How do you test a Coil? any obvious signs?

[crammy69]

How do you test if a coil is owrking correctly or not? is there any visible signs to look for to ascertain its condition?

Cheers

 

[wireman]

Not easy without lots of electronic test gear and some knowledge.
But,
An ignition coils consists of two windings one for the 12v bit and one that gets up to 25,000 or more volts for the plugs. It depends on the coil and ignition circuitry as to what resistance you should be able to measure with a DVM, the LT winding should have a low resistance of a few ohms or less and the HT winding somewhat more, 50- 1000 ohms being in the range of acceptability. Having a known good coil is about the only way to get some sensible values to measure at.

It is not uncommon for coils to fail on insulation or shorted turns, mostly this occurs in the HT winding where the change in resistance is almost unmeasurable and would not be a relaible indicator of failiure if you could do.
The cause of these insulation faults can be running with an open circuit plug i.e. damaged plug or lead off, damage from over heating and just plain old age or the ingress of damp.

Inspect the coil assembly for cracks and filth since both of these will stop it working properly.

There is nothing that can be done to repair most coils, replacement is the only truly reliable cure.

If all the ignition components are available testing the length of spark will indicate to some extent that the coil is OK, but beware that the coil could be damaged by over expectation of spark length, dont try and get inch long sparks or the insulation will get damaged, about 10mm should be enough of a spark for most normal engines.

Beware that on modern electronic systems the spark energy is very high and it will realy hurt if you let the sparks hit your skin and cause localised burning, let alone the tendency for folks to jump away from the source of shocks and fall over, hitting the floor or a shelf head first hurts even more.

 

[television]

You are talking about a normal coil and not a coil pack.


A coil has a secondary with about 15.000 turns on wire, if one turn goes short circuit, this cannot be measured on a multi meter and the other problem is that multi meters use a battery to measure ohms, and this voltage sets up a magnetic field that is apposing the measurement that you are trying to do.. There are specific transformer testers but they are not cheap.


A car coil is either 8 or 12 volt. With the 8 volt coil, when the starter is engaged, this shorts out the ballast resistor that is normally in series with the coil, giving a 30% boost to the spark. The 12 volt coil is just wired in direct.

How the coil works;;
12 volts is always on the small + terminal, the - terminal goes to the points, and these can be a photo cell or hall effect. When the points are closed this grounds the - on the coil, and a magnetic field is set up, when the points open this field collapses back into the coil, in both the primary and secondary, as the secondary has some 14,000 more turns than the primary, this is what gives the high voltage spark. Now when the points open the induced voltage will try and come out through the points, so to stop this a condenser is wired across these points, and the charge across the condenser plates is out of phase with the induced voltage and acts as a gate, thus forcing all of the induced voltage out of the high voltage secondary.

Coil failure:::
A coil can start off fine with a good blue spark on the secondary, even when faulty with a short turn. slowly the short circuit turn will appose the output voltage causing rapid heating of the coil, and slowly the output from the coil will fall away until there is a very small spark, too small for the engine to fire or run.

The correct value for the condenser is 2.2 mfd for 12volt coils and 3.3mfd for 8 volt, a faulty condenser will as from the above give a weak spark, but it would be like it all of the time.

Hope this helps.

 

[crammy69]

Many thaks gents for your detailed and clear responses (most of which I didnt understand being an electrical numpty of the first order!). In laymans terms would it be fair to say, given the cost is only £50 approx, that my best bet would be to just replace the coil ( which appers to be easily accessed and changed) and have done with it?

Many thanks!

 

[television]

Many thaks gents for your detailed and clear responses (most of which I didnt understand being an electrical numpty of the first order!). In laymans terms would it be fair to say, given the cost is only £50 approx, that my best bet would be to just replace the coil ( which appers to be easily accessed and changed) and have done with it?

Many thanks!

Thats a hard question crammy,, what is the fault on the car.

 

[crammy69]

Well its the same misfiring issue Ive had since day one really. Having put up with it for 6 months and done a few bits and pieces, there has been a number of threads where similar problems have been cured by a combination of new coil and ECU refurb. I understood that before I get the ECU examined I should at the least change the coil first as a damaged one could spike the ECU (although I have since been told that as my car has the EZL ignition as well as an ECU that it shouldnt in fact suffer from ECU damage from a duff coil).

 

[television]

Yes correct the coil would not damage the EZL.

The coil is either good or bad and no in between, and I say that from what can happen in theory, practice and experience.

If the coil is bad with a short turn the spark will fall away to zero and the car would not start, switch off and let it cool down and so the cycle goes on.

