MGB: She Died Pt2 - Coil Diagnosis

Continuing to diagnosis my lack of firing. Fuel at the carbs so I'm focused on the starter circuit. Battery charged and the starter turns over fine. I'm using the Haynes manual to diagnosis my lack of firing. Early car, positive earth.

Tried connecting a test light to the points plate and earth and no light.
Manual says to next connect the light to the "CB" terminal of the coil. Does "CB" stand for contact breaker? Anyway, don't know if that's + or - so I've connected the light to the negative terminal (wb wire to the distributor) and I get no light. Haynes says the next step is connect to the "SW" terminal and earth (does that mean switch side?). I connect the light to the positive side and I get a light. I'm reading this as a bad coil, correct?

Were your tests made with the engine turning over or were they with the engine stationary? If the points are closed and not opening you would not get a light.
That would not be proof the coil is bad until you are sure the points are working.

Clifton

Engine was not running which is the basic problem. Turned it manually so that the points were open.

The coil can be checked my measuring the resistance between the primary and secondary circuits. You can pick up a digital multimeter fairly cheap these days and I find it invaluable for stuff like this. You can check in your Haynes manual the acceptable resistences of teh primary and secondary coils.

Paul

I do believe that those were actually what was printed on the COIL (Originally!)

But, with a POSITIVE ground car, the POSITIVE SIDE of the coil should go down the the points connection on the side of the dizzy. and the negative side of the coil will go to the over to the tach and then out the tach to the ign switch. This is that looped circuit that is reversed when changing to negative ground.

Now, YOUR car is the simplest of ALL of the MGBs. There's really NOTHING that's superfluous. Just your basic stuff to run.

This should be really easy.

I'm not exactly sure what you're trying to do with the test light, but there's other ways.

Just pull the high tension wire from the coil, and touch it to ground while someone cranks the motor. If you get spark, you're good there.

Now take one of the park plug leads and do the same thing. You should get a nice blue spark there too.

I suspect that you aren't.

So, do these things and report back.

I'll bet dollars to donuts that your coil is OK.

They almost NEVER break. Yet on every single post where someone has a spark issue, the FIRST THING EVERYONE suggests is replacing the coil.

There has to be so many MGB owners out there with boxes of used perfectly good coils.

OK. If you tried what I suggested you either have spark or you don't.

It sounds like the car was running THEN just died. There are only two things that causes that. NO GAS, or Loose/Broken/Disconnected Wire or Ignition component.

Now, finding out which is which is the question, I KNOOOOOOWWW! hahhahahahaha

Here's what usually happens on these early MGBs. The wire on the starter (although this is MORE prevalent on the later starters) comes off, the wire on the solenoid comes off. MANY, MANY times, the bottom fuse works loose and fuel stops pumping as a result. The wire that goes from the dizzy to the coil works loose.

Now, the wire INSIDE the dizzy can become damaged and break or fray, although this is usually a slow thing, not an all at once death.

Bob, I'm betting that you just have had something work loose, find it!

Many times, all this fiddling actually causes worse problems than you started with, now, that may not happen with you, but, it happens with a lot of newbies.

Good Luck.

B

Ps. The spark should be nice and blue, assuming that your batteries are still fresh, and haven't been worn from repeated turning over of the engine.

B

High tension lead from coil to ground - no spark.

There won't be a spark if it's actually touching ground. Did you pull it away a fraction of an inch?
-Ed

Correct, left a gap. It was not touching ground.

Perhaps we should back up and take a run at this systematically and in the right order. Power to the ignition flows in this direction:

-Battery
-Starter terminal (catches oil, grease, grime and occassionally needs to be cleaned, just remember to disconnect the battery first)
-Ignition Switch (power to the coil goes here first)
-Coil (including the small wire to the side of the distributor)
-Distributor (including points, condenser, rotor, cap, and center coil wire
-Spark plugs

To keep from going on wild goose chases you should trace the power to the ignition in the above sequence. Can be done in less than two minutes.

First step is to use a simple test light. Connect the wire clip to ground. Turn the ignition switch to On (engine not running). Probe the Coil terminal where the power comes from the ignition switch. If it lights bright you have eliminated a dirty Starter terminal and eliminated the ignition switch.

The coil is easy to test with a multi-meter. The primary resistance of the coil as measured across the positive and negative terminals should be 3.2 to 3.5 ohms. The secondary resistance, measured between one primary and the coil tower should be around 8.45K ohms. Might vary a little bit but both measurements should be close to the two specs above. When my coil failed it failed to reach the specs. I could not start my car but I had spark at the plugs. Very faint spark. Not enough to fire off.

If the coil tests good the next step forward is to use the same test light and probe the small terminal on the side of the distributor where the small feed wire from the coil connects. Have someone turn the engine over. The test light should blink each time the points open. If not, check the points gap first and make sure the points are opening.

