Bad Coil?

[2.7RACER]

I think I have a bad coil. It's a new Bosch #25, specified for my 2.7.
Connected it up paying attention to the proper wiring of the primary side. - to ground, etc.
Runs rough, real rough. Reinstalled my old coil runs fine.
Checked the new coil with my ohm meter.
The primary read .735 ohms.
The secondary read 746 ohms.
Doesn't match this.
Not even close.
TIA,

[ruf-porsche]

If the engine runs fine with the other coil and the engine runs like crap with this coil, and the only thing you did was to replace the coil, I would say BAD COIL.

[2.7RACER]

Thanks for your response. I'm looking for confirmation of the testing method in my post.
Perhaps someone has tested this model coil.

[911quest]

I guess the windings could be bad.

Iam really starting to question BOSCH qualty any more I have had problems with there points Iam fighting a week old fuel pump that doesn't want to pump any more

[RoninLB]

Warren posted interesting info on why many Bosch coils have become unreliable mostly due to mfg being sourced in cheap mfg countries. That said, I don't trust the ohm testing for confirming a good coil. I would do exactly as 2.7RACER did with a known good used coil. I don't even trust a new coil. I don't even like the idea of mounting the coil on the fan shroud. I mount mine on the electric panel with rubber islolator feet and use the seperately ordered Magnacore long dizzy to coil wire that was around $10. I've seen so called good coils go crazy after the engine compartment heats up.

[ruf-porsche]

Originally posted by RoninLB
I mount mine on the electric panel with rubber islolator feet and use the seperately ordered Magnacore long dizzy to coil wire that was around $10. I've seen so called good coils go crazy after the engine compartment heats up.

RoinLB can you post a picture of this setup

Thanks

RUF

[Early_S_Man]

There is nothing wrong with your testing method, but the values cited in the article do not apply "to most automotive coils," and most assuredly, not the Bosch 0.221.121.001 or 0.221.121.006 coils for the CDI!

Here are a couple of cautions to observe when testing CDI system components:





What do your test leads read when shorted together firmly? Subtract that reading from the primary reading you got ...

Bosch says 0.4 Ohms - 0.6 Ohms for the primary winding, and 650 Ohms - 790 Ohms for the secondary winding.



The best test for an ignition system is to observe the waveforms from it on an ignition scope like an old Sun Analyzer!!! Alternatively, a traditional lab oscilloscope monitoring the output 'A' terminal of the CDI unit [input to the coil primary winding] can tell you if the coil is getting the proper signal to produce a 35 kV - 46 kV signal for the sparkplug.

[ruf-porsche]

What do your test leads read when shorted together firmly? Subtract that reading from the primary reading you got ...

Bosch says 0.4 Ohms - 0.6 Ohms for the primary winding, and 650 Ohms - 790 Ohms for the secondary winding.


According to the BOSCH spec, your new coil is within their spec tolerance.

But which is more believable the spec or the fact that the coil makes the car run like crap?

[RoninLB]

Originally posted by ruf-porsche

RoinLB can you post a picture of this setup

I'm may not the best example to follow and there are other pics available somewhere in "search" of other pelicanheads who have moved the coil over. The weak points in my install is that the coil is behind the panel cover out of any air-stream and the wire is close to a ground.

The coil to dizzy long wire is enclosed in small dia fuel hose and run along the enclosure for the lid lock cable.

[ruf-porsche]

Thanks for posting

[2.7RACER]

Thanks Warren and Ron,
It looks like I'll investigate further to determine what is going on with this brand new coil.
Russ has an old Sun machine that should give us answers.
Great info from everyone.
Hey, I'm just after a strong spark.
I'll report back whatever I learn.
Thanks again,

[Chris Ryan]

In checking to see what experience fellow readers have had regarding bad coils, I ran across this string. My wife's 76 911S died today for no reason. I had recently installed new wires and a new coil to freshen things up & thought somthing may have come loose, etc.. Going through the ususal fuel/spark troubleshooting, I quickly determined that there was no spark. Checked all the obvious stuff - points, CD box (making the whining noise), checked the primary & secondary coild resistances and found they were in the proper ranges. Tried again - no start. Just for fun, I then swapped out the coil with the old one I had recently replaced and the car started. Is it really true that the resistances can appear OK but the coil is bad ??

[Early_S_Man]

Chris,

While it is true that a coil can be bad and still test within nominal static specs ... that can usually be seen on an ignition analyzer like an old Sun machine.

In this case, a bad , corroded connection, aggravated by rapid changes in temperature in winter may have been the problem, and changing the coil eliminated the bad connection. I suggest putting the original coil back and retesting.

Here are some threads that may be helpful:

Ignition Troubleshooting??????

History of Bosch CDI ... toubleshooting info, parts list changes, and schematics ...

[Chris Ryan]

Thanks, Warren - Useful info in those thread links. I'll clean up all the connections and re-install the "new" coil & see what happens. One other question - the ignition switch in this car "feels loose" sometimes. Is the feed to the CD box/ ignition system coming directly from the switch making it a possible culprit for an intermittent bad power connection or should I concentrate on the connections in the engine compartment only ?