Engine dies when key switch is released in start.

[adruid]

I have a Mercruiser 3.0L Frankenengine made of components running from 1984-1996. The base engine is a recent product of Michigan Motorz while much of the external parts are 1984. It has a new alternator that has been checked out by the manufacturer, and a new high torque starter. The engine will run if jumped, but shuts down when when the key switch in released from the start position (start switch is new.) I am not a trained automotive electrician and am stumped. Does anyone experience with this problem?
Thanks for any responses.

 

[Scott Danforth]

make sure you have 12 volts on the purple wire coming off the key switch.

 

[alldodge]

The standard ignition in 1984 was for points which used power direct from the starter (purple/yellow) to the + side of the coil. This provides full 12V to the coil during starting. Once the key was released the 12V was disconnected and reconnected thru a resistance wire (purple) to the + side of the coil.

If you have replaced the points with electronic ignition then you will only need power coming from the purple 12V lead.

How many wires and what colors are they on the coil?

 

[adruid]

There is 11.55V at the purple wire on switch. I have a Pertronics ignition system with Flame Thrower II coil. Positive coil has Red from starter solenoid, Red from distributor/Pertronix, Purple from resistance wire (alternator). Negative coil has Black from distributor/Pertronix, Gray from harness (I would have to unwrap harness taping to see where the Gray connects...) Thanks!

Don't know if this helps/matters, but I replaced the water temp. gauge just before this started happening.

 

[adruid]

Don't know if this helps/matters, but I replaced the water temp. gauge just before this started happening.

 

[alldodge]

There is 11.55V at the purple wire on switch. I have a Pertronics ignition system with Flame Thrower II coil. Positive coil has Red from starter solenoid, Red from distributor/Pertronix, Purple from resistance wire (alternator). Negative coil has Black from distributor/Pertronix, Gray from harness (I would have to unwrap harness taping to see where the Gray connects...) Thanks!

Don't know if this helps/matters, but I replaced the water temp. gauge just before this started happening.

The gray goes to the tachometer, as for the rest of your wiring, it appears to be correct. Take a volt meter and measure what is at the + side coil with key ON

 

[adruid]

With key in run position there is -1.3 mV at positive side of coil. If the resistance section of the purple wire from the alternator is bypassed under the same conditions, it shows 6.1V. I have been running with the resistance section of wire in place due to overheating of wire in the past. I am going to go out and bypass the resistance section and see what the key-on reading is at the positive terminal of the coil. Will report results.

 

[alldodge]

Suggest jumping the coil to get it running and then check the voltage output of the ALT. I'm thinking the new ALT is shorted

 

[adruid]

Detached resistance wire from circuit, connected ignition wire from alternator, turned on key, got -00.0 to -00.3 mV.

 

[adruid]

Would that be the case if the key switch was bypassed and Bat., Sol. & Ign. wires would get the engine started? Under this jump condition, the engine runs with Ign. removed, but not with Sol. removed. It seems engine is running on battery at that point.

 

[alldodge]

Just need 12V (battery) to the coil to bypass everything, then just like to know what the ALT voltage is

 

[adruid]

With engine running, alternator registers .2203V. Not good sign for alternator health. We'll see what happens with a new alternator. Question: does anybody still rebuild alternators anymore? I think I'm going to look into that...

 

[alldodge]

Several places still rebuild ALT's just need to find them. Look for generator, starter repair.

The main terminal of the ALT should be connected to the battery via the main starter post. So the reading you got tells me its not connected to the starter/bat

 

[-JR-]

Just went threw this on my boat, but just the opposite . Mine will start once I let go of the key switch. Would not start while cranking but would catch just as I let go of the key.....understand !

Any ways I found out I was missing a wire that runs from the b+ on the coil to a second spade terminal on the starter. Never heard of this either .
I Guess the starter will give extra juice to your coil only while cranking .....this is why yours only runs when cranking.

You need to turn your key on and take a test light or 12 Volt meter , and find where you lose the power ,

Should start at the key switch and move all the way back to the B+ on the coil
You might find it to be your key switch or a fuse or a Ballast resistor .
The resistor is in the wirer sometimes to drop your battery voltage down to 10 volts so you do not burn out your coil.

I guess when cranking a battery may drop below 12 volts to 10 volts and then again to 8 volts going threw the resistor before hitting the coil.
And this is when the starter kicks in and gives it a full power .

Let use know how you made out

 

[adruid]

That's exactly what was wrong with my switch. That red wire was part of a daisy chain of power going through all my gauges... What a pain... Thanks for writing! Hopefully it'll help the next person with this problem find it sooner.