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MG MGB GT V8 Factory Originals Technical - No spark part 2

Back in December I was seeking advice reagarding no spark on my V8 conversion which has a 35DLM8 dizzy.

Paul Hunt and RMW both offered similar helpful advice re; checking for variation of current from the negative side of the coil to earth whilst cranking. It is my understanding that it is this "switching" that triggers the coil.I do not have a voltmeter, would a simple circuit tester with a light show the variation in current or do I need to buy a meter and learn how to use one? With the better weather this weekend i was able to get over to the car and rewire the coil to amplifier connections and make sure I had the polarity correct, but still no spark any where, there is definitely power to the coil from the ignition.

Paul and or Roger your further comments and advice would be appreciated.

Regards,

Kevin Jackson.
K.R Jackson

Kevin

It is a while back since we sent each other notes about this - remind me - is it a lumnition amplifier

(the answer to your question I think will be yes but while you are about it one or two other questions -
Current to the amplifyer ? Cleaned up the ground connection on the amplifyer ? - sorry but these little things get overlooked)

Roger
RMW

Roger.

Just to refresh your memory the original problem was that the engine would start then die after about a minute or so with the tacho dieing instantly indicating an ignition problem. I decided to replace the amplifier and the coil(new Lucas 12v sports coil).
The 35DLM8 has the amp which plugs onto the side of the dizzy body and it is a standard replacement unit.
The wiring I renewed was the old coil to amp lead which had very poor connection at the plug which goes into the amp.

You say the answer to my question is probably yes,do you mean i can probably use a lamp circuit tester to check the current variation? Can I test the circuit across the negative and positive terminals on the coil? ie if the circuit lamp goes on and off would that not indicate that the circuit was switching OK and the amp was working but possibly point to the new coil being faulty.

Hope this helps to clarify the position.

Kevin
or have I got it all wrong?
K.R Jackson

You could use your oil gauge or your fuel gauge as a voltmeter.
Anthony Morgan

Kevin
Yes that does clarify. My hesitation in saying categorically that a test lamp will give an indication of the state of the amp is caused by the worry that you might end up blowing the amp (although I do see the amp or the sensor inside the dizzi as the most likely cause of the problem). I suggest you first rule out the coil as the source of the fault.

Would it be possible to retrace your steps and refit the old coil.

I am not an expert on Lucas coils and the manual is not clear about the coil fitted when the DLM8 came along but my reading is that originally a special 'electronic coil' Lucas 22 C 12 was used with Lucas Ballast Resistor 9BR. The manual does not indicate any change with the 'new' distributor although Module 32 C 5 was shown. (the techies were very keen on MODULES at that stage).It's possible the sports coil is wrong for her. LSUK will come back with some more info idc.

Before you start her with the old coil - disconnect the tacho lead (rules out another possible prob) and inspect the wires onto the electronic assembly plate inside the dizzi and the wire upto the pickup unit (which could be snagging the timing rotor).

I don't expect her to start which leaves us with the amplifyer prob - Let us know how you get on

Roger
RMW

Kevin
Postscript - LSUK came back with the advice that you need a DLB 198 coil - the sports coil will work with points and luminition but not your amplifyer. I am hopeful your wee beastie will flash up with the right coil - Roger
RMW

Roger,

Thanks again I'll clean up the terminals on the old SD1 coil and refit this weekend. The tacho is not connected at the moment,I only recently changed it for a V8 one and in case it was part of the problem,disconnected it. Presumably LSUK is now what used to be Lucas? I do appreciate you going to the trouble of asking them to clarify the situation. Lets hope for some positive! progress before I get the big hammer out.
Regards,
Kevin.
K.R Jackson

Kevin - yes - they used to admit to Lucas - expect you have one nearby - good service centre in Portsmouth otherwise - rebuilt alternators/starters etc - Roger
RMW

Roger,

Yesterday refitted original Boch coil and checked wires in distributor which appear ok, gap on pick up only 10thou, not sure if this is critical or how to adjust,I tried loosening of the bolts that hold the unit in place but it did not seem possible to increase the gap. Also on the Boch coil which is marked for electronic systems, is not marked - and + instead there are the numbers 1 and 15
so am not absolutely certain which is negative or positive, any suggestions I initialy read the number 1 as being a minus but having thoroughly cleaned the coil it became apparent that it is not! Anyway still no improvement in the situation,no sparks anywhere. I also checked that power was getting to the coil which it is, but I also put the test light on what appears to be the negative side and also got a light which suggests an open circuit is this correct? Incidently the light I am getting from the power supply to the coil is not that bright so I may fork out for a proper voltage meter so that I can accurately measure the power supply.

As commumicating on line is a somewhat log winded business, i wonder if you could give me a call, my any time number is 0208 300 8042 however if you prefer to keep the dialogue on line I am happy to respect you pricacy.

Regards,

Kecin Jackson.

K.R Jackson

I'm not sure if this applies to the 35DLM8 but on other BL electronic ignition systems of the era the pickup triggered the amplifier which is simply a switch to handle the large coil currents that the pickup on its own cannot. As such I'd expect to see much the same conditions on the coil LT termianls as with a points system, and it is certainly worth trying with a voltmeter.

