No start. Wire harness problem?

Unchained · Post by **Unchained** » Fri Sep 23, 2011 9:04 am

I think there's a short in my harness so I want to unplug it and start the motor down below to see if she runs and narrow down the problem. What is the best way to do this? I have replaced the ignition sw., coil, ignitiion module, cap, rotor, plugs, battery and a few sensors. I had it running for a few minutes last week, but noticed my t-stat wasn't opening so I replaced it. I tried starting it up yesterday and it just cranks. I checked a few things and noticed sometimes I am getting good spark and sometimes nothing. It's like there's a bad connection somewhere. I have the Prestolite breakerless type dist. so there are no points and no resistor in the circuit. With the ignition switch on I am getting about 10 volts at the coil, 13 at the battery. When I crank it, I sometimes I get 4, 2 or 0 volts at the coil and battery holds around 11.5. I have 3 other brand new coils that I have tried and an aftermarket Pertronix ignition module but still the same results. Should I unplug the harness and check continuity between the 8 terminals on each end? Not sure what to look for. Any suggestions?

TADTOOMUCH · Post by **TADTOOMUCH** » Fri Sep 23, 2011 10:21 am

Need more information.

Boat type, age, engine type, displacement, manufacturer, type of fuel gas or diesel, any other useful info.

Unchained · Post by **Unchained** » Fri Sep 23, 2011 12:19 pm

Sorry about that. It's a gas powered 1987 Trojan 10 Meter with twin Crusader 454's model 350.

TADTOOMUCH · Post by **TADTOOMUCH** » Fri Sep 23, 2011 3:11 pm

you might try unplugging the main harness plug from in back of the engine and look for corrosion and clean it out and spray with a good contact cleaner spray and reconnect it. Sometimes those get corroded and cause all kinds of issues. when you un plug it make sure the main battery switches are off.

Unchained · Post by **Unchained** » Fri Sep 23, 2011 3:48 pm

I tried that and also checked for other loose or corroded connections. I have replaced and/or cleaned all I can find. The 8-pin connector body itself could have some corrosion inside but its a molded piece so I can't tell. What is the easiest way to start it from below with the harness unplugged? If it starts and runs, this will narrow down the possibilities a lot. Thanks. Mike

Big D · Post by **Big D** » Sat Sep 24, 2011 10:33 am

Turn on the blower. Key off. With a jumper wire, apply a 12 volt source (battery) directly to the positive terminal of the coil. Jump start the starter (remote starter switch is easiest and safest. Remove jumper to coil to stop the engine.

You should aslo know that a problem with a tach can cause ignition problems too. Disconnect the grey wire at the coil and try it again.

Check that all your battery cables are free of corrosion at the battery and block (ground, starter etc). Do a propper load test on your batteries too.

Unchained · Post by **Unchained** » Sat Sep 24, 2011 1:53 pm

Doesn't the gray wire act as the ground source for the coil? If I take that wire off it will still fire?

prowlersfish · Post by **prowlersfish** » Sat Sep 24, 2011 2:05 pm

I belive its the tach lead . the coil has no ground lead

Unchained · Post by **Unchained** » Sat Sep 24, 2011 3:14 pm

I'll give that a try. One other thing I did notice I'd that the center RPM gauge for the right and left engines pegs the needle on the port side when I place the starboard(no start motor) ignition switch in the on position. When I put the port ignition switch in the on position the needle stays at 0. Could this mean something?

Big D · Post by **Big D** » Sat Sep 24, 2011 10:10 pm

Grey wire at the coil is typically for the tach. There should also be a black wire on the same negative terminal. This is the coil's ground that's switched on and off by the points or pick up in the distributor to charge and collapse the field in the primary coil winding thus inducing a high voltage in the secondary winding. A faulty tach can ground out the coil all the time or partially thus preventing a collapse in the primary winding. No collapse, no high voltage. You should also disconnect the sync gauge as you described, it can cause the same fault and is the one thing that can pass an electrical fault from one engine to another if deffective.

Unchained · Post by **Unchained** » Sun Sep 25, 2011 9:39 am

I unplugged the harness, hooked up a direct positive lead from battery to the coil and jumped it at the slave solenoid to start it. While cranking I'm only showing 2 volts across the pos and neg terminal on the coil. I had a plug pulled out and grounded and noticed the spark was very weak. I have replaced the coil, ignition module, cap, rotor, plugs, slave solenoid and the two water temp sensors. I'm thinking there may be a short in the harness end that connects to everything on the engine so I'm replacing that today with a brand new one I picked up and will also replace the circuit breaker that is back there because it looks original. After this is done I'll be almost out of options, I think. Is there anything else in a breakerless type distributor that can fail? I really appreciate everyones input and thank you.

Big D · Post by **Big D** » Sun Sep 25, 2011 10:21 pm

You can not take a voltage reading at the coil by placing the negative lead on the coil's negative terminal. Remember that the negative side of the coil is being turned off and on by the distributor so your meter reading will not be accurate. Keep the lead on the positive terminal and put the negative lead to block ground. This will give you the voltage to the coil. But if you jumped a wire directly from the battery, it should be battery voltage. You must have 12 volts there.

When you try to start the engine in the way described, it basically takes the harness out of the picture anyway since you're feeding the coil directly.

Are you sure the engine is timed right?

Are you sure it's getting fuel?

Unchained · Post by **Unchained** » Sun Sep 25, 2011 11:33 pm

I replaced the entire engine harness with a new one and still have a weak spark. I had replaced two bent pushrods before this had happened so the distributor was removed and I placed it back in the same location. I have replaced all of these other parts since then to try and get it running to set the timing but no luck so far. I did have it running though for 5 minutes or so last week but haven't been able to get it going again. The only other thing left in the circuit that i have not replaced, short of the distributor would be the alternator. Would a short in the exciter circuit or fault in the alternator cause a weak spark? I will check the voltage as you described in the meantime and see what it reads. Thanks again for your time.

prowlersfish · Post by **prowlersfish** » Mon Sep 26, 2011 6:40 am

Make sure you dist has a good ground .

davidsmith · Post by **davidsmith** » Mon Sep 26, 2011 9:19 am

This might help.
http://www.cdielectronics.com/InstallSheet/123-7567.pdf

Trojan Boat Forum

No start. Wire harness problem?

No start. Wire harness problem?

Engine issues

slow running

Wiring Diagram