engine and drivetrain Q: rewiring to elminate ballast resistor (new Bosch Blue) ?

[72Agave]

Hello all,

I'm tackling some ignition related troubles in my 1972 2002 standard. The first thing I have done is bought a new Bosch "blue" ignition coil which, as described elsewhere in the forum, has an internal ballast resistor. To keep the engine bay as simple as possible, I'd like to remove the existing external resistor that is bolted to the fender.

What is the proper way to wire the ignition coil directly, no external resistor in the line? I think it is connect green wire from the relay on fire wall to coil terminal (+) [15], and two black wires to coil (-) terminal. One of the black wires goes to the distributor, the other off to somewhere under the fuse box on left side of car.

Did I get that right?

The green wire shows 12V hot when ignition key is in position 1 (dash lights on). I'm hopeful this is how it should be. But that's another thread.

[esty]

maybe this will help

[72Agave]

OK, thanks, that is a very useful diagram. However, with the resistor removed, should I wire the green straight through and eliminate the green/red from the relay, or wire both the green and the green/red from the relay to the (+) 15 on the coil?

The visual difference is either image 1, no resistor, but retain both wires or image 2, no resisitor and only the green straight through.

[Scott Sislane]

With the higher resistance bosch coil, aren't you weakening your spark at start-up? I thought that was why it was designed like it was, so you can by-pass the resistor for a hotter spark during start-up, then back through the resistor when running for proper resistance. Now you have constant resistance & can't get the higher voltage during start-up. Hotter spark helps for a quicker start, right?

[72Agave]

There is lots of confusing (to me) info in the forum about the coil/resistor situation, but my approach right now is based on this:

http://www.bmw2002faq.com/content/view/69/32/

"If you run a blue coil and do not bypass the ballast resistor, you will only get full power when starting and much less spark energy when running. A blue coil is actually a downgrade from stock since there is no more "boost" ability during cranking. But, on a well tuned car with a good battery, it may never matter."

I read it as: Blue coil + do not bypass = less spark. Blue coil + bypass = full spark.

[showoff07]

Hey did you ever figure the correct wiring for that setup?

[BillWilliams]

I think there are a couple of things going on here.

With a blue coil you do NOT need a ballast resistor, period. Bypass it.

One of the post talks about start-up and running voltage. That function is performed at the start-up relay. The start-up relay is a small metal device on the firewall just behind the coil. That relay, allows full voltage at start-up when there is a ballast resistor present and then cuts the voltage when the key comes off of the "start" position (so the points will not burn up as quickly).

Ballast resistors are used with black and red coils and there is a different ballast resistor for each.

However, euro cars do not have the start-up bypass. Not sure why. It was not on my touring so I thought it had been removed. But when looking at the Euro wiring schematics, it is absent.

I think the question is, with a blue coil and the ballast resistor by-passed, where or not the start-up relay is needed. I am thinking it does no harm to leave it in the circuit or by-pass it if you know how. I have seen it done both ways.

This photo shows the red coil, red coil ballast resistor, and the silver box, upper left, is the start-up relay. (The black and white wire with the connector is for the Crane)

The resistance on a red-coil ballast resistor

The resistance on a black coil resistor

Resistance on a red coil:

Resistance on a blue coil:

[John_in_VA]

Nice post, Bill!

Hmm - I'm running a red coil. Wonder if I have the proper resistor?

[BillWilliams]

You can test it with a meter like I was in the photo and see if the resistance reading is close.

Disconnect the wires from it before taking your reading.