will only start by hotwiring...ignition question.

[craig3x]

My 1987 4.3 will only start if I bypass the key ignition switch. I tried jumping the wires at the switch and it still won't work, but If i jump some wires ( not sure what they are exactly) in the engine bay, I can get the boat to fire right up. I do have power at the keyed ignition switch, but the purple wire is where my problem is. Can anyone tell me where this purple wire on the ignition switch goes to? It says "ignition on it under the dash, but I can't find it in the engine bay. Pictures or detailed desciptions would be very helpful. Thank you.

 

[craig3x]

ok, let me simplify my question. Where does the purple ignition wire go to in the engine bay? and could whatever it goes to, be faulty?

 

[craig3x]

really? No one?

 

[tdappleman]

Not sure if it would be the same as mine - but purple on mine is the 12v ignition source for the engine. It goes to the coil and I think the alternator - a purple/yellow then goes to the oil pressure switch, starter solenoid and fuel pump. Are you getting any power to the purple at the ignition when you turn it on? If not you probably have a bad ignition switch.

 

[midway]

Purple is 12 volts, it runs thru the wire harness and connects to many items, included in those items is the positive side of the ignition coil. It also connects to the starter solenoid, the alternator, electric choke, Pin 5 in the harness connector, then in the instrument panel it connects to all the gauges, ign switch, and the buzzer, It is powered off of the ign switch, it may be losing power somewhere in the harness, a good place to look is the main harness connector, after many years they start to lose connection in the harenss connector. Follow your harness from the dash and find the main plug, on my old 87 Seville it was over where the harness starts to run down the side of the boat up near the dash.

 

[docofthebay]

I'll try a basic answer without knowing your particular engine/wiring layout.
Typically to start and run an engine, your key switch does two things:
1: When it it turned once to the right, it allows electricity to flow to the engine to energize the coil or whatever form of ignition you have. This is the electricy that your engine uses to make the spark. The next key position is the start position. This sends electricity to the starter solenoid that then makes the starter turn over, engage the engine and turn the engine over. If the coil/ignition module has power to it (step one) and the the engine is turned over, then it should run. If step one is not working but the engine does turn over, it will not start.
The hotwiring you are doing at the back, is this the action that makes the engine turn over? If so does the key have to be in the run postion for it to actually crank up when you do this.
Here is a list of wiring color codes from Mercruiser's website
Old Color (BIA) Color Where Used

Black Black All Grounds

Black/Yellow Ignition kill circuit

Black Brown Alternator charge light
Pumps (bilge, live-well, etc.)
Reference Electrode - MerCathode
Brown/Orange Power feed to Auto-bilge switches

Black Orange Accessory common feed
Distribution Panel to Accessory switch
Anode Electrode - MerCathode
Red/White Orange Alternator Output to Ammeter

Blue Lt. Blue/White Trim -"Up" Switch

Brown Gray Tachometer Signal
Dark Gray Navigation Lights
Green Green/White Trim -"Down" Switch

Green Tan Water Temp. Sender to Gauge

Orange Lt. Blue Oil Pressure Sender (10 Gauge)

Pink Pink Fuel Gauge Sender to Gauge

Purple Brown/White Trim Sender to Trim Gauge

Purple Purple/White Trim -"Trailer" Switch

Red Red Unprotected Wires from Battery

Red Red/Purple Protected (Fused) Wires from Battery
Protected (+12V) to Trim Panel

Tan Purple/Yellow Ballast Bypass

White Purple (+12V) Ignition power feed from Switch
Instrument power feed

Yellow Yellow Bilge blowers
Alternator field to reg. field terminal
Starter Solenoid to Starter Motor
(MCM 470 and some Ford products)

Yellow Yellow/Red Starter Switch to Starter Solenoid
to Neutral Start switch
Tan/blue Warning system sense wire
(Audio warning)

Yellow/Black Neutral Safety switch
(transmission mounted)

White/Green Ignition module to Distributor
(Mercury Thunderbolt ignition)
White/Red Ignition module to Distributor
(Mercury Thunderbolt Ignition)
Dark Green Bonding Wires (earth ground)
Dark blue Cabin, courtesy, and instrument lights

A you can see from this list, the "new" BIA color list states that the purple wire is the one that feeds electrictiy to your ignition system to allow it to run. That wire is probably either broken somewhere in between or the problem is in the switch. My money is on the switch. They go bad with regularity. Test where the purple wire comes out of the switch and see if there is 12volts there. If not replace the switch and buy the Mercury one. Trust me. I bought several of the other brands and they were worthless. Pay the extra upfront and be done with it.​

 

[Lazy Daze]

Just to be sure... when you're hotwiring at the key... you're combining the red and purple, then using the yellow as the starter?

