Logo
FULL SITE MENU

80's inline 6 tower of power, starts when holding key but wont stay running when let go

M

Mad Mark

New member
80's inline 6 tower of power. Starts and runs while holding key in start position but as soon as you let it go to run it dies. Starter switch tests good. Good spark. Any ideas?! :mad:
 
I've tested it with two separate switches and both test open when the switch is at run. Everything is wired properly too. Everything up to and beyond the switchbox tests the way its supposed to according to the manual. Short of buying mercs ignition tester everything checks out with the proper resistance. The switchbox itself has no resistance tests I can perform so im replacing and praying at this point sigh... I'm third or fourth mechanic to look at this engine and I still have no definitive answer. I'm gonna try one more switch but I think its fruitless.
 
Starts and runs with the key in the "start" position... Does that mean it's running with your starter motor still engaged to the flywheel?
 
Yes it only runs while the starter motor is engaged, ie I hold the key in the start position. It turns and fires off and sounds beautiful up until you let the key go to the run position, where it then dies. So then its not actually running just being powered by the starter itself. Ill test the trigger again as well.
 
You say it starts, but then in the next couple words you say it's not running. Saying it starts is implying the motor is running. So how can it start but not be running?

If it's just being turned by the starter motor, you have a crank but no start symptom. Check compression, spark, and fuel. Compression and spark first because those are easy to test.
 
Like I said in the first get go... I have good spark, very good spark. I also have good compression, and plenty of good fuel, the engine does run(fires off sounds like a champ) but ONLY while you hold the key in the start postion and engage the starter. The switch is good its been tested, as well as replaced. I know it sounds like im retarded but its just a really weird symptom. Im not the first mechanic to look at this boat either, the man has brought it to four different shops and ends up with the same answer, nothing. In fact ill get a video and everyone can see wtf is happening. It doesnt make any sense and its even harder to explain.
 
http://www.youtube.com/watch?v=Hw4QvI6k0SU

If that isn't a running engine, im not sure what is. Yes the battery is somewhat dead, thats why theres such a hard start. Me and my partner have checked every system on the engine. If the trigger was bad the engine wouldn't run or produce the spark we are getting. The compression is good down all six cylinders. The fuel is good. We have also resistance tested every component that we could and it all checks out. We can't find anything that would point to not allowing the engine to continue running after it has started. And yes I do have continuity on my ignition switch when it is at run. If there's anything else I could have missed or am overlooking lemme know so I can check
 
I think it could be your switch after watching the video. I see you've checked for continuity and you have it, but it could also be sending your ignition power to ground when you flip back to "run." This will shut it down immediately.
 
I don't have a copy of the wiring schematic but it seems the circuit that supplies 12 vdc from the keyswitch in the RUN position to the ignition system is open. Test continuity between the keyswitch and the ignition source. You may need to open the main harness plug and check to that point (pin) then check from the other side (pin) to the ignition. Verify the pins are not corroded or loose in the socket so they actually make a connection when pressed together.

I used to work on HAWK surface-to-air missile radars in the US Army. This type of wire problem was common is a high moisture environment. See if it will run by running a jumper from the keyswitch (purple wire...not sure) to the primary ignition source. Good luck...let us know what you find.
 
If the trigger was bad the engine wouldn't run or produce the spark we are getting.
Click to expand...
Have replaced many over the years for this problem and Merc service school could not tell us why other than it was a feedback thru sensor. Also see below from CDI troubleshooting as it on the last paragraph.....

1. Clean ALL engine ground wires from the battery thru the engine cowling, splash pan, top cowl and support brackets.
2. Verify that there is at least 9-1/2 volts on the White wire going to the switch box (from the engine harness) while
cranking the engine.
3. Verify that there is at least 9 volts on the Brown wire going to the trigger while cranking the engine. If the voltage is
low and the voltage on the White wire from the harness is above 9-1/2 volts, disconnect the wires from the switchbox,
then check the resistance from the White terminal beside the Red terminal (from the White/Red wire on the CDI
Electronics switchbox) to the Brown terminal going to the trigger. You should read 60-76 ohms. A high reading
indicates a possible bad switchbox.
4. Check resistance between the White wire and engine ground. It should read less than 1 ohm. If it reads higher, make
sure the Black ground wire is connected to engine ground.
5. Connect a secondary ground wire (12 gauge) from the negative battery cable to the base (or case) of the switch box.
6. Using the 511-9710 Trigger Tester, check the output of the trigger according to the directions for the tester.
Only has spark as long as the starter is engaged: Usually indicates a bad trigger. Test per above.
Also in video I did not see the 2 switchboxes or 6 coils so this is a pre 1978 with later cowlings.
 
Last edited:
The cowling wrap in the boat does appear to be from a 74 / 75 model.-----Not uncommon for folks to sell / buy motors motors that are out by 10 years on the actual age.
 
Mad Mark said:
http://www.youtube.com/watch?v=Hw4QvI6k0SU

If that isn't a running engine, im not sure what is. Yes the battery is somewhat dead, thats why theres such a hard start. Me and my partner have checked every system on the engine. If the trigger was bad the engine wouldn't run or produce the spark we are getting. The compression is good down all six cylinders. The fuel is good. We have also resistance tested every component that we could and it all checks out. We can't find anything that would point to not allowing the engine to continue running after it has started. And yes I do have continuity on my ignition switch when it is at run. If there's anything else I could have missed or am overlooking lemme know so I can check
Click to expand...

Hi Mad Mark and all the other guys

I am new to this forum and from Johannesburg South Africa, hope you all are well, i do have the same problem, when cranking the boat engine it starts but as soon as the ignition go back to run the engine die, what did you do, or what is the problem, this only happen after i have change the ignition, i also had 2 guys here and the cant find the problem it is a Mercury 100 (1000) 1974 inline 6 engin

Regards
Riaan
 
Okay. That motor had a car-like ignition system that requires 12 volts to run. It also uses the 'by pass the ballast resister' nonsense to deliver a hotter spark for starting. It sounds like the 12 volts In- Run part of the circuit is dead. Easy enough to check: turn the key to ON and look for about 8 volts at the "+" end terminal at the coils.

Jeff
 
willy be said:
I have the same problem also,can not figure it out
Click to expand...
Post the serial Nr. so we can help.
serial-plate-002.jpg
 
Back
Top