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MG MGB Technical - 48D4 distributor.

So at the risk of hogging this bbs this week: I have one of those mysterious "DBL 230 ... " engines in a UK-built '73 BGT, which has a Lucas 48D4 distributor with the # 41775A. None of my researches show this as being originally fitted to 18V engines, but because of the DBL 230 engine I'm even more in the dark. A p.o. had fitted Lumenition with the optical eye (so possibly 80's or 90's). No real problems with it, but recently getting intermittent misfire which my auto leccy is suggesting <could> signal time to start thinking about a new distributor. I'd rather go down the route of checking/replacing smaller items before abandoning what "might" be a perfectly good distributor. So my question is - is the 48D4 distributor familiar and perfectly fine, should I persevere with it and look at replacing any faltering peripheral parts such as leads, coil, Lumenition connections? Or, given its likely age, would I be better looking at any current (quality) alternatives?Thanks for any views. John.


J P Hall

Interestingly that distributor and reference number crops up for Sherpa (as well as Austin Princess and Marina/Itala), and it has been wondered if DBL230 came from a Sherpa or Marina.

If the tach is flicking or dropping with the misfire then that indicates an ignition LT problem anywhere from the ignition switch through the distributor to the engine earth. If the ignition warning light is coming on as well then that points to the ignition switch and its wiring.

If there is no tach flicking then more likely to be HT, or fuel.

For all their fans electronic ignition tends to not be as reliable as points when you consider how long points were the only option. The 45DE4 used for North America was considered very poor, but the 45DM4 is seems to be bullet-proof. After-market kits are not.

With vacuum advance the 'points' plate is continually twisting back and fore as the throttle is moved. There should be an earth wire inside between that plate and the body which is a very flexible cloth-covered tinsel conductor wire, as was the internal points wire on the 25D4. The 48D4 seems to be like the 45D4 in that the condenser/points wire that exits the body is also a more flexible wire than usual. With aftermarket ignition you are dependant on the two wires that they use being equally flexible. As strands fracture a misfire can become evident. I would replace the electronic system with points and condenser first, and see what happens.

What the curve of that unit it and how it matches your engine is another matter. But if it is the original one for that engine then it should be fine.

Incidentally, have you ever done a compression test on that engine?
paulh4

Lumenition systems are generally very reliable. They do use proper, flexible wiring.

I have heard of problems with the 3-way plug/socket. It might be worth checking the female connectors to make sure that they haven't spread slightly, and close them up if they have.
Dave O'Neill 2

Wear in the dizzy or chopper can allow the chopper to hit the optical sensor and cause a misfire. You will know if this has happened as the sensor wears a little groove on the chopper.
Peter Burgess Tuning

Paul, compressions are all within the 135 to 140 range - is that low compression spec.?
Peter - you must be psychic. Put the dizzy on the bench this morning for general inspection (Lumenition bits included). Realised that the optical sensor thing only has 1 securing screw, and it had been able to move a few mm, such that its wiring came into contact with the chopper rotors! Clear witness scribing on the original heatshrink where the wires go into the sensor. I'll try and attach a photo. I suspect this has happened while I've been repeatedly firing up the ignition to sort out the carbs over the laat few days, per my recent posts.
BUT - with everything carefully put back today, I now only have 0.9volts at the coil (following Jeff Schlemmer's notes, positive lead off the coil neg)). Absolutely no spark on engine turnover. Is it likely that as the rotor blades struggled to get past the optic sensor, possibly even being stuck in the end, my Lumenition coil has cooked itself? If so, do you agree that a good brand of coil (bearing in mind I'm in regional Australia) of 1.5ohms impedance combined with the original ballast resistor is the better way to go, rather than being tempted by a commonly available 3ohm Bosch GT40?
Thanks as ever.
John.
Pic 1. After re-routing the wire to avoid a repeat. You can see the single mounting screw for the optic sensor, on which it could pivot towards the blades.
Pic 2. The original wiring, which leaves a length of wiring inside, likely to foul either plastic or metal bits. Not ideal.





J P Hall

Yes, 135-140 looks like LC if done hot with throttle wide open (although the latter doesn't seem to make much difference for me).

