What is easiest, reasonably accurate, and reasonably inexpensive way for me to test output of coil? I think mine is too weak under load, and gets progressively weaker under load after several minutes.

Is there a reasonably inexpensive device to test coil output, and then output of battery and voltage regulator as well?

I'd like to isolate electrical current and coil issues without taking vehicle in.

thanks.

Probably the only accurate way to determine coil output is on an ignition scope.
To test all other electrical components, you should buy yourself a good DVOM ( digital volt/ohm meter ).

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2007 C 230 Sport.

coils can be evaluated with a voltohmmeter.

i think it is important to do it with a coil both cold and hot.

first thing is to check for continuity. this will tell you if there is a break in the windings. if there is, the coil be stressed to provide sufficient voltages at speed and load.

it is also important to check for resistance if there is continuity. coil manufacturers will have a spec for this. your coil should measure out within their specs.

personally, i like to put a scope on the secondaries and read demand voltages for each cylinder with the engine at idle. important insights can be gained by running the engine at its max torque rpm.

if you don't have the gear to do this, find a good auto electric shop in your area. they should be able to do this.

alternatively, buy a new coil and see if there is an improvement. do check the coil before installing it, however. you would be surprised how many bad coils are out there in the "new" condition. you might want to do this before you leave the parts counter because often electrical parts are not allowed to be returned - no matter what.

a votre sante

Manny & others:

i'm going out today to get this meter. Qustions:

1. Isthis a typical item at auto parts discount stores, and how much do they run?

2. specifically, how do I check both coil output (I have old 'barrel type), and spark plu wire output? You'll ned to spoon-feed me on this.

thanks!! I don't want to make any mistakes here, especially in the testing procedure.

An ohmmeter is about useless in finding modern coil problems.

The only way to test the available output is to put it on a scope and pull a wire and see how high the voltage peak goes before the energy disipates.

The reason the ohmmeter doesn't work is that the resistance to ground of airgaps can't be measured and the common problem is internal short circuiting at some voltage. Trying to find this with an ohmmeter would be like trying to judge the size of the plug gap by measuring the resistance. Obviously a 0.010 gap will show the same resistance (infinity) and will a 0.040 gap.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician

This problem you are having could be anything in the secondary ignition system or a cloged fuel filter may also show up as misfiring under load. Typically a coil is the last thing I look for. Cap, wires, plugs are a little more likely. In my life time I have probably replaced about half a dozen coils. Only on two of those cars (Both Fords) was the coil really bad. The other coils I replaced I owe to my education in "Learning How to Fix Cars".

The best way to test a coil is with a scope. Short of a scope the next best thing I have found is a KD GM high energy ignition testor, which is nothing more than a spark plug with a ground clip on it and the center electrode removed so that the spark under no load has to arc up through the insulator and accross to the spark plug case. Place it on the coil wire first. If you spark is consitant accross this gap your coil is probably OK. You can also eliminate each wire or the cap by moving the testor from the center wire to each plug wire.

Be carefull when I got mine years ago 70's there were two versions "GM High Energy" With no center electrode in the plug (KD 2756) and the Standard ignition tester (KD 2757), for use in ignition systems with points (what are points?) or for lawn mowers. Both are available from mytoolstore.com for under $7.00.

For battery and voltage regulator, use a Digital Volt Ohm meter. With everything on if you have between 13.5 and 15 volts your fine. To isolate problems you might also want to get a clamp type (inductive) current meter.

John Roncallo

Inetd, are you sure about the 5V supply voltage to the coils? I'm using Alldata and seeing that battery voltage is supplied to terminal 15 of the coil's primary winding and primary ground is switched on terminal 1. I don't think a 5V supply to a primary winding could induce a secondary spark strong enough to support combustion.

Years ago and maybe still true today it was not uncommon to use a balast resistor to the coil primary. This resistor would allow full battery voltage to the coil during start up but cut it down to about half when the resistor/engine warmed up. So 5 volts may very well be sufficient for these newer coils it al depends on design.

John Roncallo

I have never worked on a car with plug coils. But I do remember the GM HEI distributers had a special inductive adapter and maybe that whats needed in your case. It would apear to me, that to deal with plug coils you would have to move the pickup from coil to coil to get each coil output. I hope a younger tech can enlighten us here. Also I wouldent be to quick to not replace coils on these cars since I know some systems have coil replacement service intervals in the 100,000 mile range. I dont know what Mercedes is doing with this.

John Roncallo

I checked again for the 98 S500 ignition coil wiring using Mitchell On-Demand instead of Alldata and came up with battery voltage supplied to pin 3 (yellow/green wire), ground supplied via engine computer on pin 1 (black/xxxx wire), and pin 2 (brown wire) permanently grounded on left and right sides of engine. Pin 3 constantly supplies one side of primary coil with battery voltage, while pin 1 grounds the other side through the computer controlled drivers. Pin 2 supplies ground for one side of the secondary coil, while the spark plug supplies ground for the other side.

I'm thinking the computer is providing the 5V pulse on the ground side of the primary coil during dwell to create a voltage drop of around 9-10 V across the coil.