Unidentified wire

[markhodges78]

Working with a 1998 mercruiser 4.3 I have a purple wire with a white stripe that is connected to a black wire. it comes out of the wire harness right where the fuel pump connects. My wiring diagram shows purple/white stripe wire is for trailer trim but it doesn't show that it connects to a black wire. Also when I disconnect it the trailer trim still works. So id figured I'd ask you guys

Picture below

Thanks Mark

 

[dubs283]

If the engine has thunderbolt 5 the pur/wht wire is grounded to set the module in service mode for idle ignition timing

 

[markhodges78]

I have the thunderbolt 4 ignition the module is mounted to the exhaust so no base mode

 

[markhodges78]

Anyone have any other ideas

 

[dubs283]

FYI thunderbolt 5 is also mounted to the exhaust riser/elbow

If you indeed have thunderbolt 4 then someone swapped out the module or harness and the wire is superfluous

 

[alldodge]

What's your serial number?

 

[markhodges78]

FYI thunderbolt 5 is also mounted to the exhaust riser/elbow

If you indeed have thunderbolt 4 then someone swapped out the module or harness and the wire is superfluous

I did not know that thunderbolt 5 was also on the manifold. Would the wire be at the front of the engine?

 

[markhodges78]

What's your serial number?

L095209

 

[markhodges78]

There is a purple/white wire at the ignition module so I will check between the 2 wires with a volt meter.. not sure why the previous owner would have kept it in base mode but I'm sure thats why he sold it for a song because it cranks up and idles great.. he must have seen wires unhooked and hooked it up and didn't even think it was causing problems

 

[alldodge]

L095209

Dubs is correct it's TBV
Remove the wire unless it's being timed

There is a purple/white wire at the ignition module so I will check between the 2 wires with a volt meter.. not sure why the previous owner would have kept it in base mode but I'm sure thats why he sold it for a song because it cranks up and idles great.. he must have seen wires unhooked and hooked it up and didn't even think it was causing problems

Agree

 

[markhodges78]

Ok so it is a thunderbolt 5 ignition...

 

[markhodges78]

Dubs is correct it's TBV
Remove the wire unless it's being timed

Agree

Yeah Thank you I was just reading a post about how to identify the difference between the 4 and the 5 and it was said that the 4 was mounted to the exhaust and the 5 was mounted to the distributor so I ended my search and just concluded that I had the 4 module and not the 5 but Thank you guys for helping with my lack of research..

 

[markhodges78]

This was the post that threw me off

 

[alldodge]

Best why to find out what should be on a motor is look up the serial number at a place like BAM

Here is your motor, look at Wiring harness and electrical components link
https://www.mercruiserparts.com/4-3l-gen-2-bbl-gm-262-v-6-1998-1

 

[markhodges78]

Next question ...
On the Thunderbolt 5 module and the audio alarm I do not get an alarm at key on position .. It is my understanding that the alarm is

A: constant on while key is in the on position due to the oil pressure switch at the distributor (I have touched the tan/blue wire in this location to ground and alarm was activated )

B: Activated when lower unit oil is low ( Its full but have connected the leads for testing and verified good connection )

C: In an overheat situation yellow wire from temp switch will send to ignition module and ignition module will activate alarm ( Connected tan/blue coming from module to ground and alarm activated )

If all this above is right then I would assume that I have a bad oil Pressure switch since I'm not getting an alarm at key on..

This 4.3 setup is new to me I just did a complete swap from 3.0 to 4.3 and the 3.0 did not have an alarm. I'm taking it out for the first time tomorrow and want to make sure that the alarms are working correctly

Thanks Mark

 

[Scott06]

Next question ...
On the Thunderbolt 5 module and the audio alarm I do not get an alarm at key on position .. It is my understanding that the alarm is

A: constant on while key is in the on position due to the oil pressure switch at the distributor (I have touched the tan/blue wire in this location to ground and alarm was activated )

B: Activated when lower unit oil is low ( Its full but have connected the leads for testing and verified good connection )

C: In an overheat situation yellow wire from temp switch will send to ignition module and ignition module will activate alarm ( Connected tan/blue coming from module to ground and alarm activated )

If all this above is right then I would assume that I have a bad oil Pressure switch since I'm not getting an alarm at key on..

