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MG Midget and Sprite Technical - Can't get spark!!! ARGHH...

This HAS to be the most frustrating thing in the world!

New Coil
New plugs
New rotor.

Can't get spark at the coil now using all the various tests mentioned in the archives. When the ignition is ON...it reads 12V at the coil...but when I crank it drops to 8V. It's a newer coil with an internal resistor, so I've adjusted wires to simply by-pass the external one that was with the car originally.

Does anyone have ANY idea why I'm getting NOTHING from the coil to the distributor. Last week at least I had something, then somewhere along the lines while trying to sort out the gas situation, I lost all electricity. Someone put me out of my misery PLEASE!

Chris
Chris Elkerton

Chris,
What year is the car?
Try attaching a 12v test light across the coil to see if it flashes when you crank. This should tell you something about the low-tension circuit.
If you have an inductive timing light, put it on the HT lead from coil to dizzy, then on the spark plug wire to the first cylinder to see if there is anything happening there. This should feed you some information on the high-tension circuit.
What happens if you simply run a wire from the hot side of the battery to the hot side of the coil and crank the engine over?
David "more questions than answers" Lieb
David Lieb

condenser?
JOHN HALL & JULIE ROBERTS

Sorry everyone: 1975 1500. No condensor.

I'm about to try some more tests, but I think I misstated the situation. I can't get the coil itself to do anything let alone get anything to the distributor.

Chris
Chris Elkerton

Chris. More information on the type of distributor you are running would be appreciated. Do not know about the midget, but the 75B used a points type distributor which did have a condenser.

The first thing to do is to check out your voltage problem. You note that you have 12v at the battery and 8V at the coil input. This would indicate that your car is designed for use with a coil designed for "6 volt operation", commonly marked "12V for use with external ballast resistor". If you are using a "newer coil with internal resistor" you may be using the wrong coil for this application. Hard to tell without knowing, specifically, what you are using. All coils have some internal resistance. The coils designed for full time 12V input have an internal resistance of about 3.6 ohms while the coils having about 1.6 ohms resistance are designed for a lower full time voltage input.

Detach the coil lead from the distributor cap and the lead from the negative side of the coil. Leave the lead on the positive side of the coil (input). Attach a jumper wire to the negative side of the coil and put it out of the way where it cannot ground. Turn on the ignition switch. Hold the coil lead near the block and quickly touch the jumper lead from the negative terminal of the coil to the block. As the contact between the lead and the block is broken, you should see a spark at the coil lead if the coil is working properly. If you do not see a spark, you need to replace the coil with the proper type. If you do see a spark, you have a distributor problem.

Les
Les Bengtson

Ok...spark at block when using Les' test. So I took the distributor out, cleaned it thoroughly, set everything back to the way it was when she did run last week...still no spark. New rotor. Cap in good shape.

Set timing to TDC. Rotor at plug 1. Firing order 1-3-4-2. Everything the way it should be. Still no spark at plugs.

What am I missing?

C

Chris Elkerton

Chris. What you are missing is telling us what distributor you are using.

If you had spark at the block, the problem lies in the distributor itself. The distributor's low tension circuit is a make and break circuit which charges the coil, a step-up transformer. The coil takes the system input of 6-12 volts and steps it up to the voltage needed to run the engine--about 11K volts in my tests. If the low tension circuit is not functioning as intended, the coil will not create a spark.

If you have a points type distributor, it is possible to trouble shoot the problem relatively easily. If you have an "electronic" distributor, it is a "replace with a known good unit" situation.

Again, knowing what type of distributor you have and, if it has been "upgraded" with an electronic points replacement system, the brand of the system would be of use.

Les
Les Bengtson

Hey Les,
My apologies...

According to the two "Book of Words"...(Bentley re-print and Haynes) I have the 45DE4 electronic ignition distributor with an amplifier module. It uses a "pick-up" unit, and on a side-note the vacuum advance has broken off.

The pick-up unit has 3 contacts on it of different sizes. I adjusted the gap properly, according to both books.

Chris
Chris Elkerton

Chris. The Lucas 45DE4 was known as the Opus system which the Brits took to calling the "'opeless" system. The one on a brand new 79B lasted for about three months before it was replaced with the upgraded Lucas 45DM4 system under factory warranty. If you actually have a 45DE4 that was working, you may be the world's record holder for having the last 45DE4 in working condition--up until now.

