123 ignition and coils

[dakman29]

A bit of a long story.  The fabric hood strap was coming loose on my car.  Turned out that the mounting screws for the hold down plate had stripped.  I ended up drilling new holes close to the others so that when mounted the plate covered the old holes, it worked well, but in the process I cracked the ballast resistor.  I have a 123 ignition unit in the car and I contacted the manufacturer to ask whether there was a preferred resistor that they recommended.  Here is the answer I got:

"Normally the coil being used in combination with the 123Ignition does not
need a ballast resister. Do you know the resistance of the coil you have? If
not just put an ohm meter across the two post of the coil. A reading of 1
ohm or more is acceptable. Less than 1 ohm should be addressed with a new
coil. Preferably the Bosch Blue Coil rated at 3.4 ohms works the best in my
case.
Hope this helps
Ed"

Well, after getting this response I decided to replace the coil instead of the resistor.  I had a bosch silver coil in the car.  I followed Ed's advice and bought a bosch blue coil.  When I installed it, I bypassed the resistor.  Sorry to be so longwinded but here's the question.  To bypass the resistor I disconnected each of the leads connected to it.  One was coming out of the main wiring harness and the other going to the positive post on the coil.  I spliced those leads and then just reconnected the end of the wire on to the positive post of the new coil.  I fired up the car and ran it for a few minutes and drove it around the block.  All seemed well but I just wanted to ask for responses as to whether I did the right thing in the way that I bypassed the resistor.  Thanks.  Don

[Larry & Norma]

Yes that should do it.

[mdsalemi]

As a point of reference, talking about Bosch "Blue" and "Black" and "Silver" is old school.  I think newly-purchased coils need to be bought by their rating not their color; the color is from days gone by.

http://www.ratwell.com/technical/BlueCoil.html

[dakman29]

 L & N - Thanks for the confirmation.

"As a point of reference, talking about Bosch "Blue" and "Black" and "Silver" is old school.  I think newly-purchased coils need to be bought by their rating not their color; the color is from days gone by."

Thanks Michael, I did get one of the "good" blue coils, the 3.4 Brazil-made in the old school bosch box.

[Naj ✝︎]

Hi, Don,

In my view, Bosch coils are designed to be used with their respective ballast resistors to give the correct HT voltage.
The blue coil (now silver can with yellow label) as used in the US spec 70/71 transistorised ign system and susequent MB models with M110 amd M116/117 engines are fitted with 2 ballast resistors. a 0.6 ohm with a silver band and a 0.4 ohm with a blue band. The 0.4 ohm resistor can be bypassed during cranking.

Coil and ballast resistor details here:

http://www.sl113.org/wiki/Electrical/IgnitionCoil

Naj

[dakman29]

Naj thanks for the reply. I must confess to being confused however. The info I got from the 123 folks seems pretty clear that they don't recommend a ballast resistor with their ignition system provided the coil is rated at 1.0 or higher. Your opinion is that any Bosch coil should be mated with the appropriate resistor. I guess my question is whether you feel that an external resistor is needed when the combination of the 123 ignition and Bosch blue (brazil made) is used?

[Naj ✝︎]

Hi,
I'm sure the 123 distributor can handle the extra current as the manufacturers claim.
I would still use the ballast resistors to protect the coil.
My '68 280 sl runs on a contact-less distributor from an MB M110 engine and I am using the equivalent of the blue coil with the two ballast resistors (0.4 and 0.6 ohm).
I have done nearly 8000 miles since, with no issues.

Hope this helps

Naj

[Palolo]

Don,

Short answer: The ballast resistors (and transistor ignition box) were put in as an attempt to make the points last longer (or to add reliability, from having the points not fail as often) by lowering the current passing through them, but voltage was also reduced.

Long, detailed answer: http://www.sl113.org/wiki/Electrical/IgnitionSystem specifically in the final paragraphs after the Condenser text.

Since the 123-ignition distributor no longer has points, use the right coil, and ditch the resistors for more voltage at the plugs. I think the 123 folks would have taken them in to account after all the engineering they did to bring this product to market, base springs notwithstanding. 

Cheers,

Pat

[RBYCC]

Electronic distributors are sensitive to the resistance of the coil.
The coil resistance should match the manufacturer of the electronic ignition's specification.
Incorrect resistance can cause overheating and ultimate failure of the electronic switching module.

[Naj ✝︎]

Hi, Don,

As a matter of interest, what is the Bosch Part # of the blue coil you have installed?

Naj

[dakman29]

Naj:  Sorry for the delay in getting back to you.  The part number on the box is 9 220 081 083 74U.  With the coil installed I can't read a part number on the unit itself.  By the way, I replaced and reconnected a 0.6 resistor going to the coil.  I'll see if I notice any difference in performance with or without.  Thanks again for your help.  Don