it draws from both and charges both, I BELIEVE?? The house draws from the same battery or batteries as the motors. Yes this is a new 1990 Carver Allegra to me, my first dual stern drive boat. On the back of the battery switch there are 3 studs (1) battery #1, (1) battery #2 and the third has the output with 3 cables on it, the house and I believe the port and starboard motors, guess to the starter

 

[alldodge]

Should not have 2 ALT's charging the same batteries all the time. Its ok to start and get things running, but once running they should be separated

 

[brygervais]

HMMM, Someone else suggested maybe separating and isolating also. Have 1 battery for one engine and the other motor for the other battery?? That would be interesting if that's the issue. I'll give it a try this weekend. I was planning on being on the boat but crappy weather in Mass. The plan is to go there Sunday and I'll work on separating them

 

[brygervais]

Good morning, 4th of July weekend I was not very productive. Replaced my coil still no luck, test 12v at coil with key on. Going to test ThunderBolt 4 ignition, maybe this has been going and now died??? It won't even start now, not even with starting fluid :-(...

 

[alldodge]

Maybe but the troubleshooting guide will let you know

 

[brygervais]

Good morning, has anyone used a trouble shooting guide for Thunderbolt 4 ignition module? New to me, I'm going to search but any recommendations would be appreciated.

 

[nola mike]

Good morning, has anyone used a trouble shooting guide for Thunderbolt 4 ignition module? New to me, I'm going to search but any recommendations would be appreciated.

How Mercruiser Thunderbolt ignition systems work.

Mercruiser Thunderbolt ignition systems. Mercruiser introduced the Thunderbolt IV onto their engines in 1982, alongside the introduction of the 1-R drives. The TB-IV system was used until the introduction of TB-V mid-way through 1996. Thunderbolt V was an improvement on an already exceptionally...

forums.iboats.com

 

[brygervais]

Seems like it's the ignition module. It seems like it was speractic but now died. I have 12 volts going into the module but don't have it on the white/red coming out. So now I'm going to replace my Thunderbolt 4 with something, not sure what. From what I've read Thunderbolt is no long avaliable

 

[alldodge]

Agree TB4 is no longer made, IMO best replacement is Delco EST

 

[brygervais]

My buddy on the slip next to me actually has the module in need. We are going to verify my grounds and try this out. If it does work I'm going to put this project off until the winter and replace with a newer system (maybe the Delco EST). Summers are too short here to be working on the boat, lol.