If the points are set ok but the test light does not blink suspect the condenser, rotor, distributor cap, or the large wire from the coil to the center of the distributor. Also make sure the post insulator on the points is installed correctly. If not it will ground the points and again no spark,

Do these early B's have the graphite button up inside the center of the distributor cap?

Had a situation once where this was missing and gave me fits This little piece of graphite is rather brittle and completes the circuit between the top of the rotor and the conductor to the coil wire.

Easily broken off when attaching the cap to the dizzy.

Frank,
Nice clear explanation of the steps to take.

Ok, test light from the coil terminal with the white wire to ground and the light shines brightly.

Murky results on the resistance test. Have a digital meter and the first time I connected it to the + and - terminals it slowly counted resistance down to 3.3 ohms. Did it a couple more times and it only got down to 6.5. Suspecting the meter battery it tried with another meter and it went all the way to 0. Next tried from each coil terminal to the tower and got ziltch. No reading on either meter.

Is it likely the coil just up and died?

Actually Frank, I'd do all that, only backwards, would do that IN REVERSE. Here's my thinking . . . . . .

Its much easier to start at what's BROKEN, and work backwards, then start at the battery and work forwards with what's working.

Its like when you're looking for something. MOST people stop looking for something after they've found it.

By working backwards, you can stop looking once you actually get spark. So, by starting at the plugs, then coil, and so on.

Bob - - -

When Coils go, that's what they do, they just go, but, its so infrequent, (comparatively speaking, and taking the other ignition components into question)that it makes me suspect something else.

Especially since, you put a bunch of new stuff in there, and then it ran fine to the gas station and then just dies on you. That's usually a crappy connection or something got fiddled with mistakenly or accidentally.

BUT, that doesn't mean its not the coil. what does surprise me is the different readings you're getting.


Did you replace the CAP and ROTOR TOO?

I went back and checked your past thread and you don't mention the cap and rotor, only wires, points, condenser, and nothing loose.

Do you have another coil lying around? Maybe an old VW coil, or something from an old car? Doesn'T EVERYONE have these lying around?? hahahaha

B

Everything is brand spanking new. Only got the car running a year ago (after 23 years off the road!) and it's only got maybe 200 miles on it. Jeff did my distributor so I know everything on that end is top notch.

don't have another coil lying around but it's looking like that's first thing on the list - plus new batteries for the meters. The bouncing around of the readings has me perplexed too.

Bob, Before spending a bunch of money on a nice coil from Moss or VB, or whomever, I would get a cheapy Bosch coil from Pep Boys or somewhere, and keep that around.

Like I said, I'm not thinking coil.

Bob, NEVER take anything for granted as working. What Frank and I were trying to do is methodically work towards finding the problem. We go about it slightly differently, but we get to the same spot, which is all that matters. Just because the dizzy was doen by Jeff doesn't mean that everything is still working as it should, I'm not bashing Jeff, I've just been around cars so long to know NOT to assume anything works!)

So, to recap. . . . .

NO Spark at Plugs
NO Spark at Coil, HT Lead from Coil to a Ground (Engine Block Cyl Head Nut) produces NO spark whatsoever, correct?

So, this leaves us at the Dizzy, The solenoid, the Key Ignition Switch, Batteries.


Here's what I would do, and you can do this without having to go get batteries for the tester. ( I am ASSUMING that you HAVE NOT changed the car from Positive to Negative Ground-am I correct??? - If you have, then everything is reversed)

Does your tach work??

Take a jumper wire from the NEG side of the battery to the NEG side of the coil (this can easily be taken from the fuse block too)

Take that jumper wire from the neg side of battery (which is hot) and attach it to the NEG side (HOT Again) of the coil, (I would remove the wire currently there - it comes from the tach)[/i]

Then the POS wire on the other terminal of the coil, should already go down to the dizzy.

What we are doing here is bypassing the TACH, and the KEY.

I am pretty sure that the car will start up at this point. In these cars, the HOT wire comes from the Battery to the Key, thru the tach, before going down to the coil.

IF you DO NOT get spark at this point, then you have a Coil, Dizzy, Key, problem.

I know all of this seems difficult, but, if you've done this 100s of times, all of these steps can be done in a matter of 5 minutes, telling you right where the problem is.

[i](Also, have you pulled the hose off of the carbs and confirmed that fuel is actually getting to the carbs? In some other posts, you spoke of fuel delivery leaks and problems.)

Good Luck,

Keep us updated and we'll sort this out, its pretty easy!

B

I just went through a week of trying to diagnose a similar problem. I was not getting any voltage to the coil. It turned out the starter solanoid (sp) was bad. I was able to bypass the solanoid and get the car running to test it. So $39 later and I am back in business. Perfect because the weather is mid 70's and the LPGA is in town. All is good.

Test battery forward to coil and save yourself time. Brian, sorry but I don't subscribe to your opposite direction testing, spark plugs back toward battery.

Well Frank, that's what makes this world go round. Everyone tackles problems differently.

So Bob, where are we?

B