PeachParts Mercedes-Benz Forum - View Single Post

oldtrucker · #7 01-06-2015, 06:36 PM

Not knowing what the other posts suggest on testing ignition coils, I think it can be very difficult to find a hair crack in the isolating housing.
Electro Technically, they don't go bad or lame or whatever. If the coil was sealed during manufacturing and it stayed that way, the only way you can kill that coil is to overload the secondary. On the secondary the wire is much thinner as it is on the primary e.g. low voltage side. A short spark plug could possibly overload the coil. But, I believe there is a firmware routine to detect that.
Real life, unfortunately nothing is 100% and there are always chances that contaminants, over time, can make it into areas they are unwanted. This in return can shorten the lifespan and maybe noticed in lower performance.
A coil manufacturer does specify the maximum output voltage, among other values, which in general, can range between 18,000 - 45,000 and more Volts. The coils for our cars are probably around 25,000 Volt. Here is the first problem, not everyone has a high voltage tester on hand. If you do and the voltage conforms with the specification of the manufacturer, it is usually safe to say that the coil is good. This excludes any cracks that could cause arching to any nearby metal that is attached to chassis ground or timing issues at high RPM's.
With that said, I believe any DIY testing is merely a guess and can never be 100% conclusive.

To test a coil conclusively one would have to do at least two tests.
1. High voltage output test (would require a pulse generator up to 7000 Hz or more and an Oscilloscope with high voltage probes to see if there are any misses)
2. Isolation test using a Hi Pot Tester (There are some extreme expensive ones on the market)

There are more tests that could be performed, but that is something a manufacturer would consider for QA.

Perhaps, a automotive electric shop maybe in the neighborhood and they maybe able to test the coils.

Here is something else that is overlooked. The Vehicle manufacturer does install ignition coils as a matched set. If one of them fails and needs to be replaced, the replacement may have different test values. Is this bad? possibly....
My point if you think or have confirmed that a coil or all are bad, replace them as a matched set. Bosch does test and match their coils in a set or their tolerances are very close, thats why they are more expensive.