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DSK
03-22-2014, 12:10 PM
Started up my 1987 Prostar 190 after 2 year storage. Ran fine but engine won't stop when I turn off the ignition! I have to disconnect the purple wire at the alternator to get it to stop. Also the tach is not working. I notice some sort of electronic module mounted on the back of the tach? Not sure what it's for. Any thoughts?

Bruce-ActionWater
03-22-2014, 02:45 PM
The not turning off issue would either be the starter solenoid or the ignition switch.

Table Rocker
03-22-2014, 03:03 PM
Unplug the tach completely and see if it will shut off properly. The faulty tach could possibly be sending voltage back to the coil.

I'm not exactly sure how the tach would do that, but I would still disconnect it and see what results.

DSK
03-22-2014, 11:40 PM
Thanks for the ideas guys. Dismantling the tach is where I plan to start. Ignition switch also seems like a possibility although that seems like a bizarre way for it to fail. Don't really see how the solenoid could do it since it's only in the circuit when the starter is engaged. If anyone has any other thoughts I'm all ears.

thatsmrmastercraft
03-22-2014, 11:45 PM
I'm with Table Rocker on unplugging the tach. A worn ignition switch usually fails the other way, but I have seen them fail to not shut the engine off. Had that happen on my little Toro snowblower this year. Turned out the contacts were loose and causing weird things to happen.

DSK
03-24-2014, 12:53 PM
Wierd! Completely disconnected tach AND the ignition switch while running and she keeps on running. The only way I can stop it is to disconnect the purple (exciter?) wire at the alternator. I'm thinking purple wire must be shorted to something in the wiring harness??? Or is the alternator not supposed to allow current to flow back through the purple exciter wire? I've looked at the Indmar 351 diagram that's been circulated on the forum but still can't make any sense of this. HELP

Table Rocker
03-24-2014, 01:49 PM
Per the diagrams on here, the purple wire at the alternator should be the exciter.

I wouldn't think that the alternator should be producing voltage on the exciter wire. There should be two terminals, one "voltage sensing" and one "exciter". If you are on the correct post, I bet there is a diode in the alternator that isn't doing it's job.

Is there any chance the purple wire is on the voltage sensing post? With the ignition in the run position, what happens when you disconnect the purple wire? If it keeps running, turn the switch to off and see if it shuts down.

However, I am far from an expert on alternator wiring.

thatsmrmastercraft
03-24-2014, 01:51 PM
Per the diagrams on here, the purple wire at the alternator should be the exciter.

I wouldn't think that the alternator should be producing voltage on the exciter wire. There should be two terminals, one "voltage sensing" and one "exciter". If you are on the correct post, I bet there is a diode in the alternator that isn't doing it's job.

Is there any chance the purple wire is on the voltage sensing post? With the ignition in the run position, what happens when you disconnect the purple wire? If it keeps running, turn the switch to off and see if it shuts down.

However, I am far from an expert on alternator wiring.

I think you are on the right track with this.

DSK
03-24-2014, 02:11 PM
I'll have a closer look at the connections on the alternator. By the way it does keep running with the ignition on and the purple wire disconnected. And in this case I do measure alternator voltage (slightly higher than battery voltage) at that "post". IT's actually not a post but rather a purple wire exiting the alternator with a connector about 3 inches from the alternator. I too was suspicious of the diodes but I had the alternator checked and was told they were fine? It does seem that if I put a diode in that purple wire so that current cannot flow back through it the problem would be solved. I may try that if I find no other explanation. Thanks again to all for putting your brains to this.

thatsmrmastercraft
03-24-2014, 03:01 PM
I wouldn't try to put an band-aid fix on this, but dig deep enough to find the actual cause. Have you checked that all the ground wires are clean and connected? This is likely from something that you did recently.

I had a problem last year where I attached a hot wire to the ground side inadvertently. Took out the solenoid on my new Arco starter from a backfeed. That reminds me, need to order a new solenoid.

CruisinGA
03-24-2014, 03:23 PM
I think table rocker is on the right track. When I installed a modern 100amp alternator into my '74 FJ40 I had to add a diode in the wiring to prevent it from doing exactly what you are describing. I can't remember if it was in the field or voltage sending wire.

JimN
03-24-2014, 03:57 PM
Thanks for the ideas guys. Dismantling the tach is where I plan to start. Ignition switch also seems like a possibility although that seems like a bizarre way for it to fail. Don't really see how the solenoid could do it since it's only in the circuit when the starter is engaged. If anyone has any other thoughts I'm all ears.

There's only one wire going to a tach, other than the ground and that's for the light bulb, which isn't easy to connect to the tach lead. If the gray tach lead is grounded, you'll have the opposite problem- it won't run.

New alternators are sometimes wired wrong, but I think it should be OK as long as the new one was installed by looking at the old one and copying its wiring.

If you have an ammeter, leave the key in the OFF position, disconnect one battery cable and connect the ammeter between the cable and battery post. If you see more than about 20mA of current (like, in the 300mA range), it could be an internal short.

Since the purple wire has been disconnected, measure the voltage when the key is in the ON, OFF and CRANK positions.

j.mccreight@hotmail.com
03-24-2014, 11:41 PM
The alternator should not be making voltage on the exciter wire. The purple wire is already 12v ignition switched. Its mearly there to tell the alternator to start making voltage for the orange wire, this orange wire should lead to the 50a breaker on the panel on the rear of engine above the transmission. This orange wire does splice off and go to the starter solenoid. If you remove the purple wire with key on there should be battery voltage on it. This purple wire is also used to operate the electric choke on the carburetor and for the positive side of the ignition coil, it splices off 3 ways. Perhaps the two wires are truly crossed and its back feeding through the solenoid keeping everything running, just a thought.
Do you have a fully charged battery? I did experience a low voltage situation before and the engine did run on. Battery was strong enough to start the engine but was overall weak. Bruce is correct its likely the starter solenoid contacts are now burned. I would first Fully charge the battery, replace the starter solenoid and ensure the 2 wires on the alternator, orange and purple are on their respective posts correctly. As far as the tachometer goes I would simply remove it from the negative side of ignition coil it is the grey wire and see what happens.
Now if you replaced the alternator with a non oem type you might need to contact the manufacturer of said alternator for their advice.

svrider
03-29-2014, 06:33 PM
Look at what is req for spark in the wiring diagram. Post a link to the diagram for more help. Have you checked the the ign relay?

DSK
03-31-2014, 02:10 PM
Weather finally warmed up enough to get back to this: Followed MC maniac's logic that the exciter lead FROM the alternator should not be producing voltage/current. Took a very close look at the wiring at the alternator and found that the purple wire was connected to one of the wires exiting from the alternator which appeared to be purple but in fact is a faded red or pink. The other wire exiting form the alternator is red with a thin purple stripe. It was looped back to the much larger orange wire and when disconnected it is not producing current. Don't know how they got that way but wondering if the wires are in fact reversed. So I gritted my teeth and held my breath and reversed these leads and sure enough everything is working normally now...engine starts, runs and shuts down normally and battery charging normally! Can't believe it didn't blow/burn something up but it didn't.
Thanks MC maniac and to everyone else who took the time to try to help me solve this.
Tach, by the way, was bad and apparently unrelated to this.

thatsmrmastercraft
03-31-2014, 02:14 PM
Great to hear you got that figured without letting all the smoke out of something.