Jumping the coil's resistor when starting

[dadandson02]

I always read about using Bosch's blue coil with internal resistor. I feel that using Bosch's hotter red coil with external resistor and jumping the resistor through the starter's auxiliary contact on the solenoid when starting would be a better set-up. Opinions ?

[monkeystripe]

get a pertronix and ditch the resistor all together. The point of the resistor is to provide you with full 12v+ at start up, and slowly cut back to 6v+, to keep your points from burning up. As the resistor is open at startup anyway, jumping it at start up won't do anything that it isn't doing for itself already.

[TTyMiller]

I have a Red coil ( not installed )an external resistor ( not installed ) both of which I plan to install. I also have a Pertronix which is installed. Are you saying that you do not need the external resistor if the pertronix is installed when using the Red coil? I can never seem to get a clear answer on this subject.

thanks

[Guest Anonymous]

You say "resistor provides you with 12V+ at start-up and slowly cuts back to 6V+..." and also that "resistor is open at startup anyway...". I am not sure about that. The resistor has a set value of maybe 1.6-1.8 Ohms and I dont think it is open at startup.

My understanding is that when the engine is running, the ignition switch feeds full battery voltage to coil's + terminal either directly or through the resistor when you have one, thus reducing the voltage from 14v+ down to, let's say 10V+. When starting, voltage to coil still goes to coil's + terminal fed from ignition switch but since voltage drops from cranking to about 10V+, if coil is still fed through resistor, the voltage to coil's + drops to, let's say, around 7V+. Now if you use the starter's selenoid's auxiliary terminal to feed available battery voltage directly to coil's + terminal by jumping the resistor, you get 10V+ rather then 7V+.

Now let's assume the following:

- Blue coil gives 18kV at 12V and red coil gives 25kV at 12V.

- Blue coil runs without external resistor.

- Red coil runs with external resistor that reduces voltage from 14V to 10V with engine running.

- Voltage is reduced to 10V when cranking.

- Blue coil gives 18kVx(10V/12V)=15kV at 10V.

- Red coil gives 25kVx(10V/12V)=21kV at 10V.

- If red coil's resistor is jumped by starter's solenoid at starting, coil's voltage is 10V.

Conclusion is you get 15kV at cranking from a blue coil and 21kV at cranking from a red coil.

Of course, if you run a Pertronix that can handle the red coil rather than oints which require a resistor, than run the red coil without the resistor which will give you both hotter spark than a blue coil at both startup and running.

Makes sense ?

[jgerry2002]

FYI, looking at the pertronix specs, you can not run a Pertronix with a Red coil without a resistor. Total amperage is too high.

I know people here have done it though without burning up the Pertronix.

[dadandson02]

Then it seems to me that with either points or Pertronix, red coil with resistor and jumping of resistor at cranking through starter's solenoid's aux. contact would be a better set-up than blue coil with no resistor. Agree ?

[heinrich]

From an earlier post (hope Zenon doesn't mind):

The original stock "black" Bosch coil, with an external resistor, is actually a great setup - superior to a "Blue" coil and equivalent to a "Red". I have seen the specs from Bosch that prove it but cannot find them now. You can trust me, though. ;-)

With a Pertronix, you have to run the resistor with either the stock or red coil but must bypass the resistor with a "blue" coil. If you run without a resistor with a coil that requires one, you run the risk of frying the Pertronix because it may not be able to handle the higher current draw. You can also overheat the coil itself. If you use a resistor with a blue coil, you'll get much weaker sparks but your points will last longer.

There seems to be a lot of confusion around coils and resistors so here is my attempt to clarify.

Why did BMW use an external-ballast-resistor coil, anyway?

On all North American '02s (at least from '72-on), BMW fitted the "heavy duty" ignition system that was apparently an option for carby '02s in other markets and standard on tiis and turbo. According to the factory specs and part numbers, the tii and carby external-resistor coils are the exact same part.

Essentially, the "external resistor" coil is just a coil that is designed to work properly when supplied with about 9 volts. It is then used with the external ballast resistor so it can safely operate continuously when supplied with the usual 13-14V available when the engine is running. Since a coil's current draw is directly related to the amount of spark energy it might deliver, more current is naturally better except that a coil's current has to be limited to an amount that would not make it overheat in continuous use and/or burn out the points prematurely.

But why bother with the external resistor? For normal running, it would be electrically equivalent, cheaper, and more reliable to use a coil designed for the actual system supply voltage and ditch the resistor entirely.

The answer is that the whole point of the external resistor setup is to be able to maintain or even boost spark energy during starting, when the system voltage is often pulled way down (under 10V with a marginal battery) by the starter motor and the sometimes very uneven, way rich or lean mixtures are much harder to ignite.

By using a coil designed for a lower supply voltage, the designers could then essentially "over-drive" the coil during starting by bypassing the ballast resistor and feeding the coil directly with whatever the battery had to offer. But because the resistor is only bypassed during cranking, the coil would not have much chance to overheat and the points life would not be too adversly affected by a few seconds here and there of switching the much higher current.

