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MG MGB GT V8 Factory Originals Technical - V8 ignition Troubles....


I have a 76 GT V8 sebring, fully restored.

It has a SD1 v8 now with a elctronic Ignition (Lucas) 35DM8 with the seperate ignitionunit behind the coil on the fender.

On a nice summer day I went driving but my battery was empty / broken because of a long period of storage without any care....

We jump started the Car and disconnected the start Battery and went driving. On a sudden moment (first high rpm) the engine went down, not starting any more.

I assume my ignition broke down as my battery was bad and the alternator probably overloaded the system and the ignition.

Can someone explain, or give me an advise how to proceed.... Have tried several things but I guess I have to look for another unit.

Thanks for posting!

Jos Minderhoud, The Netherlands.

for an impression of the car see:


If your battery had completely failed and was effectively open circuit then the alternator can give much higher voltages than normal which could have damaged the ignition. This is why engines should never be run without a battery connected, but they can be run with the alternator connected (for short periods until the battery goes flat anyway!).

Diagnosing electronic ignition is always tricky, virtually the only thing you can do is diagnose by replacement, which is why I don't like them.

If the 35DM8 is like the 4-cylinder version it consists of a magnetic trigger in the distributor that controls an electronic circuit in the remote module that switches the coil. The internals of the remote unit are available new from various sources, but I don't know if the distributor trigger is. However used units should be available from a number of sources, as is the later 35DLM8 unit which has the module inside the distributor instead of remote.

The first thing to do is clip a timing light onto the coil HT lead and see if that flashes while you are cranking. If it does then the ignition is working, so you would need to do all the normal checks for a non-starter.

If the coil lead isn't flashing then test the coil. You can do this crudely by disconnecting the module wires from the coil, connecting 12v to the coil +ve, and tap an earth on and off the coil -ve. You should get sparking on the coil -ve as you tap, and although the HT will be weak as you have no condenser in circuit you should get some HT spark jumping a plug connected to the coil and lying on the block.

If that's OK then it is either the module or the trigger. Reconnect the module to the coil and monitor the coil +ve and - terminals with a voltmeter while cranking, ideally with the plugs out to get as high a cranking speed as possible. This is because the DM units have variable dwell, which effectively means it supplies a fixed length pulse to the coil, which is much shorter than points would at cranking speed. This makes the pulse more difficult to see on a meter that it would be with points. In fact I have never done it on a DM unit so I don't know what you will see, but would expect to see some flicker of the meter needle if the module is switching the coil. Oh yes, use an analogue meter, as most digitals probably won't tell you anything useful.

If no pulses then either the trigger or the coil are faulty. If you can find out just what signal the trigger sends to the module you can reproduce it externally, and so determine whether it is the trigger or the module. Don't quote me but it may just be like a switch opening and closing, which you can reproduce.
Paul Hunt


I also had problems with the electonic of the distributer of my SD1 engined GT.
It was one of the first series dizzies with the board inside. As I found out it was a thermical mistake that occured frequently and was gone after the engine had cooled down.
If the tach drops when the ignition cuts off, it is a well known symptom of a faulty electonic module on the Lucas Opus.
An other poit to considre is the wiring of the vents. They must not be wired to the same circuit as the electronic ignition is. Cutting in, they are responsible for a drop in voltage in this circuit and (if it is the same as that the electronic ignition is wired too)the signal amplifier might not have enough currant any more to work well.
In my car, I have fitted a Newtronic optical trigger system with a black box meanwhile but I am planing to change to a Mallory dual point mechanic distributor and a non balasted coil soon.
I will contact C & P in Belfeld within the next few days and ask them for details.


Thank you both.

Ralph, please let me know the outcome....

Regards, Jos Minderhoud

The tach instantly falling to zero when the momentum of the car is still spinning the engine is a sign of ignition LT failure with *any* ingition system - points, DE (Opus) electonic, DM electronic etc. The failure can be anywhere from the 12v supply from the ignition switch or relay through the ignition system i.e. points or electronic module and distributor ground wire (where provided). If the ignition warning light is also glowing then that shows the 12V supply has been lost from the ignition switch.

