Loss of spark under load.

[Fun Times]

Re: Loss of spark under load.

I just need to find where it's happening. My volt gauge always reads 12.5-13.0v when running. I use the readout on my GPS which is always 13.4-13.8v.

Sounds like your GPS is connected up to your/a purple wire under the dash for 12v power? If so, like bruceb58 mentions, Check for good 12v + at the ignition switch, especially the purple wire as that is heading back to your coil.

Today I checked purple wire at the coil and found it was getting 9.5v with the key on, once the engine was running, coil voltage went up to 10.5v. I found the rest of the electrical system getting 14.3v.

Coil voltage remains the same, are my readings within spec or is it supposed to see at least 12v at the purple wire?

Since you should have the have thunderbolt IV ignition system, you should have full 12v at the coil. As a test, You may want to disconnect the purple wire at the coil, choke, alternator to make sure non of them are restricting 12v flow.

Also check for full 12v at the lanyard kill switch wiring and at the wire harness cannon plug at the engine.

 

[500dollar744ti]

Re: Loss of spark under load.

Yes....

Basic circuit is power from the battery goes through a circuit breaker, then the harness plug, then the fuse in the red wire up by the key, then through the key switch and back into the harness, then through the harness plug and into the engine harness to the '+' side of the coil...

Oh, somewhere up by the dash side of thing you might have an ignition kill switch. Should be in the remote control box, but may or may not be wired in...

HTH

I'm not sure where to look for the kill switch, is it something that would be bypassed behind the dash if unused? I don't have a lanyard or anything like that.

I'm going to check the voltage into the ignition switch then the voltage out to the coil today.

 

[achris]

Re: Loss of spark under load.

Probably on the remote control box. There might be a switch in the side of it... If no switch, then you may not have one (and therefore don't need to worry about it)

Chris.........

 

[500dollar744ti]

Re: Loss of spark under load.

I finally traced the purple wire to the back of the console and found it's tied in to the other power wires at the gauges. It actually connects at the trim gauge and I got 10.9v there, the same as at the coil. I followed the wire to the ignition switch where 12.8v is going in and I got 12.6v coming out. As I follow the wire from gauge to gauge it slowly comes up from 10.9v to 11.4v, then 11.8v, etc... I think the resistance in the switch is cutting the amperage it can provide thus dropping the voltage.

I don't see a huge voltage drop at the switch terminal but it's probably still the switch as the voltage drops by the time it gets to the coil.

I'm going to jump the ignition switch terminals and see if the voltage at the coil comes up. I don't like the way the wiring is set up by mercruiser. The coil and ignition module should have a dedicated feed, they shouldn't mooch off power that goes to all the gauges and instrument lighting.

 

[bruceb58]

Re: Loss of spark under load.

Are these measurements you are making with engine running?

 

[Fun Times]

Re: Loss of spark under load.

You have 12.6 at the purple wire at the key switch but it sounds like the power drops off at one of the gauge terminals before heading back to the engine.

Usually on most boats the purple wire gets it 12v power from the key switch before sending it back to the gauges then on back to the engine.

So you seemed to have lost some power somewhere between the gauges to the engine.

Follow wire # 5 in one of the wiring diagrams starting on page 4F-13 keeping in mind that the 12v power is heading back to the engine starting at the key switch, http://www.boatfix.com/merc/Servmanl/16/16F4R2.PDF

 

[500dollar744ti]

Re: Loss of spark under load.

I tried jumping the ignition switch terminals and that didn't change anything.

However, I did find the problem. I don't know who to blame, I'd like to blame the previous owner but it may not be his fault.

There's a big power feed coming from the main harness, it's red/purple, I think it comes from the circuit breaker on the engine. Anyway, it goes into the fuse box and attaches at one side of a 20A fuse. However, someone, in their infinite wisdom decided they should attach some of the world's finest 24 gauge wire from the ignition switch to the other side of the fuse.

So, feeding all my gauges and ignition system is this puny wire which is essentially a resistor. I ran an equivalent size wire as the one feeding the fuse to the ignition switch. Voila, my coil is now reading 12.1v with the engine off.

For me that wasn't good enough considering battery voltage was 12.6v. I looked again and found more of the finest 24 gauge wire going from the ignition switch to the tachometer, took that out and replaced it with appropriate sized wire and guess whose coil now has 12.6v key on and 13.9v engine running!

Oh and surprise surprise! Guess whose volt gauge is now accurate? I haven't seen that volt gauge read anything over 12.5v, ever. Now it's at 14v with engine running.

 

[bruceb58]

Re: Loss of spark under load.

That's great. You also learned a lot about troubleshooting voltage drops that will always be valuable.

 

[Fun Times]

Re: Loss of spark under load.

Nice job.

 

[achris]

Re: Loss of spark under load.

Mula de'yets..... :thumb:

 

[500dollar744ti]

Re: Loss of spark under load.

Thanks for the help, I printed the wiring diagram which was a huge help. I then stuffed it in the cabin with my tools in case I need it out on the water.

Don't know if I'll get out again this season but it will be ready for next year. I'm in the midst of re-sealing and securing the box joint around the boat. I did the bow last year and now the rest of it needs to be done.