Electronic Ignition Voltage Question

[Jacalore]

'85 MerCruiser 3.0L 140. Chasing down an acceleration problem...

PrevOwner converted from points system to a PerTronix electronic ignition module.
Talking with Mfg tech rep today, they gave me a voltage test to perform: Ground the neg side of the coil and turn key on (not engine on) and read voltage at pos terminal of coil, should read min 8.0V and ideally 11.0+V. Mine reads 6.5V and could be cause of acceleration prob.

Poss causes, the Mfg said, was corroded contacts, bad ignition sw - some sort of resistance dropping the voltage. Looking at the wiring diagram from the service manual, there is indeed a resistance wire going to the pos side of the coil. Why is that, and can it be replaced without damaging anything?

Wiring diag attached. Any advice appreciated!

 

[alldodge]

Re: Electronic Ignition Voltage Question

The resistance wire is used for the older point system previously installed. When the key is turned to start, the coil gets 12V, once the engine starts, key is released and voltage goes through the resistance wire. The resistance wire drops it down to 6V for the coil and points. The Pertronix 1 system was originally designed to work with the resistance wire and coil. The Pertronix 2 system does away with this but requires a 12V coil and the removal of the resistance wire from use.

So if you have a 12V non-resistor coil then the wire can be replaced.

 

[Jacalore]

Re: Electronic Ignition Voltage Question

Thanks, Dodge.
There are two wires on the pos side of the coil, which looks original. One goes to elec choke, the other goes to a separate terminal in the starter. No resistance in either wire. Must be that separate tap on starter?

 

[alldodge]

Re: Electronic Ignition Voltage Question

Thanks, Dodge.
There are two wires on the pos side of the coil, which looks original. One goes to elec choke, the other goes to a separate terminal in the starter. No resistance in either wire. Must be that separate tap on starter?

The one going to the choke is the resistance wire. The purple wire going to the choke from the harnes is stright 12V. So Pertronix is saying you can run your current setup without the resistance wire? If this is the case you will need to replace the wire coming from the choke with a new wire going to the coil. This will also require you to have a regular NON-resistance coil to replace the one you have if you have not already replaced it.

 

[Jacalore]

Re: Electronic Ignition Voltage Question

PerTronix is saying that the voltage on the coil pos side (key on, neg side grounded) should be a min of 8v... I get 6.5V but, reading your description, that resistance goes away after the engine is started. If so, than this is a non-problem, correct? Or, do I have it wrong?

 

[alldodge]

Re: Electronic Ignition Voltage Question

Not correct - When the engine is cranking the coil is getting 12V stright from the starter connection, once key is released the connection to the starter is disconnected and the 6.5V comes from the choke connection through the resistance wire to the coil. In short - there is no resistance during cranking, and resistance while running

The issues are:
Did you replace the the coil and if so with a resistance coil or non-external resistance required coil?
If you did not replace the coil what type is it?

 

[thumpar]

Re: Electronic Ignition Voltage Question

The resistance only goes away when the starter is cranking. While in the on position the voltage comes through the resistance wire. I just ran a new wire for mine. Do you have an internally ballasted coil?

 

[Jacalore]

Re: Electronic Ignition Voltage Question

Not correct - When the engine is cranking the coil is getting 12V stright from the starter connection, once key is released the connection to the starter is disconnected and the 6.5V comes from the choke connection through the resistance wire to the coil. In short - there is no resistance during cranking, and resistance while running

The issues are:
Did you replace the the coil and if so with a resistance coil or non-external resistance required coil?
If you did not replace the coil what type is it?

I did not replace any of these components, trying to work my way thru prevowners work. The wire from coil to choke has no resistance. How do I determine what coil is in this circuit? I assume prevowner matched IG module (I know).

 

[Jacalore]

Re: Electronic Ignition Voltage Question

The resistance only goes away when the starter is cranking. While in the on position the voltage comes through the resistance wire. I just ran a new wire for mine. Do you have an internally ballasted coil?

I don't know what coil I have, how do I identify it? Both wires going to coil are just regular copper wire... Where is the resistance coming from?

 

[Fun Times]

Re: Electronic Ignition Voltage Question

Both wires going to coil are just regular copper wire... Where is the resistance coming from?