This 4.3 setup is new to me I just did a complete swap from 3.0 to 4.3 and the 3.0 did not have an alarm. I'm taking it out for the first time tomorrow and want to make sure that the alarms are working correctly

Thanks Mark

yes I would think your oil pressure alarm switch is bad. You can check it for continuity. It should have continuity to a good engine ground when engine is off and open when running and you have oil pressure

When these go off its too late anyways you should watch your oil pressure and temp gauges constantly - especially first time out with a new or new to you engine.

Depending on what thermostat is in (140 v 160F) you should read 150 ish or 170 ish degrees.

Oil pressure as long as you have about 10 psi per 1000 rpms the engine is at it will live.

Gear oil bottle keep an eye on it periodically. I notice a little level (1/2" or so) drop when I am first out after a gear oil change, then I top it off and it will hold level until the next season. Even if the level is down at the bottom of reservoir there is still lube covering the gears and bearings in the upper

 

[markhodges78]

yes I would think your oil pressure alarm switch is bad. You can check it for continuity. It should have continuity to a good engine ground when engine is off and open when running and you have oil pressure

When these go off its too late anyways you should watch your oil pressure and temp gauges constantly - especially first time out with a new or new to you engine.

Funny you should say that.. I bought a complete boat with this engine in it and a rotten floor just for the Y-pipes and the upper unit with plans to convert my boat to a 4.3. The guy said that he shut the boat off to pick up some friends but then the boat wouldn't start back up. He popped the hatch and seen oil everywhere so he sold the boat saying engine was shot. I got the boat home filled it with oil started the boat up and noticed that the oil was shooting out of the oil pressure switch that controls power to the fuel pump... I replaced the switch started the engine up and it sounded great no noises and purd like a kitten. Holds about 45 psi idling. So I converted me boat from 3.0 to 4.3 I don't know why the engine survived because with no oil pressure the engine should have ran for a bit with just the fuel in the carburetor. Also when he attempted to restart the pump should have filled the carburetor again. So mybe just lucky for me idk..

Depending on what thermostat is in (140 v 160F) you should read 150 ish or 170 ish degrees.
I'm at about 150 on the muffs

Gear oil bottle keep an eye on it periodically. I notice a little level (1/2" or so) drop when I am first out after a gear oil change, then I top it off and it will hold level until the next season. Even if the level is down at the bottom of reservoir there is still lube covering the gears and bearings in the upper

Yeah my 3.0 had the oil bottle but the alarm was never hooked up on the boat I had to fish the tan/blue wire out of the condiute as it was just stuffed back in there at the gauges.

 

[markhodges78]

Took the boat out yesterday and it ran great I do have a few questions though as the 4.3 is new to me

1st when I start the engine cold it has 40psi of oil pressure idling and goes up to 60psi while under power and maintains 60psi the whole time under power. But after a 30 minute run down river when brought back to an idle the oil pressure is around 20-25 psi but goes back to 60 if rpm is increased to 1500-2000 psi is this normal for this engine?

2nd question is about propeller.. The propeller that I got with the 4.3 engine was a 4 blade stainless steel 21 pitch so that's the 1 I put on it. At 4200 rpm I was running 34.8 mph this is at maybe 3/4 throttle if I put that in a prop calculator that is 24.57 % slip. SO on the way back up I put the 19p stainless steel prop I had on there and at 1/2 to 3/4 throttle are so I was running 4600 rpm and getting 39.50 mph that is 13.60% prop slip .
Why so much prop slip on the 4 blade 21p? Should I try another 4 blade 21p or a 23p 3 blade ? What are you guys running on your 4.3 ?

 

[alldodge]

Getting into many things on this thread, all good

First a 3 blade has less resistance then a 4 blade. A 4 blade 21 equals a 3 blade 23. Going to a 19 3 blade is dropping a lot (equal 17 4 blade) and wonder why you didn't over rev

You should prop to get closer to 4800 at WOT

You get more slip with more blades because more friction and requiring more power to turn the prop

 

[markhodges78]

Getting into many things on this thread, all good

First a 3 blade has less resistance then a 4 blade. A 4 blade 21 equals a 3 blade 23. Going to a 19 3 blade is dropping a lot (equal 17 4 blade) and wonder why you didn't over rev

Yeah i knew that a 21p 4 blade was equivalent to a 3 blade 23 and idk how the 4 blade did on the original boat it was on with this same engine i never really went full throttle because i was watching my rpm and i didn't over rev because I made sure to keep my eyes on the shop tach because testing was the main reason I was out there. I was just throwed back because for that prop to be the same as a 23p it shocked me at the speed lose between the 2. I'm just not sure weather to try a 21 or a 23 3 blade next ..

Also what did you think of the oil pressures ?