Mercruiser 7.4 from 1999 will not start (starter not turning), sometimes

[shellone]

Hi ! Can someone help me troubleshoot the following issue: I have a perfectly running Mercruiser 7.4 (serial OF 437348). Only problem is, sometimes turning the key will not start the engine, the starter will not turn, the voltage does not change (drop). It is as if I tried to start the engine while in gear. Initially it seemed that moving the throttle around the idle position would help, as if there was a bad contact in the switch behind the throttle, but now I have found it has nothing to do with the throttle and the switches therein. Some time ago I had an issue with a 5.0 V8 where the engine would start and go off after approx. 10 seconds ... it was a bad contact at a sensor for the oil pressure. Is there some sensor that might be the cause now preventing the starter to turn? Randomly, sometimes after a couple of minutes, sometimes just trying again, will start the engine as if nothing was wrong. The Key lock is OK, hot wiring there does not help, hot wiring at the relais always helps - means the engine always starts and then runs fine, with the right oil pressure and all.

 

[Eric 3.7LX Alpha 1]

Click on this link for a troubleshooting guide. If you go to the Adults Only thread at the beginning of this forum it's full of great info.

https://forums.iboats.com/threads/how-to-troubleshooting-your-engines-starting-system.166663/

 

[Scott Danforth]

start with cleaning all the battery cable connections between the battery and the motor. alsocheck the quality of the slave solenoid.

if that doesnt fix it, then pull the starter and have it tested

 

[shellone]

Click on this link for a troubleshooting guide. If you go to the Adults Only thread at the beginning of this forum it's full of great info.

https://forums.iboats.com/threads/how-to-troubleshooting-your-engines-starting-system.166663/

Thanks, went through the list and checked everything all to work but the neutral switch. I found two cables (yellow and yellow red) come out of the throttle box but do not know how to test the neutral switch ... and do not find the info on that great guide ...

 

[shellone]

start with cleaning all the battery cable connections between the battery and the motor. alsocheck the quality of the slave solenoid.

if that doesnt fix it, then pull the starter and have it tested

Have checked major connections, the solenoid and the starter to all be good ... in fact many times it works just fine and cranks fast. It is just that, sometimes, the system is interrupted, just like if I tried starting with the gear engaged. Doing nothing it might work at the next try. Do not know how to test the neutral switch and do not know which sensors would interrupt power to the starter ... guess that is where I should be looking ...

 

[Scott Danforth]

Intermittent connection

 

[shellone]

Intermittent connection

Yes, obviously there is some connection that at times is interrupted ! Do you have any suggestion where I might start looking for the cause ? I checked the ignition key lock, the neutral switch (shorted it temporarily), the solenoids and all contacts and cables bringing power to the starter. My feeling is some kind of sensor that inhibits the starter to turn, or better the contacts with some sensor ... as said, I had this oil pressure sensor that had a bad contact and caused to engine to shut off 10 seconds after start. In this case it could be a sensor that caused the engine to just not start, but which and where ?

 

[nola mike]

Yes, obviously there is some connection that at times is interrupted ! Do you have any suggestion where I might start looking for the cause ? I checked the ignition key lock, the neutral switch (shorted it temporarily), the solenoids and all contacts and cables bringing power to the starter. My feeling is some kind of sensor that inhibits the starter to turn, or better the contacts with some sensor ... as said, I had this oil pressure sensor that had a bad contact and caused to engine to shut off 10 seconds after start. In this case it could be a sensor that caused the engine to just not start, but which and where ?

Naw, no sensors from what I see on that engine that would do it. The yellow/red should energize the slave solenoid. That wire gets 12v when the key is turned to start. The neutral safety is between the ignition switch and slave. You're saying you jumped across the safety switch and it still didn't start? You need to see if you have 12v at the yellow/red on the ignition switch when turned to start, at the front and back end of the neutral switch (if you have easy access--if not, I'd check the slave first), and at the slave. Since it's intermittent you'll have to do it obviously when it isn't starting.

 

[shellone]

Naw, no sensors from what I see on that engine that would do it. The yellow/red should energize the slave solenoid. That wire gets 12v when the key is turned to start. The neutral safety is between the ignition switch and slave. You're saying you jumped across the safety switch and it still didn't start? You need to see if you have 12v at the yellow/red on the ignition switch when turned to start, at the front and back end of the neutral switch (if you have easy access--if not, I'd check the slave first), and at the slave. Since it's intermittent you'll have to do it obviously when it isn't starting.

Commander Mike ! YES ! You made my day. The confidence with which you ruled out sensors gave me a new push and I actually and finally made a good connection of the neutral safety bypass (welded and isolated) which probably was as intermittent as the switch itself and after that no more failures. Primed engine came up running as new, restarted twice, waited two minutes, new start, waited five minutes, last start for now ... statistics tells me that was it ! THANKS !!

 

[Scott Danforth]

so basically the troubleshooting in post #3