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post #1 of (permalink) Old 04-16-2002, 01:07 PM Thread Starter
 
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alternator diode for MSD 6A

87 YJ 258 (4.0 head, bunchaotherstuff, etc...)
The electrical system is stock except for:
MSD 6A box and accompanying alternator diode.
Nutter bypass, but I'm guessing that doesn't matter anymore seeing as I have a new ignition box anyway(?) and none of my emissions stuff is hooked up anymore.

When I installed my 6A the first time about a month ago, I had the run-on problem. Well, I goofed up and fixed it by using a ballast resistor. Long version of the story here. Short story is my 6A cooked shortly thereafter and was repaired by MSD (kudos to them for customer service).

I got the box back and re-installed it about a week ago along with the diode for the brown alternator wire. The diode is installed in-line in the brown wire with the stripe on the alternator side. No more run-on problem.

As soon as I did this, I noticed my volt meter behaved differently. Usually now (but not always), when I start the Jeep the voltmeter comes up from pegged low to sit just shy of 12 volts. Sometime later it will jump up to it's normal reading of around 14 volts and stay there for the rest of the time the Jeep is running. The length of time it waits to do this varies from immediately to a couple seconds to sometime after I am trundling off down the road. Usually it does not take long. Sometimes (but not always) the jump to 14 volts coincides with a blip in the throttle, sometimes revving the motor doesn't do a thing.

I figured this was just a byproduct of installing the diode. All of it was casual concern for me until yesterday morning. Yesterday morning the battery was almost completely dead. It would give that awful anemic "bzzzt" solenoid sound, but not turn over. I couldn't even jump start it. I put the battery on the charger and it easily took a full charge and started right up. The volt meter continues it's antics.

It kind of behaves like I am leaving a really small light on, like a super-slow drain on the battery. But I have not left any lights on and the installation of the 6A and diode are the only things that have changed about the electrical system recently. These changes also exactly coincide with the new behavior.

Any particularly likely places I have hosed this up? Anybody else have any similar experience?
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post #2 of (permalink) Old 04-16-2002, 02:33 PM
 
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Re: alternator diode for MSD 6A

Diode in backwards.

A Diode is a one way valve for electricity, so it has a flow direction that has to be observed.

If you want an easy fix, make sure the stripe of the diode is facing the alternator.
If I were you, I'd disconnect the Brown resistor wire all together, and run a wire from what used to be the positive side of your ignition coil connector...
(where the small Red MSD wire connects now)
And run that over to the alternator.
I'd put a Diode inline, with the stripe facing the alternator, and connect the other end of the diode to the #1 terminal of your alternator.

If you really want to get jiggy with it, use a new alternator plug (available from any discount auto parts store for around $4.).
The large wire (#2 terminal) will connect directly to the 'BATT' terminal on the back of the alternator. Use an 'Eye' connector, and have at it.

Connect a Radio Shack Diode (P/N 276-1143 or P/N 276-1144) with the stripe end facing the alternator to the small (usually white #1 terminal) wire.
The wire from the Positive side of the factory ignition coil connector will connect to the other side of the Diode.

If you want to do it the easy way, try Radio Shack P/N 64-3036A butt connectors for really small wire. These make the job a snap!

Don't forget the heat shrink tubing to seal everything up.

Factory Coil Connector, Positive side-----20 Ga. wire-------- Diode w/Stripe Facing > ------- White wire of alternator plug.

You are looking at under $10 for everything.
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post #3 of (permalink) Old 04-16-2002, 04:09 PM Thread Starter
 
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Re: alternator diode for MSD 6A

Just went out to my Jeep and verified that the stripe is already facing the alternator. Could my problems be explained solely by still being hooked to the brown resistor wire as opposed the positive side of the coil?
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post #4 of (permalink) Old 04-16-2002, 04:28 PM
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Re: alternator diode for MSD 6A

I too just installed the diode for the same reason in my jeep. I am using a Crane HI6-R ignition unit similar to the MSD. I too have been noticing that the voltmeter would only register a charge when you rev the Rpm’s up past 1500. After that the charge will say on at any rpm. This happens all the time without any variation. I am not having any discharge problems that you are experiencing. I left the jeep parked for a few days without any starting problems. In fact, with the ignition off there should be no current flowing thou the diode. It acts like a small switch.

