checking dwell and testing coil
 

i was trying to check my dwell angle and the #'s are all over the place, the same w/ the rpm's. i used the same meter on my range rover and was able to get a gd rpm reading. then i decided to test my coil reading and they were way off, i compared it to a coil i had sitting in my garage (was off a landrover discovery) and those #'s were fine. 1- can i use the discovery coil in the porsche (76 911S), 2 - any suggestions! thanx

Mybee I noticed in my Gal freind 740 volvo uses the same coil Bosch coil transformer as my stock coil for my 72S. I thought, I wounder if she would know if I traded.. CHEAK PART NUMBERS FIRST!!!

Unsteady dwell values can be a sign of worn distributor bushings or loose/mechanically damaged points (the latter is not very likely). Perhaps it is time for a distributor cleaning and refurbishment. Do a search; Early_s_Man has some good information posted. Cheers, Jim Sims:D

anc,

There are many things, including points and other worn distributor parts that can cause dwell/tach readings to go crazy. Your distributor is supposed to be the same as that on '73 911T engines, so everything I posted in the following thread (except for RS 2.7 specs) applies to your distributor:

http://www.pelicanparts.com/cgi-bin/vb/showthread.php?s=&threadid=5432&highlight=distribu tor+lube

Your coil is supposed to be a Bosch 0.221.121.001 model, and factory specs of 0.4 to 0.6 Ohms for the primary winding can be hard to verify because of design problems with many of the inexpensive digital multimeters on the market today ... which give a reading of '1' or sometimes '0' with any value 1.5 Ohms or less! There are seldom ever any problems with the primary winding, so you don't really need to worry about it. Shorts between turns of the secondary winding is the most common problem, along with oil leakage, with the CDI-system coil. So, if you get a reading below the specified 650 to 790 Ohms, so a substantially lower value indicates a problem. You probably should verify readings against a second multimeter before deciding to order another $100+ coil, though! One more thing ... when cheap digital multimeters give you a low battery signal or ennunciator, <b>DON'T TRUST ANY MEASUREMENTS! Go change the battery, before making any more critical measurements!</b>

Since that same coil was in use continuously by Porsche for 21 years from the fall of 1968 thru the summer of 1989, it does not seem unreasonable to find it used on other model and make vehicles that used a CDI-ignition system!

i'm using an actron cp7676 that i bought friday, i measured .2 on the primary and .6 on the secondary. i then tested another coil that i have sitting around from a range rover, also a bosch, and it tested .6 on the primary and 7.4 on the secondary. i'm using the meter on 200 for the primary and 200k for the secondary. the range rover bosch is within specs, the one in the porsche isn't. couldn't this be a reason for the #'s being all over the place.
ps the rr bosch coil is #0221122392 there is then underneath that the #'s 467 965 , thanx for your knowledge, i'm not afraid to work on my vehicles, but i don't want to throw away $, regards, Andrew 76 911S

Andrew,

That is not the correct coil for your car! Does your car have the stock Bosch CDI-unit, or a Perma-Tune box? So, has your car been running OK with that coil?

The Bosch CDI-unit can be damaged by using coils of the wrong impedance, and that coil is DEFINITELY the wrong impedance!

Perma-Tune claims to be compatible with any coil that does not have an internal ballast resistor, like VW Beetle coils, but their reputation and reliability has come into question lately!

That Actron/Sunpro is an'OK' meter, especially if bought at the web clearance price, and doesn't have the 1 Ohm problem I described. But, you don't have the capability of 'zeroing' for the test lead resistance, so you have to subtract the 0.2 Ohms or so of lead resistance from whatever reading you get for low resistance tests on the 200 Ohm range.

thanx for your input, the coil i mentioned is the one off a range rover and is sitting in a box, the one in the 911 is 0221121005, that's the correct one isn't it, i think the coil is no gd, i'm going to try to pick up another one

Andrew,

No, it is supposed to be 0.221.121.001. Recheck your part number!

Unless I misread your post, you don't seem to be using the correct range on the meter to test the resistance. You want to use the LOWEST range that will give you a reading! Use the 200 Ohm range for the primary, and the 2K range for the secondary. What were your results on those ranges for the Porsche coil?

the primary is .2 (the meter set to 200) and the secondary .548 (meter set to 2k), the coil # is 0221121005, thanx, Andrew

The good thing about working on a 38 year old car is an 11 year old thread still answers my question.
The old girl hadn't run in a while and was mis-firing badly. Before I dropped $125 for a new coil I threw the ohm meter on it and it tested okay. Cleaned up the contacts and voila! runs like a champ.
-C

Gotta love those free fixes!

Yep, best kind!

Warren has left a treasure trove of wisdom to all of us. What a wonderful gift.