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MG MGB Technical - Pertronix puzzler

Ok; had a little money left over after Christmas and thought I would treat myself to an electronic ignition upgrade for my B. I have been running a Mallory dual point, which has been working fine, but I figured that not having to adjust points on a bi-monthly basis might be kind of nice. I purchased the pertronix Ignitor kit for the positive ground Mallory dual point ignition as this all fits inside the distributor and is one of the few positive ground electronic ignition systems.

The kit arrived Friday and I installed it Saturday (Jan. 31). The parts went together without any problems and the kit instructions were very clear. I then re-installed the distributor and tried to start the engine, but had no luck after half an hour. I changed the timing around during this attempt (both advance and retard) just to see if anything would kick over, but no luck.

I then went through a problem solving session. First test was to switch on the ignition, put a test light at the coil (-) terminal and see if there was juice flowing to the coil. Yep, that worked. The next test was to see if the Pertronix module was working. I lined the engine up to the static timing mark and then twisted the distributor back and forth to see if the current flow was being interrupted (using a test light). Yep, that worked too, I could see the light go off and on as I twisted the distributor, therefore the Hall effect module was picking up the magnet sensor.

I rechecked the wiring and double checked the ground and everything tested out correct. I then decided to perform the spark test to see if juice was getting to the plugs. I grounded the #1 plug and then had my wife crank the engine while I observed for spark. First two rotations of the crank and the plug fired, then nothing after that. I then tried all 3 other plugs with the same test and ended up with the same result. Two fires of the plug and then nothing. I checked the coil and noticed that it was somewhat warm to the touch.

My next step was to search the Internet (including the archives here) for any problem solving tips on the Pertronix. I found a few, but when I went back and checked my installation, everything was correct and none of the symptoms I was seeing matched what had been previously discussed.

Today, I re-installed the Mallory dual points and the engine fired up the first time. I adjusted the timing and then ran the engine half an hour and had no problems. Therefore, I am now stumped. It appears that the ignition system itself is working fine, but there may be something about how the Pertronix wiring is called out in the instructions that may be a bit off. Therefore, I am asking if anyone has had experience converting a positive ground Mallory dual point to Pertronix, if they had success doing so, and if they could fill me in on any tips, tricks, and/or corrections.

I had wired the Pertronix exactly per the factory instructions (black wire to switch, black/white wire to (-) terminal on coil, ground wire from (+) terminal to ground). All wiring was checked for continuity prior to trying to start the engine.

This is a real stumper and any help will be appreciated.
Ron Kluwe

Some of these come DOA. Try calling them on Monday (Petronix) I have heard they back thier stuff well enough (which is good with a few DOA units).
Carl W French

"not having to adjust points on a bi-monthly basis might be kind of nice"

What?! Mine don't need adjusting after 3k! Or are you doing 12k or more per month? Either your Mallory has something wrong with it or you are not doing it right. It is true that whilst I was checking and setting by gap I used to do them at each 3k service interval, but since I started checking by dwell they just don't need doing. The difference is that whilst they are still developing a spike, which affects the use of a feeler gauge in the gap, it doesn't affect dwell to anything like the same degree.

The fact that twisting the distributor back and fore interrupted the current through the coil indicates that it is working, all the Petronix is is is 'electronic points' which fire the coil in mostly the same way. Fitting a Pertronix has been known to change the timing significantly even if the distributor remains in position in the block. You don't say if you set the timing up statically after fitting the points, you should have if you didn't using this twisting method with the appropriate pointer on the TDC mark as a rough guide to get you going before doing a dynamic setup.

A positive ground car needs either a different Pertronix or connecting the other way round. AFAIK these units only have two wires - one to the coil +ve or SW and another to the coil -ve or SW. The Pertronix ground is picked up from its physical fixings to the points plate.

I know there have been problems replacing late-model factory 'electronic points' with Petronix or similar, or even conventional points, as the factory electronic distributors don't have the 'points' plate ground wire that these other systems need. But going from points to Pertronix shouldn't have made a difference unless you disturbed that connection.

And carry your points and condenser with you, failure of these electronic units is well known. Personally I would never bother with them. And to be honest a dual-point distributor is also a waste of time on a low-revving engine like the MGB, if single points are good enough for the V8 with half the dwell time they are certainly good enough for the 4-cylinder.
Paul Hunt

I believe Pertronix for 4 cyclinder engines require a coil with 3.0 Ohm resistance. You might check that. The directions for mine are a bit confusing on that point. One place it says 1.5 ohms when speaking in general terms but in another place specifically addresses 4 cylinder engines and says 3.0 ohms there. Comments. FWIW.

JTB
J.T. Bamford

Yep, Pertronix units need resistance of at least 3 ohms just like points.
Mike MaGee

Both 6v and 12v coil systems exhibit an overall resistance of about 3 ohms - wholly in the coil in the case of 12v systems, about equally divided (i.e. 1.5 ohms each) between coil and loom ballast in the case of 6v. That shouldn't make any difference as the Pertronix is switching the current in series with the coil, or the coil and ballast, and it is much the same in both cases/ But note that 12v Sports coils typically measure 2.4 ohms. The difference between 12v and 6v systems can be in where the Pertronix picks up its operating supply from. In the case of a 12v system this would be the coil +ve or SW, but on a 6v system this point is continually switching between 12v and 6v as the Pertronix opens and closes, which is why it may have to pick up a 'clean' 12v supply from the white at the fusebox for reliable operation.
Paul Hunt

This thread was discussed between 01/02/2004 and 03/02/2004

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