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MG MGB Technical - Crazy tach readings and misfiring

For the past 3 years I have been using a Crane electronic ignition system in my '72 MGB-GT. Until recently this has worked well. In the last few weeks, the tach needle occassionally starts swinging back and forth, and then the engine starts misfiring. The problem usually only lasts for a few seconds, and then the engine and tach return to normal. The problem occurs whether the engine is cold or warm, and sometimes it does not occur at all. Otherwise, the engine runs great. Since the misfiring occurs in conjunction with the swinging tach needle, I am 99% sure it is an ignition problem.

Any ideas what might cause this? I have checked all the electrical connections, and each appears okay. Is this a possible warning sign that the ignition module is going to pack up? Any ideas would be appreciated.
Gordon

Gordon. The electronic ignition systems are much more difficult to troubleshoot than the points type systems. Most of us do not have the test equipment, schematic diagram or the knowledge to check out semi-conductor systems. Since the Crane system is the old Allison system, you might check to see that the chopper is secure on the points cam, the light system is tightly secured to the base plate and that the electrical wires running to the light and receiver are not being crimped or damaged. You can also check for power input to the coil, do a resistance check on the coil and check power output from the coil. You are correct that, usually, when the tach swings erratically and the engine misfires it is some form of low tension circuit problem. You say it happens both when the engine is cold and when it is warm. Does it happen at any particular speed or when accelerating? Could it be caused by movement of the baseplate (caused by the vacuum advance) or of the points cam (centrifugal advance)? Could something be grounding out under these circumstances? A chaffed wire? Is the ground wire from the base plate to the dizzy body in good condition? Sorry, no real answers here. Only ideas to be examined. Les
Les Bengtson

Les,

Thanks for your thoughts. The problem occurs randomly, although more frequently when the engine is at a constant speed (annoyingly while I was on the highway yesterday). It seems to occur more often at higher revs, but, not always. The vacuum advance long ago ceased to work, so that removes that possibility. I'm thinking that a loose wire (or bad ground) is the culprit. While the Crane unit might be the cause of the problems, I'm guessing that as a solid state unit, if it had a problem, it would cease to function, altoghether. I'm going to check the low-tension circuit--thanks.

Gordon
Gordon

Gordon,
I had the exact same problem: random tach needle going crazy and misfires. Turned out to be the connection of the main battery cable to the starter. I looked underneath at the starter connections and I could pull the wire from its connector! I cleaned up the wire end, and crimped on a new connector, and re-attached the wire to the starter, and no problem since. This just happened to be my problem, but it is another place you can look for problems; check all the connections at the starter, and the fuse box. The only reason I checked the starter connections in the first place is that I could hear a slight sound on the lower right side of the car whenever this misfiring occured.
Good luck!
Craig
Craig

Gordon,
I have a '72 that had the same problem last summer.
Try main Ign. feed off key switch where it splits just under dash. It splits here for ign.-tach and fuel pump. Just a bit of corrosion here gave me fits.
Bill
William Scullion

Gordon- For me it was the lack of ground at the tach head at dash on the 1971 MGB. Tach swung back and forth whenever it passed 3kRPMs. Did it before and after pertronix. I think Les has the answer above. Vic
vem myers

Electronic ignitions can be really hard to troubleshoot since so much of the discrete electronics is not available to check out. Units like this Crane/Allison XR-900 "Fireball" have any of their electronics "potted" in resin so that you cannot even see them, much less fix them. The "black box" becomes a simple throw away replacement issue if it goes belly up.

As the others have said you'd do well to check for loose/bad wiring with a random intermittent like this. Often, if you observe a coincidence of a bump on the road with the intermittent cut out, then it is probably a bad ground or other connection. If this happens however, while the car is parked and running in the driveway, it may well be a component in the black box giving up the ghost.
Bob Muenchausen

This thread was discussed between 09/01/2003 and 12/01/2003

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