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1977 MG Midget 1500 no spark...

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cursichella Charles M
St Helena, CA, USA   USA
Hi,

Please, I would be so grateful for any help with my latest drama, 77 MG 1500. I've replaced the coil, distributor cap (Mallory dual point), points, rotor and plugs but:

No spark at plugs
No spark from coil
Power IS going to coil
Negative terminal at ignition coil is giving positive polarity (should be grounded?)

The plugs had a weak spark before I
replaced the points and rotor.

I've read through and have performed various tests suggested in older forum posts:

Plugs
Coil
Ground to battery

I'd be grateful for any suggestions. :/

p.s. edits were to clarify and fix typos.



Edited 3 time(s). Last edit at 2017-05-19 01:47 PM by cursichella.

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dickmoritz Avatar
dickmoritz Gold Member Dick Moritz
Philly 'burbs, PA, USA   USA
The Mallory dual-point distributor is quite easy to diagnose.

First, suggest you remove any wires from the coil - terminal. Then use a test light or volt meter to confirm that you have power to the coil + terminal when the ignition key is turned on.

If you do not have power, then you'll have to trace the wiring back to the ignition switch.

If you have power to the coil +, then you should also have power to coil - with the key on and all wires removed from coil -.

Remove the fat coil wire from the distributor cap while leaving it attached to the coil. Connect a spark plug to the end you just removed and lay the spark plug against a good ground/earthing point.

With the ignition key on, connect a small jumper wire to coil -. alternately touch the other end of this jumper to ground/earth. Each time you remove this jumper from ground/earth, you should see a healthy spark across the temporary spark plug. If you do, then your coil and primary wiring to coil + are good.

Next reconnect to coil - the small wire that goes to the distributor. Remove the distributor cap and insert a small piece of thin cardboard between both pairs of contact points. Now check for power. You should have power to coil +, coil -, and to one of each pairs of contact points.

If you do, then remove the cardboard from one pair of points and have a helper crank the engine with the distributor cap off. You should see a spark at the points each time they open. Repeat this process for both sets of points.

Try all of this and report back...

Dick



Errabundi Saepe, Semper Certi
(Often wrong, but always certain)

cursichella Charles M
St Helena, CA, USA   USA
Thank you so much for your quick reply!

Currently charging battery because all of my earlier testing has probably weakened the charge.

Will report back soon!

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Havenor Avatar
Havenor Richard Costa
Fairhaven, MA, USA   USA
Ground strap to engine?

It's always my starting point. The engine moves on mounts which can pull on it, and oil can foul it. Make sure it's clean and sound.

cursichella Charles M
St Helena, CA, USA   USA
In reply to # 3514861 by Havenor Ground strap to engine?

It's always my starting point. The engine moves on mounts which can pull on it, and oil can foul it. Make sure it's clean and sound.

The ground straps looked pretty dirty! However, engine block seemed grounded when I checked with the power probe. Is that sufficient to say that the ground strap is okay?

Havenor Avatar
Havenor Richard Costa
Fairhaven, MA, USA   USA
Dick, ....Jeez, excellent diagnose post. It's worthy of a bookmark, and I need to work on my old boat motor this weekend. I'm going to use it. It's very similar.

66jalopy Avatar
66jalopy Silver Member Phillip Jolliffe
Lake City, FL, USA   USA
Last Mallory I worked on, 50 years ago, I removed one set of points, got the other set working and then put the second set in. Did you replace condenser? Big failures with bad condenser.

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cursichella Charles M
St Helena, CA, USA   USA
In reply to # 3514893 by 66jalopy Last Mallory I worked on, 50 years ago, I removed one set of points, got the other set working and then put the second set in. Did you replace condenser? Big failures with bad condenser.

I didn't replace the condenser. Tested it with a multi meter a couple months ago and it seemed okay. Would this be causing the symptoms I'm experiencing?

