Ignition coil question on my 1983 W123 2.0

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gassygassy

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Joined
Jun 2, 2023
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35
Location
Lutterworth
Car
1983 W123
I had a 2006 Volvo V70 for a short while which incurred several thousand pounds down the drain to incompetent garages mis-diagnosing faults, doing what The Diagnostic Computer told them instead of having two brain cells to fix the real problem.
So it got dumped in favour of a lovely restored ?1983 -the VIN says it was manufactured 1981 - W123 2.0 petrol which has no computers, in fact hardly any electronics that Faraday would have difficulty understanding. I suspect it is actually 1981 as the fan heater blower motor is at the top centre of the dash, not above a footwell.
I drove it for several hundred miles and it went just fine and dandy until today when it suddenly coughed and spluttered just as I entered a long dual carriageway. I managed to stutter to a layby when I looked for an electrical fault under the bonnet. I couldn't find any loose wires. I removed the HT coil low voltage leads and measured the various resistances. The HT terminal to earth or either of the LT terminals was about 10k ohms. The LT coil seemed to be about 2 ohms which I think is what it should be. I am suspicious that the coil might be on the way out. Years ago when I did mobile car servicing, and having been an electronics engineer I found that HT coil windings tend to break down their winding resistance with age. A new one is about 18k ohms and when they get down to 11k, the voltage produced starts this symptom that I have, intermittent firing.
Having pulled into the layby, removed the wires, measured the resistances and replaced the wires the car went correctly, for about 25 miles when it started misfiring again. I don't mean going bang, I mean just not firing at all when it should. Again I pulled over and this time I looked inside the distributor cap and found to my surprise there are no points, instead a hall effect sensor and rotating steel 'cross' which pulses the sensor at each ignition point. All seemed well so I replaced the cap and off I went and got home OK.

So I have diagnosed that the HT coil needs replacing. I have a question: is it a straight 12v coil or does it have a ballast resistor, and if so where would I find it? I didn't see one when I was under the bonnet.
Were some of them straight 12v coils or did all of them have a ballast resistor? Ford, in case you didn't know introduced 10v coils in about 1970, and dropped the supply voltage when running by using a resistive 'wire' made of string impregnated with carbon. The carbon would bunch up into islands and cause misfiring. Typical Ford disgusting build quality.
 
today when it suddenly coughed and spluttered just as I entered a long dual carriageway.
How far into the journey when that happened? W123s are notorious for dampness inside the distributor cap causing misfires as the engine warms.
 
Thanks for the reply Bellow. I am confident that it isn't damp because I removed the distributor cap and inspected it inside. The day before this happened I had driven the car from Tilbury to Canterbury and then on to Rugby where I live. The journey was faultless. Yesterday when this non-firing happened I had driven it from Rugby to Newark faultlessly and it was only after a couple of hours in Newark when I set off back home that it started. On both days, when it ran perfectly for hundreds of miles before non-firing, the weather was warm and dry.
I do have a spare 6v Bosch coil in the garage, (probably from a Beetle but it looks brand new). I might go and investigate the existence or otherwise of a dropper resistor and the voltage and current drawn by the 123 coil and see if I can substitute the 6v one with a fat resistor . . . .
 
Thanks for the reply Bellow. I am confident that it isn't damp because I removed the distributor cap and inspected it inside. The day before this happened I had driven the car from Tilbury to Canterbury and then on to Rugby where I live. The journey was faultless. Yesterday when this non-firing happened I had driven it from Rugby to Newark faultlessly and it was only after a couple of hours in Newark when I set off back home that it started. On both days, when it ran perfectly for hundreds of miles before non-firing, the weather was warm and dry.
If you are absolutely certain then....
I do have a spare 6v Bosch coil in the garage, (probably from a Beetle but it looks brand new). I might go and investigate the existence or otherwise of a dropper resistor and the voltage and current drawn by the 123 coil and see if I can substitute the 6v one with a fat resistor . . . .
I wouldn't if I were you. Get it wrong and you'll overload the LT side (ask me how I know) and burn it out.

BTW - duplicated threads on the same subject don't help anyone.
 
Dont know if they have a ballast resistor or not ,but if it as it will be close to the coil or bolted to the bulkhead .
 
