Welcome to our resource for MG Car Information.
MG MGB Technical - coil resistance?
I have two ignition coils, a Lucas DLB101 and Unipart GCL100. Both have been on my standard 1966 BGT at some point or another. The former has a resistance of 2.7 Ohms (cold) the latter 3.1 Ohms. The manual suggests a resitance of 3.1-3.5 Ohms for the HA12 coil. Is the difference (2.7 to 3.1) significant?
The reason I ask is that I am planning to install a pertronix Ignitor electronic ignition module and it requires a coil with a minimum of 3 Ohms. One thought, when coils fail to do they go from fine to fail straight away or do they fail gradually (possibly indicated by the reduction in resistance)?
Any advice gratefully received.
If the Pertronix says a minimum of 3.0, I think I would use the one with 3.1 Ohms.
I think a coil with no internal ballast is about 1.5 Ohms, so I think the diff between 2.7 and 3.1 could be significant.
Coils can certainly fail gradually, and may make you think you are starving for fuel. They may work cold and then not hot. I don't know if the resistance is an indicator/predictor.
|C R Huff|
|Cheers Charley. It would be good to know if a fall in the resistance in a coil indicates that it is failing - it is an easy thing to check (presuming that is that the coil I have had a resistance of 3.1+ Ohms when new. I'll see if I can get some info from Lucas on this.|
|For info: according to Lucas technical staff the ignition coil DLB 101 has a resistance in the range 2.7-3.2 Ohms. If it falls below this it indicates that the coil is failing.|
|Welcome to the hornets nest. Firstly forget internal ballast - it is irrelevant. Coil resistance and suitability for a particular application depends on its *overall* resistance and not whether that resistance is achieved purely by turns of wire or a combination of turns of wire and an internal resistor.|
The 2.7 is probably a sport coil, mine measures about 2.5 ohms. About 3 ohms is standard for the chrome bumper unballasted ignition system, about 1.5 is standard for the ballasted rubber bumper ignition system. A 2.5 or so sport coil is for an unballasted i.e chrome bumper system, for a ballasted i.e. rubber bumper system it would be about 1.2 ohms.
A Pertronix should run any of them, although on a rubber bumper car you may need to supply the Pertronix with a full 12v but leave the ballasted connection to the coil, and on a 72 and earlier car the Pertronix may cause tach problems.
But when deciding on a coil you must also determine whether a PO has added a ballast resistance to a chrome bumper car, or bypassed it on a rubber bumper. Using the incorrect combination will either result in weak sparking, or an overheating coil and damage to the trigger especially points.
|Thanks Paul. |
I had bought the Pertonix positive earth unit for my positive earth car. A timely Q/A in August Safety Fast suggested possible problems with the tacho with the Pertronix unit (also in the archives I think). I emailed Pertronix in the states and they confirmed that their unit would *not* work with the tacho. I returned the unit to MOSS! That together with concerns over the coil resistance plus cost convinced me to stay with points.
|I converted my tacho to voltage input and it runs fine with a +ve earth electronic ignition. I do miss the elegant one turn current pick up of the OE fit which menat that a short in the tacho did not stop the car but something had to give. Of coures you have to move the tach pick up to follow the switched side of the coil.|
|The beauty of the voltage-triggered tach used from 73 onwards is that by putting a hidden switch on the pickup wire behind the dash to earth it acts as a very effective immobiliser.|
could you clarify please the advice that the Pertronix ignition would not work with your tacho? Is that because it was negative earth, of did it have something to do with your model?
The reason I ask is that I installed a Pertronix unit in my 72 B and from time to time the engine would die, and the tacho drop to nil. The car would start again immediately as if nothing had happened. I have not had the same problem when running with points. The Pertronix distributor insist their unit is OK and suggest my coil might be to blame. Is this kind of problem common in Bs?
|J H Crighton|
|It is the shape and duration of the Pertronix current pulse that causes problems with the 72 and earlier current triggered tachs, it doesn't seem to be a problem with the 73 and later voltage triggered tachs. Sometimes the tach pickup can be modified to work with the Pertronix, sometimes people have had to fit the later voltage triggered tach to earlier cars.|
If the engine dies and the tach suddenly drops to zero at the same time that indicates there is a problem with the ignition feed or the LT circuit. If the ignition warning lights comes on as well that indicates it is a problem with the ignition feed through the ignition switch, if not then the feed to the coil, through that to the trigger (of whatever type) to ground. Could be bad connections between any of those components, earlier coils with rivetted spades (later coils have threaded studs and nuts) are known to have this problem, it can be the wire(s) going through the distributor body to trigger on the 'points' plate (either points or electronic trigger of various types) as these are being continually bent back and fore under changing vacuum, or it can be the distributor ground wire which does the same thing, or indeed just a defective Pertronix (or points/condenser). The fact it comes back straight away points (ho ho) to the wire(s) going through the distributor body and the ground wire more than anything else.
Thank you for that explanation. What puzzled me was (a) the car starts again immediately, and (b) the problem does not occur using points. Same coil. Same ground wire in dizzie. Same ignition switch.
I take it your helpful point about the shape and duration of the Pertronix pulse affects only the tacho? (i.e. an is not related to the LT/ignition issue)?
I will try it with another coil, just in case there is something related to the unique combination of the coil I have (a relatively new Bosch GT40) and the Pertronix system.
The 'good' news is that a Pertronix system works perfectly on my 54 TF.
|J H Crighton|
|The shape and duration of the pulse *should* only affect the tach, unless there is something wrong with the Pertronix itself. That's the problem with electronic ignition, you can really only diagnose it by substitution, which is expensive.|
If the problem doesn't occur with points that would tend to eliminate the coil as the cause. It's possible the Pertronix wires are fractured internally, but it works OK in the TF because its vacuum signal is different i.e. flexing the wires differently or not at all. I've just had that problem on a TV where the conductors had fractured inside the insulation at the cord-grip in the plug. Plug it in one mains socket and it worked OK, plug it in a different socket and it didn't. But other things worked OK when plugged into either socket.
This thread was discussed between 01/08/2008 and 16/08/2008
MG MGB Technical index
This thread is from the archive. The Live MG MGB Technical BBS is active now.