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MG MGB GT V8 Factory Originals Technical - No spark
|Help please am getting desperate, After trying to remedy fault in ignition system I am now getting no spark at all, when holding HT lead from coil approx .25-.5" from an earth position. The car has a 35DLM8 distributor which has the amp on the body of the distributor, I have replaced the amp and the coil with a new Lucas 12v sports coil and now get nothing at all, I have checked that there is power to the coil and that is ok. The feed to the coil was also renewed to bypass any ballast resistor that might have been lurking in the loom. Any suggestions?|
|Kevin. Doesn't sound too good. Assuming there is no spark at all (shorter distance ??) and you have changed like for like - try first another lead, then your original coil and amp. If this is an entirely new set up - what did it replace ? confirm also you do not have a linked FI system.|
No FI and leads were new 300 miles ago but could be faulty so will try a different coil lead. Original problem was engine would reluctantly start but cut out after about 1-2 minutes. tacho went straight to zero indicating a breakdown (I believe) in the ignition system hence the replacement of coil and ampifier, they were the original parts that came with the SD1 engine so were getting on a bit after 18 years.
Could it possibly something to do with the electronics inside the distributor ie the trigger which sends the signal to the coil?
The fault could be one of a number of things but on these occasions it usually something that I have done to make a difficult situation worse - assuming you have no spark at all - the dizzy has not been touched (more on that next time), the question is whether the amplifyer is trying to trigger the spark - could you put a meter on the coil and look for a voltage change as you turn the engine.
Thinking laterally I am also interested that you were losing the tacho - the tacho won't run without the engine but the engine will run without the tacho. It is fed by the second fuse from the bottom - the fuze that seems to blow regularly - also taking your fans and indicators - check it is in good order. If it isn't and you have taken your new coil feed from it, your problem could be sorted.
Let us know how you get on.
Thanks for your input so far, there is definitely power to the coil i put a circuit tester on it and got a light. Your suggestion re. the amplifier I do not have a meter but could get one but would not be sure what and where to test for the voltage change. I will pull the car out on Sunday it's in a friends garage a couple of miles away in Bromley so its unfortunately not easy to nip out to the car and check something. I'll double check the wiring to the coil and dizzy and try a different HT lead for the coil and see if that changes anything. If you could let me know what i should check re current variation from the amp i would be grateful.
|Kevin - Given you may not have too much power in your battery, I suggest you start by retracing your new wiring which cut out the ballast resistor.Have you disconnected anything else ? I believe the fuze (see above) is a red herring but check it anyway - check in particular your connection to the amplifyer (that there is not a broken wire and that inside the dizzi, with the flashguard removed, the distance between the timing rotor and the static pickup is 14 to 16 thou). If all is well, check that you have power on the coil still, remove the lead (coil to dizzi) from the centre of the dizzi and hold the metal end connector with a dry rag 1/8" from the cylinder block.|
Have your friend turn the starter - you should see a series of blue pulse sparks. If not you need to put a voltmeter between the negative of the coil and earth -if there are variations as the engine turns, the chances are that the dizzi and amplifyer are working but the coil or the lead are not. If there are no fluctuations, suspect the amplifyer - perhaps refit the old one and try again.
If there are sparks the first time, repeat the process with the lead refitted to the dizzi but using a plug lead. You should see less frequent sparks from a plug lead.
Good luck - Let us know how you get on.
Went over to the car on Sunday to check out your suggestions, inverted the coil to make it easier to see what I was doing, I had bought a crimping tool with a set of connectors with a view to making up some new connections from the amp to the coil, the original plug in socket does not fit on to the two spade connectors on the amp very well and I though this might be a good place to start. Unfortunately my friend whose garage the B is in had thrown away my wiring diagram so was somewhat stuck as I am not absolutely certain that I have the wiring to the coil correct, Will pick up another diagram from Dave Vale tomorrow. I did give the engine a spin over to move the oil around a bit it comes up to 40psi just on the starter motor so that probably did some good.I rechecked the HT lead abbout .25"from a good earth and there was nothing at all, so once i have fitted the new leads and made sure its wired correctly will check everything as suggested.To get it going again would be a nice Christmas present.
|Good luck - you are looking for a good quality blue spark 1/8" from the block remember.|
|I recently fought a simular problem. My problem was no power out of the dist. Turns out the points were grounding out on the body of the dist. hope that might help?|
|I haven't been able to test this theory out but I think the 35DLM8 uses a relatively simple low-power switch (the pick-up) controlling a high-power switch (the amplifier) to control the coil. If so then using a voltmeter as the distributor rotates you should be able to see voltage changes on one or other of the wires with respect to earth, or between both of the wires going from the pick-up to the amplifier. If so the pickup would seem to be working meaning the amplifier is faulty. If not then possibly the pickup is faulty. Likewise the amplifier just replicates the points, so again as the distributor rotates you should see the voltage on the coil -ve changing between 12v and earth. The spark should occur when the voltage rises from earth to 12v.|
This thread was discussed between 14/12/2004 and 23/12/2004
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