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look at the wiring diagram for the 280SE which is typical of the merc ignition wiring of that time. It involves a distributor with 2 ballast resistors and a transistorised ignition unit. 52 53 54 55 and 68 in the diagram. The HT coil will be less than 12 V operating voltage. [9-10v ?] It sounds as if one or more of these items has been replaced by an aftermarket hall effect triggering unit- this may include the transistor unit and ballast resistors depending on the kit.. Ballast resistors may have been removed completely and the coil replaced. Without knowing the manufacturer of the kit it's difficult to know. One thing to check immediately is a live feed to your current setup from the fuse block with the bullet type fuses-usually located on the bulkhead. They tend to corrode and make poor contacts but will respond to cleaning with some abrasive. good luck
 
If the coil is ballasted, with the contacts closed so that current is flowing, you should see 6-9 volts at the switch (positive) side of the coil. This will go up to near battery voltage (10-11V) when the ignition switch is turned to the start position and the car is cranking.
If it isn’t ballasted you will see nearer 12 volts with the contacts closed.
 
Thanks for the replies. I have done some more investigating. I don't think it has an aftermarket kit, it doesn't have points and condenser and it doesn't have a ballast resistor. It has a hall effect pickup on the outside of the distributor body, fitted into a recess in the cast alloy body so I don't think it's an aftermarket kit. (see photo). A co-ax cable runs from the pickup coil to the transistorised ignition box bolted to the inner wing near the coil. (photo). I measured the supply voltage with the engine running and it is well over 12v so I don't think it is a ballast resistor coil. I think that would read about 10v. I tried to add photos of the HT coil environment but the system said the photos are too large and for the life of me I can's see how to resize them smaller on my MacBook.
When poking about I noticed that the co-ax cable was touching the chassis rail, and had been rubbing against it sufficiently to discolour the paint - but not to rub it down to the metal. So I felt with my hand to see if the cable insulation had been rubbed through. It isn't somewhere you can look it has to be done by feel. I couldn't feel any screening wire coming through the insulation. However it seems logical that if this screen wire had been touching the chassis this would cause the non-firing so I have cable tied it to a position where it cannot touch the chassis. The co-ax cable from the pickup coil on the distributor would be carrying AC pulses and so if one phase of the signal was earthed, it would screw up the electronic ignition system. I have driven it about 35 miles since then with no trouble. I proceed with crossed fingers.
And a small tool kit in the boot . . . .
Fuse box: the very first thing I did was to dump the aluminium fuses and replace them with what was described as copper - but they are brass. At least brass is going to corrode slower than aluminiumIMGP5202.JPGIMGP5203.JPGI haven't had the car long and I am getting used to the fact that every time I stop, people come and admire it. The other day at the council tip, the operatives were taking photos of it. A white van man overtook on a dual carriageway, he flashed his lights and waved a thumbs up out of the driver's window after he had passed. Someone said they reckoned it is worth 10 or 12k. I resisted the urge to say you can have it for 9k.
 
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Hi Optimus. I looked at that distributor and it is different to mine. That one has what you would call the 'normal' cap retaining clips. Mine has the inevitably superior (!) Mercedes system of having sprung loaded J clips. To release the cap you push down a 'Philips screw' head and rotate it 90 degrees. The J shaped hook at the other end of the 'screw' rotates around a cast catch on the distribtor body and when both 'screws' are rotated the cap is released.
 
I can't remember my 1980 200 having points so I'd reckon electronic by 1980. What I remember at the time was burning up the points (melted insulation bush on the spring) on a Ford Kent with a '6 Amp' coil I was told was for electronic systems only.
 
Yeah, well with Fords, everything that worked burnt. If it didn't burn it came off in your hand (gearsticks) or melted. Or just plain wore out in 50k miles.
 
Yeah, well with Fords, everything that worked burnt. If it didn't burn it came off in your hand (gearsticks) or melted. Or just plain wore out in 50k miles.
It wasn't Ford at fault but me for fitting an inappropriate coil. Nonetheless, it provided an insight into the folly of fitting the wrong coil.
 
I can't be totally against Fords because I am just about to buy a new motorhome with a Transit chassis. Also, a Ford holds my World Record for speed. I went from Swindon in Wiltshire to the roudabout outside Buckingham Palace in 58 minutes, in a Mk ll Zodiac. I don't care what race car you have now, you could not beat that.
However I wouldn't want the embarassment of owning a 1970's Ford car now . .. . .
I think I read somewhere the reason that classic Fords are expensive and classic Mercedes are cheap. It goes something like this: There are only six Cortinas left running on the roads but there are 3,827 Mercedes W123. Insert your own numbers to correct that but the gist is that Mercedes were built to last decades, Fords were built to last until they bring a new model out.
 
My distributor number is 002159940...and then three or four more numbers which are hidden by the cap lid crank. It also has ZV 0007 above the 002 number and under the cast-in Merc star logo and another 0237 on the other side of the retaining clamp also above the line of big numbers.
 
Ive just had another poke about with my phone camera. The number under the merc star is 0021589401. The other number starts 0237 but the rest of it is hidden behind the fuel pump. So I guess it is 0237002539 as in optimusprime's post above.
 

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