"I now only have 0.9volts at the coil (following Jeff Schlemmer's notes, positive lead off the coil neg))"

Not sure what you mean here. With no wires on the coil -ve with the ignition on you should have 12v on both the coil +ve and the -ve, and that is for ballasted and unballasted ignition. If you have 12v on the +ve but lower on the -ve then the coil is bad.

Adding an earth to the coil -ve if you have ballasted ignition the +ve voltage will drop by about half and for unballasted it should remain at 12v.

If you get less voltage than that on the +ve then the feed from the ignition switch or relay is bad. Note that even when you have an ignition relay the feed to the coil or ballast can still come direct from the ignition switch, it depends on the year and market.

Ballasted ignition i.e. 1.5 ohm coil with a similar resistance ballast in the +ve supply is always better than a 3 ohm coil and no ballast in circuit. Assuming the bypass circuit from the solenoid is working you will get a boosted spark when cranking.
paulh4

John

Do you have enough slack in your re-routed wiring, to allow the base-plate to move?
Dave O'Neill 2

Yes absolutely, thanks Dave - I had seen your and other comments on that, and improved the wiring layout to make sure. A new exit hole near the optic sensor made life easier, and there's plenty of wiggle room as the plate moves. Both vacuum and mechanical advances checked and operating.
All I need now is some spark.
There's also no voltage reading at the connector between module and dizzy,I'm guessing because of a dud coil.
John.
J P Hall

Most electronic systems have a connection to both sides of the coil - the +ve to pick up 12v and the -ve to trigger the spark. However with ballasted systems it's better if the module picks up 12v from the fusebox ignition terminal so it gets a steady voltage instead of one continually switching between 12v and 6v. With an external module the voltages between that and the trigger in the distributor will be something else altogether.
paulh4

I'll do those tests today, thanks Paul. I certainly don't rule out reverting to points & condenser even temporarily, but with the B having the random 48D dizzy, can you cofirm that 45D points & condensor would be fine? I'd also put in a new 48D dizzy cap if anyone still sells them?
All of this leads me to ponder whether, ultimately (though I hate spending money), I should be looking at a new distributor/coil setup, be it points or electronic. Food for thought. John.
J P Hall

Doing the Paul H. tests, gives the following results:
12.5 v on both pos and neg coil terminals.
5.5 v with earth wire to coil negative.
Coil primary resistance 1.5ohms.
So - coil might not be fried after all., at least on the primary side of things. Looks like next step will be to check whether the absence of spark comes from a problem in the coil h.t. side or whether the Lumenition module is not functioning. Not sure how to check the Lumenition module - will have to do some homework.
Thanks all. John.
PS - one option to replace the old 48D and Lumenition setup might be a "Hot Spark" 45D4 with or without electronic ignition gubbins. I'm a long way from the UK, so anyone with a view on Hot Spark?
J P Hall

If you have a dwell meter, or an analogue voltmeter (digitals can give different results) you should be able to see if the Lumenition module is triggering the coil when cranking with the plugs out.

Dwell should give a reading, as should a voltmeter connected to the coil -ve - it depends on whether your system is fixed dwell or variable. Fixed Dwell should show around 50-60% and volts about 5-6v. If variable dwell that will give a lower dwell reading but a higher voltage reading when working correctly. If the trigger is open-circuit volts should be about 10v, and if short-circuit zero volts, dwell maybe zero in both cases.
paulh4

I read that twice and then nearly understood, Paul! Thanks heaps - I'll run through it with my leccy mate tomorrow; any view on whether the 45D points & condenser will fit in my 48D?
John.
J P Hall

I've not been able to find points or condenser for a 48D4 to compare! However this Magnetronic system indicates it's suitable for a range of distributor types including 45D4 and 48D4 https://www.gsparkplug.com/magnetronic-ignition-lumenition-mtk-007-lucas-a-c-45d-4-cylinder.html, which may or may not mean the points and condenser are the same. Not expensive to get them and try though.
paulh4