When you say 'vents', Ralph do you mean electric cooling fans? The factory V8 powered its fans off the ignition supply, albeit with points ignition. Still not a good idea as it pulls down the voltage to everything else as well as supplying low voltage to the fans. Since beefing up my fan wiring with a direct connection to the alternator and local grounds my fans are noticeable faster and the temp has never got past mid-way between N and H, before that it could get up to the H.
Paul Hunt

@ Paul,

sorry for that mistake, please read fans not vents :-)

@ Jos,

The problems seem to be cured by changing some wiring so the ignition is now seperated from any other 'loads' in this circuit.

First I thought there was a problem with the Holley and I found a bad secondaries diaphragm and renewed it. It did not cure the hesitation of the engine, so I put in new HT wires (planed since a couple of months allready) and a new Airtex fuel pump to substitute my set of two Mikuni pumps that had become noisy within the last years.
The car performed very good but got problems when warmed up. Checking the timing, the strobe died at +3000 rpm without load, flashing again at lower engine speeds.

As I fitted a Newtronic ignition some years ago, there seemd to be not enough power to run the black box of the ignition?!
During the past 4 month, the whether was not that charming to take the car for a ride, so I renewed the wiring of the fans with additional emergency sensors in the lower hose to the radiator and run a new wire from the relais of the fans to the fuse box where I only checked for a permanent +12 Volts post. Crazy not to have a look into the wiring diagram, I connected my installation to the post where the brown wire is added that serves the ignition switch.

The problem did not show up when starting but when the temperature rises and the fans cut in, there was a drop of power in that line, so the ignition was not within it's voltage range (8 Volts min.) :-(

After I routed the wire for the fans to the alternator, everything was fine again.
As Paul allready mentioned he did the same mod on his works specs V8 (with points) and found out that the fans started working more efficiently and I can confirm this from my findings.



I think I need to do that mod of wiring the fans direct to the alternator.

I have a relay to switch the fans so presumably i disconnect the power feed to the relay and then run a decent size wire from there to one of the terminals on the alternator,

Which to Which please. and what do I do with the disconnected wire that previously fed the relay?

Kevin, (not good with wires)

Kevin Jackson

Ralph - 8v is very low and there would have to be something very wrong with the wiring or alternator to have the voltage drop that low, or fans that took a huge amount of current. Mine were 'only' losing about 2v (from 14v) but even getting that back still made a noticeable difference.

Kevin - the original *factory* V8 cooling fan relay is a bit of a weird one as it only has three terminals - one for the input from the sensor, one out to the fans, and a common 12v supply from the green circuit (fused ignition) which powers both the relay and the fans. To power the fans direct from the alternator you have to change the relay to a more conventional one with four terminals, or the fans will run with the ignition off which is not a good idea. The existing three wires are connected as before with the green wire now just being used to operate the relay. The new connection from the spare output terminal on the alternator via an in-line fuse is used to power the fans via the relay contacts. As the fuse is only there to protect the wiring I use a standard 17amp rated 35amp blow tubular fuse as you (should) have two spares for that in the fusebox anyway, changing the fuse type or rating means you will have to carry different spares. If you have a look at you will find the original and modified schematics which should help. The modified schematic shows the new brown feed to power the fans coming off the fusebox, but it is more effective if it comes direct off the alternator, you will have to obtain a suitable large-sized spade connector to fit in the alternator plug. If you use a modern 'cube' relay the terminal numbers translate as follows:
These relays are available in fused as well as unfused versions, whilst the fused version saves you wiring in an in-line unit, they are blade-type fuses so you will need to carry a spare of that type. I'd also strongly advise adding the local earths under each fan mounting bracket bolt, they probably make more difference than the feed direct from the alternator.
Paul Hunt

Thanks Paul, I'll add that to the to do list for when the weather gets warmer.

I had a lot of electrical upgrading done when I first built the car so need to have a look to see what my car electrics guy has done.

Kevin Jackson

This thread was discussed between 02/03/2009 and 06/03/2009

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