engine and drivetrain Where does my resistor wire go?

[peterschop]

And just when I thought I understood this resistor wire and coil concept....

I think my coil is going bad on my 1974 Automatic because the car starts cutting out after driving 10 to 15 miles and the coil gets really hot. So before I buy a new one, I thought I would check the value of the resistor wire so I can buy the right coil.

I found the resistor wire where it plugs into the coil. The insulation that was once clear is now yellowing. I can not find any markings on the wire as to what the resistance is. I tried to trace the wire back but it is difficult when it is in the harness. I pulled the fuse box out to see if the wire was there but could not find it. I checked the wiring under the steering column and into the ignition switch but still could not find it. Where does the other end of the resistor wire go?

Also while I was fiddling with the car, I checked the voltage at the coil and am now even more confused. With the engine off and the ignition on, the voltage from the coil 15 + terminal to ground reads 8.6 volts. This tells me the resistor wire is working. When I unplug the resistor wire from the coil and test, it reads 12.16 volts from the resistor wire to ground. How can this be? Also, where the resistor wire plugs into the coil there is also a black/red wire. When I trace the wire back through the harness I find that the black/red wire is unplug from where ever it is supposed to go.

My other problem is that I think my Pertronix is not wired right. Both the black and red wire from the ignitor are plugged into the coil. The Pertronix instructions show that with a resistor ignition, the red should be powered by a 12 volts or before the resistor wire. I thought I would run a wire from the fuse box to the red Pertronix wire. How do I attach the wire to the fuse box? Should I spice it to one of the existing wires? I hate to cut into one of the wires.

Thanks,

Peter

74 2002A

[Napes]

Yhis may give you some helpful info:

http://www.bmw2002faq.com/index2.php?option=com_content&do_pdf=1&id=69

Bob Napier

[Scott Sislane]

if you want to locate your "resistor wire" this should help: http://www.bmw2002faq.com/component/option,com_forum/Itemid,50/page,viewtopic/t,352446/highlight,/

Scott

[peterschop]

Bob, I did read Zenon's excellent article which is one of the reasons I wanted to measure the resistance before buying a new coil. There is a lot of good information but some of it seems to contradict itself.

My coil is Black with KW12V stamped on the bottom. The factory manual list this coil for my car. Zenon states that '74 and later has the resistor wire and all resistors for all years were 1.8ohms, according to the factory manual. He then goes on to state about some damn footnote that some earlier actually had 0.9ohm resistors but did not know the cut off date.

He then said that if you add a Red coil or later "stock" coil to earlier car, you have to replace with a 1.8ohm or add 0.9ohm.

He added archives from Bosch listing four types of coils.

- Type KW(Black) 12V-24V - for 12V operation use ballast resistor 0.6-1.0 ohm.

Not to take away from his article but it leads to some confusion on my part.

I have a '74 with a Black KW12V coil. Is this coil suppose to have a 1.8ohm resistor as he first states or a 0.9ohm as the information also seems to show? I have found a Bosch coil which is suppose to be the replacement for the Black coil but it says to use with a 0.9ohm resistor. Or should I buy a Red coil which is to be used with a 1.8ohm resistor?

This is the reason I want to figure out the value of the resistor wire, only I can not seem to find the starting point of the wire. Also my black/red wire that ends on my coil is unplug on the other end and I do not know what it should attach to. I think that this is the wire that supplies 12 volts while the car is cranking. Maybe this wire is not needed since the car seems to start fine without it although that may change in cold weather.

[peterschop]

Scott, I had found your pictures about removing the resistor wire. My car is a '74 and the wires are either black taped or in tan sheathing not blue. You said that the resistor wire started on the drivers side. Was it attached to another wire or was it plugged into something like the fuse box or ignition switch? I hate to start removing all the sheathing because I am not planing to remove any wires.

Peter

[Scott Sislane]

Scott, I had found your pictures about removing the resistor wire. My car is a '74 and the wires are either black taped or in tan sheathing not blue. You said that the resistor wire started on the drivers side. Was it attached to another wire or was it plugged into something like the fuse box or ignition switch? I hate to start removing all the sheathing because I am not planing to remove any wires.

