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How is this possible??

527 views 5 replies 3 participants last post by  rick2718 
#1 ·
1989 300se, 250,000 miles


I pull the coil wire off the center of the distributor cap, stick a spark plug in, ground it, add a strobe timing light to
the mix for good measure - when I crank the engine, the plug fires constantly as does the strobe, as expected.

When I plug the coil wire into the center of the distributor cap, I get no spark at any
spark plug. The plug wires were all fine 4 days ago, and the multi-meter does not show any significant resistance in any of them.
The distributor cap and rotor are new.

how is this possible?

thanks

(ran fine every day for forever, got hard to start when cold, now won't start at all after sitting 4 days.)
 
#2 ·
I don't believe it is possible. I'd sure be closely inspecting that new cap and rotor. Maybe check for continuity on the center electrode?

Also check for grounding somewhere. A new track inside the cap leading to the distributor? There's got to be something amiss. Maybe look at the center electrode and the rotor with a magnifying glass.
 
#3 · (Edited)
I would check continuity of the center contact in the cap, if that is good then take some emery paper and clean the glaze off the center of the rotor. Also check the continuity between the center of the rotor to the tip of the rotor also before reassembly. If anyone of those steps test negative it's probably best to return the parts and install genuine Bosch.

Good luck.
 
#4 ·
Thank you for the replies.

The meter shows continuity from the center of the rotor to the edge contact of the rotor. Each pin has continuity through the cap.

The one thing which is harder to check is whether or not the spring-loaded center
contact on the inside of the cap is actually touching the contact at the center of the rotor.

I tried to test by turning the engine over until the rotor pointed to a plug wire contact then checking for continuity
from the center pin (outside the cap) to that plug wire pin (outside the cap) - the meter shows no connection.

but then I thought that there must be a jump-the-gap spark going on each time the rotor passes a plug wire connection.

Is that true?

I always thought of it as making a physical connection from the rotor edge to each plug wire contact in turn,
but even if they could be machined so precisely and the rotor did touch, it soon wouldn't any more.

So a meter from center pin of
cap (on the outside of the cap) to plug wire contacts (also on the outside of the cap) would show no connectivity even when
the rotor is passing by (assuming the sample rate of the meter was fast enough to catch it).

is that logic correct?


and if it is, could coil strength matter? given that a wire from coil output to a spark plug works - jumps the gap in the spark plug,
is it possible that it might still not jump the gap from the rotor edge to the plug wire contacts inside the cap?
 
#5 ·
Check the continuity of the king lead, that's the one going from the center of the cap to the coil, then install the king lead to the cap and check from the inside center of cap to the end of the king lead; the end that would go into the coil. That checks the connection and continuity of the center of cap also the coil lead at the same time. Even with a weak coil you should be seeing some kind of spark at the plug unless the coil is totally fried.

A few years I had a 94 Toyota Camry come in with no start condition, turned out to be the center contact was worn down and not passing current to the rotor. To confirm I just pulled on the center contact a little to stretch the spring a little and it started right up first crank. Replaced the cap and rotor the plug leads tested good job done.
 
#6 ·
That was it, thanks for the suggestion.

Even though it was a new cap, I had to stretch the little spring under the center button to get it to make solid contact with the rotor.

The spark showed up afterward... it was a dull orange spark and she still wouldn't start, but a new universal off-the-shelf coil and
she starts instantly and cruises again. ready for the next 250k


thanks again
 
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