A lucky eBay find: NOS SWB Coil

[edmayo]

So you figure nothing electronic could ever fail ?

[304065]

304065, Is it this you wanted to see?

Attachment 400071

and after installation.

Attachment 400070

Nice. October, 1967 date coded voltage regulator. A keeper!

Panel looks great. How come the two "wye" connectors? The red wire from the ignition goes into the single end, out the double end come the fuel pump and the power to the resistor single end. (The double end of the resistor has one wire to the coil and the other one to the 87 terminal of the resistor relay- that way when the relay is energized by the yellow wire (to 86) power flows directly from the two-terminal block to the 30 terminal of the relay, and to the 87 terminal, where it then goes to the coil, bypassing the resistor entirely)

[304065]

So you figure nothing electronic could ever fail ?

And not only fail, but fail on the way home from the Parade, in the pouring rain, at night, with the handheld oscilloscope left at home on the bench!

All Petronix (or Hot-spark) users would be wise to carry a backup complete cast-iron distributor with points in the spares kit, a dwell meter and timing light.

At least that's what I keep in my trunk . . . instead of golf clubs!

[CarmelSkip]

So who has a current source for the green decal ?

[718RSK]

I have some left

[71gold]

I would love to have one as well
Thanks
Frank

[CarmelSkip]

I have some left

. PM sent re purchase

[Merv]

Love the green Bosch decal. Already have one on my 911. I will never take out my BR18 iron bodied distributor from my 356 either.

[CarmelSkip]

I cleaned up the coil and it's ready for a nice green label. Any help would be appreciated

[911MRP]

Old thread and post, but one that folks might look at when testing coils. Has some useful information. I haven't seen much Bosch data on the spec of coils but do have a datapoint for 0 221 121 001.

I tested a few more coils for comparison purposes. Useo pop these data at your own risk of death, property damage or personal injury. No kidding there Tesla, you can kill yourself or burn the house down if you don't treat high voltage with respect.

These are measured specifications with an LCR meter and an ohmmeter. Your results may vary.

Bosch "Black" coil 0 221 102 006 8.5mH, pri res 2.1 ohms. Original SWB coil as above.
Bosch "Red" coil 0 221 119 030 8.2mH, pri res 1.3 ohms. Requires external ballast resistor.
Bosch "Blue" coil 0 221 119 027 12.8mH, pri res 3.2 ohms. This has an internal resistor.
Bosch "CDI" coil 0 221 121 001 1.8mH, pri res 0.2 ohms. This is for Bosch CDI only.
Bosch "Motronic" coil 0 221 118 322, 4.5mH, pri res 0.4 ohms. Original on Motronic cars.
MSD "High Vibration Blaster" coil 8222, 8.0mH, pri res 0.7 ohms, sec res 4.5k ohms, turns 100:1

As you can see there is a huge difference in Coil specifications over the years.

A few observations.

Points had a max current limit of about four amps. With a +12V nominal voltage at the coil, that meant that three ohms of resistance was necessary in the ignition circuit to keep the points from burning up. Hence the SWB 006 coil was used with an external resistor for normal operation, which resistor was bypassed during cranking when the voltage was lower anyway.

Same thing applies to the Red coil. This must be used with a 1.8 ohm external ballast resistor for points operation. That gets you 3.1 ohms, see above. For electronic ignition it's a different story, see below.

The famous "Blue" coil teaches us a lot. It has both an internal ballast resistor and a very high inductance. This will produce more ignition energy, but this coil's output will fall off dramatically as RPM increases. This coil would be fine for four-cylinder, low RPM motors such as the VW beetle, in which community the Blue coil is frequently found.

The CDI coil has very little inductance because in the Bosch CDI system, the coil does not store ignition energy, it's stored in a capacitor, which discharges through the coil. So the coil's purpose is to serve as a transformer only to step up the voltage from the 460V output of the CDI.

The Motronic coil is an interesting one. Looking at this, we see the tradeoffs made by Bosch and the Factory to get the right balance between ignition energy and high-rpm performance. Fairly low inductance and very, very low resistance-- this is due to the fact that the coil is switched by a transistor inside the Motronic unit, which can probably switch 10 amps or so. Having an onboard coil driver probably isn't great for the longevity of the control unit, however, which is probably why the Factory and Bosch started using external coil drivers with the 964 and 993.

The in spec on the primary winding of the Bosch coil 0 221 121 001 is 0.4 -0.6 ohms range according to an original paper I have for the Carrera RS of 9.72. It is a document from Bosch , the one in my files is the original paper not a photocopy.

It is consistent with the figure given in the original post.

While this excerpt is pertaining to the early RS specifically with the Bosch 121.001 coil type is not unique to the RS

This is similar for 2.4S dated a year before with same resistance range


This is a used spare 001 coil I keep for my RS. It is Bosch date code 227, July 1972 so also made in 1972 but a tad earlier than the original fitted to car. The test results of that primary winding resistance was at the mid point in the Bosch published range.

Attachment 613651

If it helps while I didn’t have a Bosch spec range the measured secondary winding of this old coil test results between terminal 1&4 were 683 ohms.

This result is ok for a spare — I’ve never seen a specified range for this secondary winding from Bosch printed in black and white. Seems to be within the range mentioned in first post for this type of coil 650 to 790 ohms.

In addition to the spare the .001 coil that is in my car, is almost certainly its original, based on date code being car’s manufacture date minus a month. I’ve not touched it in over thirty years. The prior owner was a racer over lunch he has assured it wouldn't have even thought about date matching back then only would have ensured it was correct Bosch part if needed. Haven’t measured the presumed original one.

The original poster did more comprehensive test it seems. I have several Bosch automotive oscilloscopes plus lots of other matching vintage Bosch test equipment from late sixties and seventies besides but I only did a quick check with a simple pocket multimeter that I keep to hand. One of the Bosch oscilloscopes can be glimpsed in this photo I happen to have

From this leaflet if seems it was the top of the range oscilloscope from Bosch around time the cars were new


The other Bosch oscilloscope I have was a cheaper reduced function model from that era ) but this is still decent old school bit of kit.


How many do I need (actually just sold third one like this to a pal) ? Unlike people who work on these old cars for a living for me just a little hobby but do like to have tools and equipment of the period.

It seems the particular coil .001 (in list on the post quoted see underlined) falls outside of the spec if measured 0.2 ohms between terminals 1 & 15 or perhaps the result needs to be rechecked as value seems too low compared to range on the paper ?

Adding nrw information to the old thread to help others who might like to see funfurnatiin of primary circuit range on a paper printed by Bosch back when the model 72/3 MFI S and RS were new plus a couple of measurements from a vintage survivor coil from summer of 72, at the cusp of the 72 to 73 model year refresh.

Both the coils I have still have their original sticker and match photos of 73 RS taken at new, so another of those small details where repair and restoration isn’t faithful to how it was as many stickers I see on RS these days are different. Less important than if a coil not working to the required spec but since this thread at one point was about a stickers I mention it.

Steve

PS for eagle eyed the battered old Purolator oil filter sitting on the oscilloscope came from a car at my friend’s workshop that was a dusty garage find. Car hadn’t been serviced in decades so I retrieved the mangled oil filter from the dumpster because I wanted an example of an old original one as a reference but haven't got around to looking at how it differs in graphics etc vs the current Classic reproductions.