No spark 4.3 Mercruiser

[Wayneg13]

I’m an auto mechanic, I don’t claim to be an electrical expert by any means. I got a 2002 Sun Tracker party deck 21 with a 4.3 Mercruiser, it’s carbureted. GM12552422 I think is the serial #. No spark from the coil, it uses the really old school 8140 coil. No power to the coil either. Fuel pump doesn’t turn on. Only way I can start it is running 12v to the coil but, when I crank it it starts on the acc position not the crank position Also the fuel pump turns off because obviously I have to turn the key back to off so the starter doesn’t rip apart. I’m sure there’s a short somewhere. I replaced the ignition switch and bought a universal kill switch I’m trying to figure how the wires on the kill switch are supposed to work. I assumed there was power running to the kill switch but there’s nothing. I can’t even find a wiring diagram which would be helpful. Also none of the gauges turn on unless I run that jumper wire directly to the coil.

 

[Wayneg13]

Only fuses I can find anywhere are under the helm. The wiring looks super simple compared to a modern vehicle. I’m worried it may be the ignition amplifier which apparently is like $700.

 

[Wayneg13]

This ignition switch has 4 prongs, but only 3 wires connected. No grounds which doesn’t make sense to me. Red is 12v constant, yellow/red is starter signal, purple is I don’t know. I assumed purple would be run signal or “On”. Any help would be much appreciated. .

 

[Scott Danforth]

first and foremost, welcome aboard

second, look at the stickies at the top of the forum and go thru the troubleshooting for the TB4 and TB5 ignition.

DO NOT JUST THROW MONEY AT IT. do a proper diagnostics

most likely the trigger is bad (its about $40)

 

[nola mike]

This ignition switch has 4 prongs, but only 3 wires connected. No grounds which doesn’t make sense to me. Red is 12v constant, yellow/red is starter signal, purple is I don’t know. I assumed purple would be run signal or “On”. Any help would be much appreciated. .

I can't get you to the right link on my phone, but go to http://boatinfo.no/ and look for manual #26. You can find a wiring diagram there

 

[Wayneg13]

I can't get you to the right link on my phone, but go to http://boatinfo.no/ and look for manual #26. You can find a wiring diagram there

Thank you. I just paid $90 for a service manual strait from Mercury, hopefully that has a good wiring diagram when it shows up.

 

[Scott Danforth]

you also need to go thru the troubleshooting tree.

see this thread https://forums.iboats.com/threads/how-mercruiser-thunderbolt-ignition-systems-work.676476/

 

[Rick Stephens]

What I am reading is we has no voltage at the coil - and if he jumps voltage to the coil, the motor will start. He does not have a Thunderbolt ignition problem or it wouldn't start.

I would start by cleaning all connections to from the battery. Pay especially close attention to the ground at both ends. Get them shiny clean.

Then do the same thing with the rubber multiprong connector that is on the main harness going to the engine. Pull it apart, clean each of the contacts - I use a little stainless brush made for cleaning big truck electrical plugs.

After reassembly, check that you have 12 volts at the ignition switch. Should be red coming in, and when the key is on, purple heading back to the motor. If you have power there, then check that purple is powered at the motor. Purple goes to the electric choke, the ignition and to the oil pressure switch that powers the fuel pump.

 

[Scott06]

This ignition switch has 4 prongs, but only 3 wires connected. No grounds which doesn’t make sense to me. Red is 12v constant, yellow/red is starter signal, purple is I don’t know. I assumed purple would be run signal or “On”. Any help would be much appreciated. .

Red is power from engine to switch constant as you observe is correct Yellow red is to starter , purple is ignition on. You don’t need a ground on the switch you are only switching the positive side of 12 v up there. There will be a ground bar under the helm for the negative side of the garages and accessories. One stupid thing I see is the run / ignition power gets daisy chained through the gauges on its way back to the engine which is a bunch of connections to get corroded or loose.
u should have 12 v on yellow red when key is starting, 12 v maintained on purple when in run, if you do look down stream from there

sidnt see ricks post sorry if this was redundant ...

 

[Wayneg13]

Red is power from engine to switch constant as you observe is correct Yellow red is to starter , purple is ignition on. You don’t need a ground on the switch you are only switching the positive side of 12 v up there. There will be a ground bar under the helm for the negative side of the garages and accessories. One stupid thing I see is the run / ignition power gets daisy chained through the gauges on its way back to the engine which is a bunch of connections to get corroded or loose.
u should have 12 v on yellow red when key is starting, 12 v maintained on purple when in run, if you do look down stream from there

sidnt see ricks post sorry if this was redundant ...