It should be a purple wire located between the coil primary (+) post and wherever your engine model spliced into the purple 12v ignition key power on wire. Some engine models were located by the cannon plug and others had a junction near the choke wire.

Here's some other ideas for you to where to look,
http://forums.iboats.com/mercruiser-i-o-inboard-engines-outdrives/resistor-wire-55709.html

Item number 25, http://www.mercruiserparts.com/Show...desc=WIRING+HARNESS+AND+ELECTRICAL+COMPONENTS

Here's a bad picture of one,
Quicksilver Mercury Outboard Bravo Mercruiser Resistance Cable Kit 84 94227A2 | eBay

 

[achris]

Re: Electronic Ignition Voltage Question

I don't know what coil I have, how do I identify it? Both wires going to coil are just regular copper wire... Where is the resistance coming from?

The resistive wire LOOKS like a normal copper wire, but the wire is set-up in the factory as a resistive wire.... Replace it with a piece of normal wire... That's where your problem is.

Most electronic ignition systems don't care whether you have a ballasted or un-ballasted coil.

Chris......

 

[Jacalore]

Re: Electronic Ignition Voltage Question

achris:
I took resistance readings on both wires, zero, none. They are straight copper wires.

Fun Times:
I'll check those links for more info, thanks!

 

[airshot]

Re: Electronic Ignition Voltage Question

I have the same setup, the purple wire going to the coil from the wiring harness is a resistor wire, if you peel the coating off it you will see a wire wrapped around another wire (resistance). Go directly to the elec choke or you can take apart the harness to locate the junction where the resistor and choke wire connect, I went right to the elec choke and spliced in a normal wire to run to the coil so as I now have a full 12v directly at the coil...problem solved. Check voltage at choke you should have 12v there or you have other problems.

 

[Jacalore]

Re: Electronic Ignition Voltage Question

I will check this tomorrow, thanks!

 

[achris]

Re: Electronic Ignition Voltage Question

achris:
I took resistance readings on both wires, zero, none. They are straight copper wires. ...

Then the voltage should be as the Pertronix tech said. If it isn't, you have resistance somewhere... Maybe not that wire, but you need to find it...

Chris.....

 

[Jacalore]

Re: Electronic Ignition Voltage Question

I have the same setup, the purple wire going to the coil from the wiring harness is a resistor wire, if you peel the coating off it you will see a wire wrapped around another wire (resistance). Go directly to the elec choke or you can take apart the harness to locate the junction where the resistor and choke wire connect, I went right to the elec choke and spliced in a normal wire to run to the coil so as I now have a full 12v directly at the coil...problem solved. Check voltage at choke you should have 12v there or you have other problems.

Where does the 12V come from? And what is the resistance of the choke to coil wire?
This is my wiring setup (pic attached). Thanks.

 

[achris]

Re: Electronic Ignition Voltage Question

Where does the 12V come from?

12 volts is available at all times on pin #6 of the big plug (that goes to the engine instruments and the key switch), via the 'circuit breaker'. When you turn the key to ON, that 12 volts comes back into the engine harness on pin #5 of the big plug.

And what is the resistance of the choke to coil wire?

Typically, around 1.8 to 2Ω (it's in the manual, same one you got the wiring diagram from)....

Chris....

 

[Jacalore]

Re: Electronic Ignition Voltage Question

Thanks, Chris. I'm going to take some readings this morning, try to find where the resistance in the coil circuit is located. I don't see how replacing the wire from the elec choke to pos side of the coil will get 12V there, tho - what am I missing?

 

[Jacalore]

Re: Electronic Ignition Voltage Question

Aha! I read 1.8 ohms between elec choke (purple) and coil pos terminal (purple)!
I'll splice in a wire between those two points and retake the voltage test.

 

[Jacalore]

Re: Electronic Ignition Voltage Question

Ran the straight wire, bypassing the resistance wire, and the throttle now accelerates the rpm!
Couple questions: The coil seems to be running hot, took a voltage reading - all within PerTronix specs. Coil normally runs hot (never checked before)? And, once in a while, when giving it throttle, the engine hesitates. Normal?
Thanks!