Peter

the wire comes off a relay on the firewall, and is soldered to the "reistor wire" which I spictured in my thread. I removed that section of the wire as I have a hot spark box. (MSD box / Streetfire Box)

Having too much resistance should not cause your coil to get hot, as far as I know. If that's why you're looking for the wire, I think you're heading in the wrong direction.

Scott

[peterschop]

I just found the starting point of my resistor wire. I unwrapped about 18" of black tape from the harness. Right in front of the driver on the firewall I found where the green wire is crimped onto the resistor wire. I was able to measure the resistance of the wire. Zenon was right. It is 1.8ohms. This is the place that I will splice the wire going to the red Pertronix ignitor.

This should help in selecting my new coil. I believe the Red coil requires 1.8ohms.

Peter

[esty]

old thread i know but it never hurts to ask and learn...above, Peter says he found the resistor wire crimped to a green wire...on a 74 we are working on, the green wire originates at fuse 12 and some distance out of the fuse box the clear resistor wire is spliced to the green, fuse 12 wire...is this normal, typical?

 

the car has a black coil now, if we changed to a blue coil i assume we would just remove the resistor wire and splice a regular wire to the green wire...right...wrong?

 

['76mintgrün'02]

Normal, typical, right, but what's wrong with just leaving the resistor wire in place? 

You can run a new wire beside it, to keep it company. 

 

I think they are a cool feature and one in working condition has value.

If you snip it you could flip it along to someone in need. 

 

Assuming it still measures the proper ohmage, I say pay homage to the ingenious design and keep the German-made black coil sparking, as long as it's warking the way it should. 

 

Still good is good enough for me.  Newness is overrated and overpriced. 

 

Black Coils Matter!  (no-offense)

Tom

[albatcha]

I took a few pictures last year when I changed the wire on my 74 tii, I don't know if the fact that it's a tii makes a difference to whether or not they would have spliced a wire.

 

On my car it was the clear/yellowed resistor wire all the way to the fuses.

 

Maybe someone measured the resistance, it was too high and they cut out the amount of resistor wire they needed until it was low enough.

[albatcha]

In case of any questions yes I know the car is carbureated but the wiring harness is the original tii harness

['76mintgrün'02]

I think the resistor wire that runs to the fuses is correct for a '74.  That came up in a discussion a while back, when I was trying to describe where to find the splice in on my '76.

 

I wonder if it has to do with the placement of the coil.  The coil was moved to the front in '75, correct?  To avoid the heat of the nuclear reactor exhaust manifold?  Maybe they just added the length of wire to adapt to the new position.

 

The Blue book lists proper coils per model year and there is variation in the resistance over the years.  I think some even had built in resistance, like the blue coil, but it was before blue coils were blue.

Blue coils matter too!

 

Tom

[esty]

how critical for a driver is precise resistance anyway?...what are the ramifications of wrong resistance?

 

[JohnS]

3 hours ago, albatcha said:

I took a few pictures last year when I changed the wire on my 74 tii, I don't know if the fact that it's a tii makes a difference to whether or not they would have spliced a wire.

 

On my car it was the clear/yellowed resistor wire all the way to the fuses.

 

Maybe someone measured the resistance, it was too high and they cut out the amount of resistor wire they needed until it was low enough.

 

I can confirm that the resister wire on the 74tii is twice as long as that used on the '74 and up "regular" 2002s.  It's twice as long and thus has twice the resistance.  The resistance wire on the 74tii runs from the fuse all the way to the coil.  It's designed for use with the "red" coil.  I kept mine stock on my 74tii instead of butchering the original wiring.

[JohnS]

56 minutes ago, esty said:

how critical for a driver is precise resistance anyway?...what are the ramifications of wrong resistance?

 

Depends if you're using points, or Pertronix, HotSpark, 123, etc.  Bypassing the resister wire and installing a blue is an easy way to go.

 

Edited May 4, 2021 by JohnS