If the coil had some odd fault and low output, then it would be hard to start the car, this could also be caused by a bad condenser, or incorrect value though the whole engine performance would be affected and not just idle where the coil is running hardly under any load.

By all means buy a coil,,but take care,,my local man just bought a box of 12 all faulty.

It must be better to go to a breaker and buy a second hand one of a good make,,,Bosch,Lucas etc they will only cost a couple of £ each as no one buys them these days,, and do get the correct voltage. take care on this as I have seen 12 volt coils wired through a ballast resistor, this would give an all time low voltage output

 

[crammy69]

Thanks Malcolm, the misfiring only starts when the engine is warm and up to normal running temperature. The Misfing is present throughout the rev range not just at idle, its just more noticeable at idle ( once the cold start valce has cut out). I do get hot starting problems (reluctance to start, it always does, it just needs an extra few turns of the starter motor to get it to catch), it normally stalls when started from hot (its fine from cold ).

Im a little unsure what to do now, I was working on the basis that the coil would either sort it, or be the trigger for me to send the ECU (not the EZL unit on the inner wing but the actual ECU in the bulkhead area) off to the IOW or Alan at ECU Clinic to diagnose the ECU in case this is the cause of my lumpy running/misfire. What do you think?

 

[kth286]

If you buy a new one get it from Mercedes dealer for two reasons.

ALL ignition parts for cars should be replaced with dealer parts. It is a well known fact that copy electrical parts do not perform as they should over the long timeframe required.

AND you get 2 years warranty with genuine Merc parts.

 

[kth286]

Two critical components are :

1. coolant temp sensor (they alter resistance with temp, but go out of spec and are cheap). Critical for correct fueling.

2. lambda sensor (if your car has one)

 

[crammy69]

Two critical components are :

1. coolant temp sensor (they alter resistance with temp, but go out of spec and are cheap). Critical for correct fueling.

2. lambda sensor (if your car has one)

1. I changed the Colant temp sensor early on ( On Malcoms advice) as it was easy and cheap!

2. I dont have a catalytic convertor so shouldnt have one of these ( luckiliy!)

Cheers

 

[kth286]

Previous owner may have removed cat but you may still have lambda sensor (oxygen sensor) on that generation car, have a look.

 

[crammy69]

Its a 1990 model, would it have a at originally, I though that was post '93, mind you I guess Mercedes would have ben ahead of the game on that one....

Where would I find it do you think? I know thier is an air temperature sensor that connects to the AirFilter box tubing, I guess its not that that you mean.

 

[crammy69]

So Malcolm, is there really not much chance that my coil may need replacing given what i have said?

 

[television]

So Malcolm, is there really not much chance that my coil may need replacing given what i have said?

Very little chance, if it was the coil it would spit back when accelerating.

The only thing that changes on your car is that the mixture is weakened once warm. If it was the coil, the car would not pull at all, your problems are all hot running and start.

I know that when coils go short circuit, that they do heat up and rapidly fail after a few mins, they go so weak on spark output that the engine stops.

As earlier,,pick one up for a couple of pounds from a breaker and try,,but I still do not think that this is the problem, at idle the coil has a little bit longer to recover.

When the fault is there,, why not set up one of the sparking plug leads so that the wire is about ½" away from the metal part of the engine,, start the car and there should be a fat blue spark from the lead.

I often wondered how you were getting on with your fault and I do remember that you had the Bosch diagnosis done

 

[crammy69]

Thanks Malcolm, its funny you mention spitting back - shortly after re-starting a warm engine the engine does sort of putputput until its got lots of revs then settles a bit - is this what you mean?

As regards the diagnostic, other than swapping over the EHA with one from a breaker ( which caused the car not to start at all!) all I have done on the engine side is oil change, and changed the evaporation filter in the wheel arch. Ive pretty much been just living with the car as is. I'm actually bidding on another EHA at the moment on ebay which is only a year old so may well be better than mine, but I dont feel its that even though the diagnostic identifeid it ( i did do as you say and replaced the O rings in it and it appears to no longer weep at all).

 

[television]

If I remember correctly your dad used to do a few things with cars,,ask him if he can set up a spark gap with a plug lead,, you just need to push a bolt or screw into the plug cap and wedge it so that the end of the bolt is just ½" away from ground and check the spark, if its weak and yellow then thats no good.

In electronic no matter what you think that you know, something new can come along and make nonsense of everything.
Sure the coil being OK when on a cold start fits the bill, the rest does not follow.

With a bit of luck I hope to pick up some more info on your system, within a few days