Maybe it went bad from all the starting attempts!

dickmoritz Avatar
dickmoritz Gold Member Dick Moritz
Philly 'burbs, PA, USA   USA
There have been many bad condensers out there. One nice feature about the Mallory is the fact that the condenser is mounted externally. So instead of paying 12-15 bucks for an MG or genuine Mallory condenser, just go to any auto parts store and ask for a condenser for a 60's or 70's General Motors V-8 application. Three or four bucks, it's plug and play, and will work just fine...

Dick



In reply to # 3514905 by cursichella
In reply to # 3514893 by 66jalopy Last Mallory I worked on, 50 years ago, I removed one set of points, got the other set working and then put the second set in. Did you replace condenser? Big failures with bad condenser.

I didn't replace the condenser. Tested it with a multi meter a couple months ago and it seemed okay. Would this be causing the symptoms I'm experiencing?

Maybe it went bad from all the starting attempts!



Errabundi Saepe, Semper Certi
(Often wrong, but always certain)

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dickmoritz Avatar
dickmoritz Gold Member Dick Moritz
Philly 'burbs, PA, USA   USA
Thanks for kind words, Richard. <blushing>

There are few more steps depending on the outcome of the above diagnostics, but we have to see what the OP finds first...

FWIW, all of the above only takes five or ten minutes or so, works with any points-type distributor, and is one of the primary reasons I prefer points over any of the electronic setups, which generally have no viable diagnostic procedure at all...

Dick

In reply to # 3514867 by Havenor Dick, ....Jeez, excellent diagnose post. It's worthy of a bookmark, and I need to work on my old boat motor this weekend. I'm going to use it. It's very similar.



Errabundi Saepe, Semper Certi
(Often wrong, but always certain)

66jalopy Avatar
66jalopy Silver Member Phillip Jolliffe
Lake City, FL, USA   USA
I have had three bad condensers in the last 2 years, weak or intermittent spark, works cold then quits hot.As suggested change it, they are cheap My next will be a Chevy one, I will mount outside my 25D distributer.

cursichella Charles M
St Helena, CA, USA   USA
Got it, I'll try and grab a new Condenser in the mean time and see if that does the trick!

cursichella Charles M
St Helena, CA, USA   USA
Hi,

Ok, first tests tested ok. Next, I tested for the spark from the coil that goes to the distributor cap with spark plug on ground. I got a spark but it was weak and didn't spark with each try. Maybe 1 out of 5 tries gave off a very tiny blue spark.

By the way, battery is at 12.6v.

dickmoritz Avatar
dickmoritz Gold Member Dick Moritz
Philly 'burbs, PA, USA   USA
You say your battery is at 12.6 volts, which is nearly fully charged. Did you confirm that you also have 12.6 volts at coil + ? Do you have another coil that you can try this same test on? You should be getting a healthy blue spark...

Dick



In reply to # 3515095 by cursichella Hi,

Ok, first tests tested ok. Next, I tested for the spark from the coil that goes to the distributor cap with spark plug on ground. I got a spark but it was weak and didn't spark with each try. Maybe 1 out of 5 tries gave off a very tiny blue spark.

By the way, battery is at 12.6v.



Errabundi Saepe, Semper Certi
(Often wrong, but always certain)

jmac Avatar
jmac Silver Member Jere McSparran
Greenup, IL, USA   USA
1978 MG Midget "Therapy"
1978 MG Midget "(SOLD)"
cursichella,
You asked if the neg. side of coil should be grounded in your first post. The answer is no, not by running a wire from the neg. to the body of the car. The neg side of the coil is grounded through the distributor and your points. The coil is a type of capacitor. When the points are closed the coil collects and builds voltage. When the points open that voltage is released by the coil giving you a powerful spark. If you ground the coil directly to the body the coil will build voltage but will not release the charge properly when the points open.
It has been a while since I dealt with points. I got this right didn't I Dick?



JMac
JMAC Engine Shop
http://www.jmacengineshop.com

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