Anyway I was wondering too if somebody else has been experiencing this same occurrence with this installation.

Other than that it works great! Know I don’t have to drop the clutch in gear to stop the motor.
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post #5 of (permalink) Old 04-16-2002, 04:50 PM
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Re: alternator diode for MSD 6A

O.K. flipped the diode too and no difference. In fact I can not find a stripe on mine just a serial number on one side of the diode. In fact this diode is shaped like a flat disk (brown in color) w/ two bear wires coming out of the bottom. From the looks of it you can see one of the incased wires coming out from the center.
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post #6 of (permalink) Old 04-16-2002, 10:33 PM
 
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Re: alternator diode for MSD 6A

This does not sound like a diode to me. Sounds like a capacitor or a MOV (metal oxide varister for surge protection). Diodes should be black in color and the band is silver on one end-the same color as the writing on it. SOme higher power ones look like bolt studs. Diodes block the current in one direction turning an alternating current wave into to a direct current wave. Batteries don't like alternating current. Where did you get this?
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post #7 of (permalink) Old 04-16-2002, 10:46 PM
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Re: alternator diode for MSD 6A

Mine has been doing the same thing. I would start it and after the oil pressure came up, I would rev it up until the voltmeter showed a charge. I replaced the fusible link with a maxifuse and the problem went away. I think it's just too much resistance from the fusible link to the main harness.
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post #8 of (permalink) Old 04-17-2002, 12:43 AM
 
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Re: alternator diode for MSD 6A

It could be a lot of things...

BUT...
1. When you have to rev the engine up to get the charging started, that means the excite wire is not working as designed.
By reving the engine high enough, you are getting the alternator to self excite from the residual magnetism left in the rotor, and the excite wire isn't doing it's thing...

2. If the resistor is restricting the forward voltage to under 3 or 4 volts, the diode isn't turning 'ON'.
It takes a set amount of forward voltage to do the job and get the Diode to open the 'Valve'.
If the resistor value is too high, the Diode isn't getting enough voltage to do it's job correctly.
Run a new wire with just the Diode in line and try it that way.

3. The MSD Diode is of an unknown value to me. The specifics of that particular Diode are not supplied, so I don't know how much forward voltage it takes to open it.

The Radio Shack Diodes I recommended will do the job with flying colors, are under $2, and have the specs. on the back of the package.

4. Here is a simple test...
Take a test wire connected to the positive side of the battery, and with the engine started, but not charging, just touch that wire to the #1 terminal.
There is lots of space to reach the terminal with the plug still plugged in, so just touch the back side of the plug connector for #1.

If your alternator IMMEDIATELY starts to charge (and it will) you need to eliminate the brown resistor wire.
You can cut it out of the harness, you can leave it in the harness and bypass it, or you can do as I suggested and build a new, clean supply line for the excite terminal.

While you are at it, you can add a charge warning light too!
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post #9 of (permalink) Old 04-17-2002, 01:24 PM
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Re: alternator diode for MSD 6A

I really don’t know if it is a diode or not. It came with the kit for this “run on” ignition problem.
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post #10 of (permalink) Old 04-17-2002, 01:45 PM
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Re: alternator diode for MSD 6A

Opps!! You are right I put the wrong device in! I just re-read the installation instructions again and what I used was a SURGE ABSORBER and not a diode. This is used for ‘noise suppression on the stage limit input’. Going to Radio Shack tonight to get a “diode” now that I know what it looks like.

Thanks for the clarification Brhino.

TeamRush, Thanks so much for the great info! I makes so much sense now!!!!!!!
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