Time to get a new distributor. It is possible to replace the internals of the 45DE4 with various aftermarket "points replacement systems". All that I have seen have been so modified. But, considering the fact that the vacuum advance is non-functional on your unit, a new distributor might be more cost effective.

That is where you need to go next.

Les
Les Bengtson

Hey Les,
How did I know this was coming.

I'm just so frustrated as to why it did work (albeit only for a few quick bursts) to now not a thing. What would stop "juice" getting from the coil lead to the sparks? Is it simply a matter of I fried the "pick up unit"? Is there a way to test it, or am I pretty much screwed?

Thoughts?

Chris
Chris Elkerton

Chris. I do not know how to test the pick up unit, nor do I know of anyone who does.

Distributor works like this:

The power comes into the coil and flows through it to the distributor. When the point are closed, the power flows through the coil, creating a magnetic field in the primary windings. When the points open, the magnetic field in the primary windings collapses and induces (creates) a charge in the secondary windings. (Since there are more secondary windings than primary windings, the effect is to step up the voltage being produced in the secondary windings.)

As the higher voltage is being built up in the coil, it is transmitted though the high tension circuit--from the center terminal of the coil to the center terminal of the distributor cap, through the top of the rotor and to the terminal for the spark plug being fired. This voltage continues to build up until it is sufficiently high to cause the current to jump from the center electrode to the outer electrode, causing a spark. The voltage only builds up high enough to cause the spark, no more. This is, almost always, well under the 20K volts that the standard coil is capable of providing. Somewhere between 11K volts and 13K volts in my experiments. Thus, a 40K "Super Chicken Pluck--High Performance" coil seldom, if ever, operates at anywheres near its rated capacity and, if it did operate there on a regular basis, would be very hard on the high tension ignition system.

This is the basic operation of a coil type ignition system regardless of the triggering device--points and condenser, optical trigger or magnetic trigger--they all perform the identical function, but in slightly different ways.

If your make and break circuit is not operating to make and break the electrical connection, you have no spark produced by the coil.

Since you have tested the coil and seen that it is capable of causing a spark when tested, the only remaining cause is the distributor itself. I

f the coil lead produces a spark when a jumper wire is touched and removed from the block, the coil and the coil lead are functional.

If it does not produce a spark when hooked up to the distributor, the distributor is at fault.

If the coil lead produces a spark, when hooked up to the distributor, the low tension circuit is good.

If the coil produces a spark at the coil lead, but not at the spark plug leads, the problem is with the cap, the rotor, or the lead(s) for the spark plug(s).

From this description, you should be able to define your problem exactly. Then, it is your decision as to how to correct it.

My way is installing a points type dizzy because they are so easy to trouble shoot. But, I have the equipment to check them out and make sure they are in good condition.

Look at your options and let us know what you find out.

Les
Les Bengtson

Hey Les,
You sound like my dad yelling at me for not paying attention, but I think it finally sunk in! LOL (this electrical stuff really does twist me up mentally)

So...spark at coil lead when "-" terminal grounded and lead is close to block.


"If it does not produce a spark when hooked up to the distributor, the distributor is at fault."

When I re-connected the distributor to the coil, and test the HT lead at the block when turning over... nada. I'm assuming this is the result that shows me the distributor is pooched. Correct?

Chris
Chris Elkerton

Chris. Sorry about sounding like your Dad, but there is a good chance I am his age--if not older.

Yes, the distributor is the problem.

If I may continue to sound like your Dad, I would recommend that you obtain a points type distributor. It will make your trouble shooting easier. You can trace the current path through it and, if there is a problem, correct it easily. If, after you have the car running properly, you want an "electronic ignition" you can either install one of the after market points replacement systems or install a purpose built electronic distributor.

You need to look at the mechanical advance curve for your engine, then, compare it with the mechanical advance curves of the various points type Triumph Spitfire distributors. You might be able to use a Lucas 25D4 or 45D4 distributor from a Midget, but I am not sure about this. I know that the Triumph engines used a different type of drive gear than the BMC A and B series engines did and do not know how it attaches. (Someone here will, however, know and provide this information.)