I was getting nothing from the purple “on” signal, no power in any position, I’m gonna have to trace that wire and find out. If it is daisy chained like that i’ll bet there’s bad connections. I’m just nit sure where that purple wire is supposed to go. No gauges come on or the fuel pump which I’m assuming the purple “on” wire is for.

 

[Scott06]

I was getting nothing from the purple “on” signal, no power in any position, I’m gonna have to trace that wire and find out. If it is daisy chained like that i’ll bet there’s bad connections. I’m just nit sure where that purple wire is supposed to go. No gauges come on or the fuel pump which I’m assuming the purple “on” wire is for.

generic merc helm diagram

Mercruiser Marine Engine Harness Schematic | PerfProTech.com

Large selection of quality mercruiser parts, marine engines, marine parts, boats parts and accessories, such as Mercuiser engines, exhaust, motors, and other boating supplies and accessories. Find affordable parts for leading brands including Volvo marine parts, Mercury marine parts, Quicksilver...

www.perfprotech.com

generic merc 4.3 carb engine diagram attached hopefully it is legible

 

[Wayneg13]

generic merc helm diagram

Mercruiser Marine Engine Harness Schematic | PerfProTech.com

Large selection of quality mercruiser parts, marine engines, marine parts, boats parts and accessories, such as Mercuiser engines, exhaust, motors, and other boating supplies and accessories. Find affordable parts for leading brands including Volvo marine parts, Mercury marine parts, Quicksilver...

www.perfprotech.com

generic merc 4.3 carb engine diagram attached hopefully it is legible

I was looking all over for that, I think I wasted $90 ordering the service manual.

 

[Rick Stephens]

First thing - unless you have fuel injection, the pump does not ever come on with the key. Fuel pump has two sources of power - the purple wire from the ignition goes through an oil pressure switch is primary. Secondary is the purple/yellow coming off the starter powers the pump during cranking.

First thing for you to check is all the connections - and clean em. Next, the red wire at the ignition switch is power from battery - that is the source for ignition switch and dash. No power to red, no power to ignition switch.

When the ignition switch is turned to on, that connects purple to power. Purple goes from there, through the emergency man-overboard switch, through the neutral safety switch and back to the big plug on the motor. On the engine side of that plug the purple wire goes to the ignition, the choke and the the oil pressure switch to power the pump.

I consider the Alpha1 Gen2 Service Manual essential to own. The Service manual for the motor, not so much. Is just the same as any SBC anywhere.

 

[Wayneg13]

I’m confused maybe this is different than a car IGNITION switch. Red is constant and I got 12v there and 12v to start signal when cranking, does the ignition switch power the purple “on” wire in the ignition switch, that’s what I was assuming. I’m not getting power to the purple wire at any position. Or does power come from somewhere else to purple?

 

[Rick Stephens]

Power to purple comes from ignition switch. Sounds like you found your problem.

 

[Wayneg13]

That’s what I thought, I just bought a new switch from basspro, hooked it up the same way but not getting any power to purple. I’ll mess with it again and swap wires around and find out.

 

[alldodge]

Key switch might be wired wrong
Red/Purple should have 12V full time, even when key is OFF. This wire is connected to the common on switch

Purple should get 12V when key is turned to run (or ON). Wire goes to kill switch then to coil and oil pressure switch

Yellow/Red gets power when key is turned to start. Wire goes to neutral safety switch then to slave relay on motor which energizes the starter

 

[Bt Doctur]

B=battery power
I=purple, ignition
S=solenoid, yellow/red slave

 

[nola mike]

Sounds like op might have the wrong switch. There shouldn't be an acc position. Unless you are doing something outside of factory wiring, nothing should be attached there. Purple is powered in the run position.

 

[Wayneg13]

Sounds like op might have the wrong switch. There shouldn't be an acc position. Unless you are doing something outside of factory wiring, nothing should be attached there. Purple is powered in the run position.

This switch, clicks once to the left for accessory. Middle is off, one click left is on and two clicks left is start. This is no position gives power to purple. Maybe it’s the wrong switch , basspro shop sold it to me, if it’s anything like the auto parts store I know these mess up sometimes.