Whilst having the coil wires the incorrect way round is said to result in a slight degradtion of spark performance the engine should still start and run, and you should certainly be able to get a spark out of the coil on static tests.

With the voltmeter between the 12v supply terminal of the coil, nominally the +ve terminal with the white or white/light-green wire, and ground with the ignition on turn the engine by hand. Alternatively take the distributor out and turn the shaft on *that* by hand. You should see about 12v all the time on a chrome bumper car, or a rubber bumper car that has been converted to a 12v unballasted ignition system. On a rubber bumper car with the original ignition wiring you should see it switching between 12v and 6v as the trigger is ... well, triggered.

With the voltmeter between the coil -ve (distributor wire) and ground you should see 12v when the trigger is 'open', and 0v when it is 'closed'. The trigger may have additional electronics to give a very short 'open' pulse and hence gain dwell, I don't know. These readings should be the same for both chrome and rubber bumper wiring.

Come back with the readings you get and we can take it from there.
Paul Hunt

Paul,
Thread was not meant to exclude others but as I had an ongoing dialogue with Roger it was addressed to him.
Thanks for the further advice. I will pick up a voltmeter tomorrow, theres a halfords across the Rd from where my car is garaged and see what readings I get. When I used the test lamp on either - or + I was getting the same strength light and that was with the pick up in the dizzy between the points of the star rotor, I would have thought that there should only be current on the -tive side when the point of the rotor passes the pick up or have I got it wrong?

Regards,

Kevin.

K.R Jackson

Paul, Roger
Bought a volt meter and did the tests you both suggested and found no voltage variation when cranking. Changed the amp back to the original and repeated the test and got the required current fluctuation and the engine started to fire. As the petrol has been in the car for about 15 months, pumped it out, approx a gallon and a half and put some new petrol in. Allowed the fuel to feed the carb turned the key to start and it went 2nd turn and ran perfectly LOUD CHEERS. So the ignition is back to original spec apart from the new wiring between the amp and the coil, I ran the engine long enough to warm it up about 10 Mins, without any problems,I was amazed at how well it actually ran after its long hibernation.I am now wondering what caused the original problem, obviously the replacement amp was faulty and the old one seems ok at the moment. Will give it another run next weekend to see if I get any problems, it could be the old amp breaks down when it gets really hot under the bonnet if so it can be replsced without major difficult or maybe it was just the connector cable, time will tell. Discovered that I had a couple of other electrical problems, no indicators or low beam on one headlamp but I can sort that next week. One other thing that became apparent when pumping out the tank was that I was getting a vacuum in the tank, is it supposed to vent through the cap or is there a seperate vent from the tank itself, the car is a 74 early R/bumper.
Anyway many thanks for your time, patience and assistance to someone who hasn't a clue about electrics.
Hope to get other problems out of the way, get the car MOT'd so I can use it to go the the Brands mtg. in April.

Regards,

Kevin.
K.R Jackson

Back in town after the break - glad to read the good news. As for the original fault, I rather agree with you - you may find the amplifyer does not like getting hot. You can be sure that at some stage the amplifyer will pack up and the only way to be sure that it does not let you down is to carry a spare. Same goes for the coil. I suggest you keep an eye open for another pickup unit as well.

the pickup gap should be 14 - 16 thousand (inches).

On the subject of petrol tanks, there are no separate vents on the OE tank and I guess the vent should be through the cap - there is usually a vacuum on mine when I come to fill it up - doesn't seem to mind !

remember to chnge the oil now.

Best wishes

Roger
RMW

Roger.

Will swap the faulty amp for a new good one hopefully,
Re the gap on the pick up is that adjustable?
The vacuum in the tank was high enough to cause the bottom to lift when I was pumping out the old fuel with the ordinary fuel pump, May drill a new vent in the cap can't see that could do any harm.

Will change oil and filter, engine still has running in oil in it but has done 300 miles which should be enough for bedding in, I will feel happier with some decent oil in the sump.

Also dicovered that the vacuum unit on the dizzy has gone,probably from fuel or vapour getting to the diaphram whilst the car has been standing so will replace this asap, engine actually started more readily and ran very smoothly without the vacuum advance working. I also want to check the mechanical advance because with the engine mods I am informed that the standard advance is to slow,this can poosibly wait until I put the car on a rolling road to optimise settings.

Thanks again,

Kevin.
K.R Jackson

Kevin - the SDI pickup certainly was adjustable (two small bolts at the base) in towards the rotor and out - take care not to move it along the circumference of the rotor or you will have to redo your timing.

The vacuum advance on these engines seem to have a limited life - for exactly the reason you suggest. There is a vapour trap apparently but I have never managed to find someone who either recognises or sells it.

I would get a new tank cap - they only cost 6 - should do the same myself probably.

Roger
RMW

This thread was discussed between 20/03/2005 and 30/03/2005

MG MGB GT V8 Factory Originals Technical index

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