You'll have to be more specific as to what you mean by "won't start". Do you mean won't actually start, or won't even turn over?

I'm going to guess that you mean it won't even turn over (like it's dead).

Do this: 1st, look at the shift interrupt switch on the engine and make sure the micro switch is released.

Next, try jiggling your throttle/shift lever - to make sure it is actually in neutral.

I'm not sure exactly what you're doing back at the engine, so these are just some general comments.

 

[craig3x]

Thank you for the responses. We might be getting somewhere. I took some pictures to better clarify:

1st pic. Here is the back of my ignition switch. Orange is 12V. Purple is the ignition wire (it is marked). If I jump these 2 together, the boat will not turn over. This tells me the problem is elsewhere...not my switch. ( I know this because I had to jump these wires together before when I lost my key) The yellow wire is "key on" after the boat is done cranking.


The 2nd picture is of my coil. The left side is "neg" and the right side is "pos". However, I am only reading low voltage when the boat is not running, not 12V.

3rd picture is ...I don't know, but it has a purple wire going to it. can anyone identify it?

and the 4th pic shows how I am hotwiring it to start. I turn the key to on, then jump these two bare terminals together. what is this, and is it bad that I'm doing that?

 

[craig3x]

Not sure if it would be the same as mine - but purple on mine is the 12v ignition source for the engine. It goes to the coil and I think the alternator - a purple/yellow then goes to the oil pressure switch, starter solenoid and fuel pump. Are you getting any power to the purple at the ignition when you turn it on? If not you probably have a bad ignition switch.

Yes, same. purple is ignition wire. it get's 12v when the key is turned to start. I bypassed it though on the back of my switch and gave it direct 12V which should turn the engine over. But it's not. So I need to know exactly where this purple wire goes in the engine bay. I believe it does go to the "pos" on the coil, but it has to go somewhere else to. 12v to the coil won't make the engine turn over. Does it go directly to the starter, or does it go through some other stuff first?

 

[midway]

If the engine does not turn over, then the Purple wire is not the problem, your problem would be with the starting circuit, if the engine turns over but will not run then you are looking at the ignition circuit. Purple wire should be on the I terminal, that powers the gauges and ignition system, the Yellow wire should be on the S terminal and the Red(Orange) wire on the B terminal on the back of the switch. Purple,I terminal, has power anytime the switch is in the on or run position, the Yellow, S terminal, has power when you turn the switch to the start position, Red, B terminal, is power all the time per Merc wiring diagram. If your wiring does not match this then it sounds like maybe someone has rewired somewhere in the past and did not stick to the color code. Now the Yellow, S Terminal, goes thru a neutral safety switch which is located in the shift lever control, if the neutral safety switch is bad you will not start the engine from the key switch, you would have to go to the engine area to start. There is also a magnetic switch in the engine compartment that could be bad, it excites the started solenoid to start the engine. My first post was ignition circuit related, this post is related to the starting circuit.

 

[tdappleman]

and the 4th pic shows how I am hotwiring it to start. I turn the key to on, then jump these two bare terminals together. what is this, and is it bad that I'm doing that?

If the 4th picture is the bottom right - then it appears you are jumping the slave solenoid which is fairly easy to replace. They do go bad - sometimes the warning signs are having to turn the key off and on a couple of times prior to it starting and then eventually it will fail.

 

[craig3x]

thanks for all the advice. FIXED IT!!! It was a bad connection at the main harness

 

[midway]

Glad to know you have it fixed. Now go have some fun in the sun. :grin: