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MG Midget and Sprite Technical - Coil query

Can anyone explain the following.
I have been running with a coil from an MGBGT V8 ( the old coil was thrown away by accident)and I bought a new coil to replace it and immediately the rev counter stopped working.Reverting back to the old coil the rev counter worked fine.I made sure I connected the terminals correctly.Engine starts and runs fine with either coil.
My sprite is 1966, 1275, negative earth. I have tried both a new sports coil and new standard coil from local mg specialist.The specialist cannot tell me why the new coils stop my rev counter working.
Any suggestions would be appreciated.

thanks,

Donald
D M Campbell

The very first thing I would check is to measure the resistance of the low tension circuit of the coil that works the tacho. Then compare that to the ones that do not work the tacho. I wonder perhaps if the coil that works the tacho is actually a ballasted coil?
Bob (robert) Midget Turbo

As Bob says, check the resistance of the coil. The relevant figures for resistance for various kind of coil are:

Coil type /// Primary windings /// output voltage

12 volt basic coil/// 3 to 6 ohms/// 20,000 volts
12 volt ballasted/// 1.2 - 3 ohms/// 30,000 volts
12 volt electronic ign// 0.3 - 1.2 ohms/// 60,000 volts

Excuse the crap attempt at formatting but it always goes haywire when posted, anyway.

But for all that I think the coil voltage is a bit of a red herring. The tacho simply counts the number of "switching to earth"s and compares them to engine rotation giving a value.

I wonder if the condenser is allowing a points message that isn't actually opening properly and thus not giving a message to the tacho circuit.

Are the points properly set? perhaps a smaller than "book" setting might not give the tacho circuit time to Count with.

I think I would be looking more deeply inside the dizzy for this answer Donald
Bill

Yep heard what you said Bill and time and testing will tell.
However take into account that the common thing to the problem is the coil, and to be honest I have never known the tacho to fail because of the items you mentioned except when those items have caused erratic engine running as well.
I will be very interested to find out the cause of this problem.
Bob (robert) Midget Turbo

I'm looking at where Donald says that they have tried two new coils to fix it. A new sports coil and a ornery one. Now I think I remember that you have been as dismissive of Sports Coils as I am in the past (a coil is a coil!), so the basic unchanged bit is the dizzy, Points and condenser (and of course that blooming pigtail wire that feeds it) but if it runs there ought to be something we could put our fingers on.

I wish Donald was close enough for me to have a proper look at it for him, but much as I love Tayside, I ain't about to take a hike up there for an hour's nosing about in his wiring...

Yup I'm agog about this one too

So the next logical thing is volt drop and resistance checks on the tacho to coil wire. As this is an early car the tacho feed ought to be the "into tacho with a female terminal sleeve and out with a male one" type. I have seen thses litle bullet ended wires get very poor internal connections due to corrosion.
Bill

In fact Bill I always use the high powered sports coils but I was intigued by the statement that refitting of the original coil cured the problem.
Bob (robert) Midget Turbo

Ah, couldn't remember whether you had said "for or agin" but the old coil back on syndrome has happened with jobs I've done back in prehistory too. Never did track it down, but as a roadside breakdown it wasn't THAT important...

(Must remember Bob does like Sports Coils...)
Bill

This thread was discussed on 01/03/2008

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