On earlier '02s with a ceramic-block ballast resistor, there is a relay near the firewall that does the bypassing based on a signal from the circuit that energizes the starter solenoid. On '74 and later, they simplified the design and eliminated the relay, instead supplying battery voltage directly to the coil via a second, smaller spade terminal on the starter solenoid (which is itself a relay anyway). Also on '74 and later, the obvious ceramic resistor was replaced by a sneaky, "special" resistance wire that runs in the harness from somewhere over the engine to the coil's positive terminal - it has clear insualtion and a moulded marker at the coil end with the resistance value on it (by now those are usually just a blob of crispy rubber). All resistors for all years were 1.8ohms, according to the factory manual.

Some final notes:

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.

As mentioned above, if you run a stock or red coil without the resistor, you run the risk of overheating the coil, burning out your pertronix, or going through a lot of points. I have heard some people say red coils work just fine without resistors but I have a hunch they have later cars that have the sneaky resitor wire and they are running them correctly without realizing it.

Other aftermarket coils? I dunno - you have to look at each one's specs. You have to be sure the coil you use will not draw too much current for the ignition system you have - points, pertronix, or crane. If you add a resistor, make sure you are not inadvertantly adding it to the stock one or disabling the starting boost feature - it could make all the difference one cold day.

regards,

Zenon

_________________

'73 2002i Verona (Megasquirt/318i EFI conversion, daily driver)

'72 2002 Golf (bone stock, rusty but still pretty, sleeping)

'73 2002 Fjord (first '02, now parts car, almost gone)

http://www.zeebuck.com

[monkeystripe]

yes,

I agree, I didnt have the numbers in front of me, so I just cut to the chase of the concept. I would say however, if you have points, that running a blue coil without the resistor at all is a recipe for burning points faster. my setup- flame thrower with pertronix for 2 years now with no problems.

[monkeystripe]

Come spring when it melts up here and I can get the '02 out of storage I will take the actual measurements and post them, so we can put this to bed. I too, have seen this bit come and go without a diffinitive answer.

[heinrich]

Okay, here's another post that supports Zenon's pretty definitive answer:

http://www.bmw2002faq.com/component/option,com_forum/Itemid,57/page,viewtopic/t,270062/highlight,red+coil+resistor/

Blue Coil = Internal Resistor (2.8ohm resistance) = Remove External Resistor

Red Coil/Stock Coil = No Internal Resistor (1.5ohm resistance across coil terminals) = Retain External Resistor

Whether you're running an electronic trigger (Pertronix or Crane XR700) or the original points, the above hold true. It's not just about burning up points--it's about burning up coils and the Pertronix unit itself. Of course, the situation may change if you're running an MSD or HI-6 unit or something; but I don't know about that.

BTW, I run an XR700 and Pertronix Flamethrower coil and have retained the ballast resistor, with no negative results.

[heinrich]

Okay, here's another post that supports Zenon's pretty definitive answer:

http://www.bmw2002faq.com/component/option,com_forum/Itemid,57/page,viewtopic/t,270062/highlight,red+coil+resistor/

Blue Coil = Internal Resistor (2.8ohm resistance) = Remove External Resistor

Red Coil/Stock Coil = No Internal Resistor (1.5ohm resistance across coil terminals) = Retain External Resistor

Whether you're running an electronic trigger (Pertronix or Crane XR700) or the original points, the above hold true. It's not just about burning up points--it's about burning up coils. Of course, the situation may change if you're running an MSD or HI-6 unit or something; but I don't know about that.

BTW, I run an XR700 and Pertronix Flamethrower coil and have retained the ballast resistor, with no negative results.

[Zenon]

All resistors for all years were 1.8ohms, according to the factory manual.

As in the original thread, I gotta correct myself. Right after I wrote the above, I looked at the manual more carefully and saw the footnote along the lines of "early 0.9ohms". So that means there were in-fact two values of ballast resistor used on '02s and thus a different external-resistor coil for earlier cars than later.

Unfortunately, I cannot tell when the change was made but suspect it was no earlier than '75. The late stock coil that required a 1.8 ohm resistor is essentially the "red" coil, while the earlier stock coil was slightly less powerful but not enough to really matter as far as I can see.

Running a later OE or "Red" coil with the early 0.9 ohm resistor may or may not kill your pertronix. On the later cars, the value was moulded onto the resistor-wire's end near the coil but in most cases this is now illegible.

regards,

Zenon

[jpcapoc]

Zenon, after reading your post a week or so ago, I changed my .9 ohm resistor to a 1.9. I was having intermittent stalling. I suspected the resistor was the problem, as the XR700 manual specifies a minimum of 1.4 ohm if the coil does not have internal resistance. So far so good, hasn't stalled since I changed it. I also move the unit from my original mounting under the coil to the plenum under the winshield. I also change the coil to a bosch Red.

[Zenon]

Zenon, after reading your post a week or so ago, I changed my .9 ohm resistor to a 1.9. I was having intermittent stalling. I suspected the resistor was the problem, as the XR700 manual specifies a minimum of 1.4 ohm if the coil does not have internal resistance. So far so good, hasn't stalled since I changed it. I also move the unit from my original mounting under the coil to the plenum under the winshield. I also change the coil to a bosch Red.

I wonder if the Crane unit has some form of protection in that it will shut itself down when it overheats but work again when cooled off, rather than just burn itself out. If so, that would explain why it works OK now that your coil is drawing less current. Your total resistance (red coil plus 1.9 ballast) is now over 3 ohms which I think meets both Crane and Pertronix requirements.

regards,

Zenon