I have been looking at a couple of magnetron style products, Paul - "just in case". But my leccy mate promises me that tomorrow he'll test the Lumenition optic eye on his oscilloscope; if it has survived the impact of the chopper, and IF the power module is OK, we might be able to restore the spark. The internal radius of the chopper has clearly been worn down where it engages with the shaft (binding when the optical sensor shifted), so possibly it was not rotating as it should. Interesting!
One more thing : is there anywhere in the UK I might source a new/replacement cap for my 48D4 distributor? Mine is "ok", but is probably 50 years old.
Thanks all.
John.
J P Hall

is he side entry John--- would be best choice to clear the steering uni. on your B
is it clip on or screw on--

https://www.ebay.com.au/itm/202434819418?chn=ps&_ul=AU&norover=1&mkevt=1&mkrid=705-139619-5960-0&mkcid=2&mkscid=101&itemid=

Don't really know but I always thought a 48D with screw on cap was always an electronic dist. using the 45D dist as a base, that would explain why you can't find a listing for 48D ign points etc. the caps are the same and you 'could possibly' retrofit 45D points,condensor etc---but why

willy
William Revit

Awesome Willy, thank you! - I tried and tried, but your googling skills are clearly far superior. I've seen documents describing the 48D as fitted to Austin, Vanden Plas and other 70s vehicles, but not to MGBs, and no reference to electronic ignition at the time. I'll definitely get one of those new caps if the Lumenition comes back to life. My cap at the moment is top entry, clip-on. John.
J P Hall

The distributor I saw online did have points and condenser, and a screwed cap with right-angle connections.

The straight lead clip-on cap for 25D4 is a pain on RHD, the Mini 45D4 right-angle cap would seem to be suitable for that distributor as even though on a Mini it is screw-on the cap has the lugs for the clips as well.

Why? Reliability, simplicity, ease of diagnosis, cost :o) I've never had points or condenser fail in over 50 years. (So far ...)
paulh4

Back to your original post---intermittent misfire-
Do your plug leads have rightangle ends where they plug into the cap or straight.
There's a well known fault with pre/rubbernose MGB's that have a straight leadset and top loaded caps
The steering universal wears away at the insulation on the plug leads and causes a missfire-- Initially it's intermittent depending on the angle of the steering but as it wears/gets worse and ends up a continual misfire
Rightangle ended leads are needed for a straight cap or use a side loading cap.
With the cap you have at the moment ,is that clip on or screw on---any chance of a pic of your cap

willy


William Revit

I have lead 4 routed round the others to hold it away from the UJ, the rubber cap had worn through to expose the insulation.

'Off the shelf' leads were all straight when I bought these. Right-angles are dearer and are right-angled at both ends. Right-angle cap and straight plugs are harder to find and dearer still. Maybe next time, these have been on since 1999.

Had a similar problem with the V8. On the cap the left-bank leads are straight but originally the right-bank right-angle because of the angle of the distributor on the engine. A replacement set had straights on all and the right-bank ones were pressed up against the bonnet. From somewhere, possibly the original 4-cylinder set, I could pull the right-angle connectors off and fit them into the straights. That was in 1995. Again correct configurations available if you look, but possibly only by specifying what you want for each lead and made up specially.





paulh4

Thanks for that Willy. Here's my existing cap - it works, but only just. My leads have (had) right-angled boots, so no problem with interference.

Update. The Lumenition optic bit has today been pronounced deceased by oscilloscope test. It is no more;it has shuffled off this mortal coil and gone to meet its maker... .

So it's goodbye to Lumenition, and I have ordered an Accuspark set-up with baseplate that fits inside my perfectly-good old dizzy body. Plus new leads, coil and rotor. Just thought I'd give a different system a go.
I do want to thank Willy o' the south, Paul H. and others for your valued input. Living where I do, it sometimes feels a bit lonely!
John.





J P Hall

That cap looks ok from here, just needs the posts cleaning up a bit.
If you were going for a side entry cap you'd need to check it was multi fit to accept the clips or check your dizzy housing to see if it has holes for a screw on one,
On the lonely thing--just think, if you sold me that TC i could get a mate to drop in and talk to you while he loaded it up -I could get him to bring beer- LOL
Stay safe up there-more weather coming i believe
willy
William Revit

Ha! I love your persistence, Willy. Like a dog with a bone. Let me drive the b..... TC for a bit, will you! Then I'll keep in touch.
Off to batten down.
John.
J P Hall

This thread was discussed between 19/01/2024 and 24/01/2024

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