It would, also, be of interest to know if the 1975 North American specification cars (which were sold in both the US and Canada) used the Lucas 45D4 points type distributors. The MGB did and I do not know why the Midget would have had the Lucas 45DE4 installed two years before the MGB got it. Does anyone have any information on this?

Les
Les Bengtson

>>>>>>>It's a newer coil with an internal resistor, so I've adjusted wires to simply by-pass the external one that was with the car originally.<<<<<<<


Can you elobrate a little more on what that means, and how you achived this???

prop
Prop

Hey All,
Prop...this is probably wrong, but I just connected the two ends that ran on opposite sides of the resistor. (if the new coil has one built in, then this one is not needed?) Feel free to yell at me like Les and tell me if that was dumb or not! I'm happy to learn by my mistakes as long as someone explains WHY it's a mistake.

Everyone...a local MG garage has a used working 25D distributor that he says will definitely work. I trust he know what he's talking about, but if anyone sees a problem with this, please JUMP in before I buy it.

Chris
Chris Elkerton

If you are talking about the finned aluminum resistor, that is not the ballast resistor that goes in the coil feed. It is a "run" resistor, which has to do with the internals of the OE electronic system; I don't know what effect jumping this might have, but it's probably not good. As Les said, if you had one of these that actually worked until now, it's a record, and it's probably over now. The OE system was hopeless, unreliable, and highly variable in it's "partial failure" state - which usually happened in a year or so.

Convert it all to points or aftermarket electronics, with the correct coil.

FRM
FR Millmore

Chris, email me. I have a coil you can use and I have the ignition test manual here you can use to troubleshoot your system.

I have a 45DE4 distributor on my car and it's still operating well.

Somewhere around here, I have a spare distributor with a complete ignition system on it. I will have a "look see" to see if I can locate it. If I do, you are free to try swapping it over.

Clive Reddin 76 Midget

Wow...

Clive just scored some beer! I'll e-mail you to talk! Thanks again!!

Chris
Chris Elkerton

Update:

- Acquired a 25D Distributor.
- Hooked everything up and tested for spark.
- Affirmative on good spark.
- Aligned timing to TDC.
- Adjusted to #1 plug.
- Filled carb bowl with gas.
- Sprayed some gas into throat.

Contact... ignition...and at 6:47 EST. Toronto, Canada...


SHE RAN!!!!!!!!!!


Much more to do to adjust mixture, test fuel pump (cuz I think it's not sending enough gas), but after 14 years, she ran!!!!!!!!!!!!

Can you see me all dancing?! Thanks for everyone's help.

Chris

Chris Elkerton

Chris. I appreciate your letting us know that things have turned out well. Too often, people fail to do so and we are left wondering how they are doing.

Glad you got it running.

Les
Les Bengtson

Thanks to everyone for their troubleshooting help and suggestions! She's been idling nicely for a while now, but like all things sitting for 14 years it's taking a while for things to sort themselves out. The emissions system and all that fun stuff was removed years ago so I'm having fun trying to sort out what to adjust and what to leave alone on the carb. Where does the vac line from the distributor plug into directly on the ZS carb?

Also, I'm being cautious since, as I was fiddling, the temperature sender cable split letting all it's "gas" out so I'm worried she might overheat without me having anything to gauge it by. I see in the VB/Moss catalogues I can get a new sensor cheaply, but is it electric? (I don't see the metal cable with it) If so do I need a new gauge?

I'll probably start a new thread titled "ok...she's running...NOW WHAT?!"

Chris
Chris Elkerton

Great news on your car "living" again Chris!!

That thing in the catalogue is a sending unit for the later electric temperature gauge and won't be of use if you have, well perhaps had is the more correct term here, mechanical temperature gauge setup. I'm afraid those are rather expensive.

However, nothing says you can't keep the original gauge and add an electrical temperture gauge elsewhere in your car.

As for the vacuum part, there's a tube at the top of the carb behind the cap where you add the oil to the damper on the carb. Normally there's a plastic T there but since your emissions system is missing, no reason you can't run the vacuum line to there.
Clive Reddin 76 Midget

This thread was discussed between 01/07/2008 and 05/07/2008

MG Midget and Sprite Technical index

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