On 2/11/2021 2:24 pm, keithr0 wrote:
> On 1/11/2021 11:05 pm, Clocky wrote:
>> On 1/11/2021 12:58 pm, keithr0 wrote:
>>> On 31/10/2021 5:57 pm, Clocky wrote:
>>>> On 31/10/2021 11:55 am, keithr0 wrote:
>>>>> On 30/10/2021 9:57 pm, Noddy wrote:
>>>>>>
>>>>>> OT: Car post
>>>>>>
>>>>>> As most of you are aware I own a 1956 Ford F-100. It's a cool old
>>>>>> thing and I love it to death.
>>>>>>
>>>>>> But....
>>>>>>
>>>>>> It was put together by a bloke who really had no idea what he was
>>>>>> doing, and I've been chasing problems with it ever since I bought
>>>>>> it last year.
>>>>>>
>>>>>> It's coming good, and I've got most stuff sorted on it, but one
>>>>>> problem thing it's had since the day I got the thing is an
>>>>>> intermittent ignition stumble, and because it's been *very* random
>>>>>> it's been terribly hard to pinpoint.
>>>>>>
>>>>>> Until yesterday :)
>>>>>>
>>>>>> The truck has it's original 272 cube Y Block V8 that was
>>>>>> reconditioned not long before I bought the thing, and is standard
>>>>>> apart from having an alloy 4 barrel manifold, an Edelbrock 1405
>>>>>> carb, a nice set of ceramic coated headers and an MSD "ready to
>>>>>> run" distributor.
>>>>>>
>>>>>> One of these:
>>>>>>
>>>>>>> https://www.vpw.com.au/parts/MSD-8383
>>>>>>
>>>>>> These ready to run things are a quick and easy convert to
>>>>>> electronic ignition for older shitheaps like this, as they're all
>>>>>> fully self contained with a single pole reluctor trigger and an
>>>>>> ignition module mounted under the base plate. All they need is a
>>>>>> power feed from the coil and they're good to go. They also have a
>>>>>> couple of cool features such as a built in rev limiter which is
>>>>>> programmed by momentarily earthing the tach signal wire while the
>>>>>> engine is running, and their best feature in my opinion is that
>>>>>> the mechanical advance mechanism is *above* the base plate under
>>>>>> the rotor which makes changing the advance curve a five minute job.
>>>>>>
>>>>>> On the downside, I found out not so long ago that MSD gear is now
>>>>>> made in China, and that gave me cause to be concerned as Chinese
>>>>>> automotive electrical components don't have a stellar reputation.
>>>>>>
>>>>>> Anyway, that theory was confirmed yesterday morning when I was
>>>>>> coming back home from a trip into town when the thing suddenly
>>>>>> started carrying on like crazy and would refuse to run for more
>>>>>> than a couple of seconds. Long story short after much pushing,
>>>>>> swearing and short stop start driving I managed to get it home and
>>>>>> start my diagnosis.
>>>>>>
>>>>>> Checked everything. Coil, wiring, relays, temperature, the lot.
>>>>>> Nothing seemed to be out of order. Last thing left to do was pull
>>>>>> the distributor and have a look at it, but given that it was
>>>>>> fitted brand new and has only *just* clocked 2000 miles I wasn't
>>>>>> expecting it to be where the problem lay.
>>>>>>
>>>>>> Murphy's first law of automotive diagnosis: Assume nothing :)
>>>>>>
>>>>>> The distributor shaft needs to be removed to remove the base plate
>>>>>> and get to the ignition module. As I was peering under the module
>>>>>> when lifting it as far as the wiring harness would allow, I saw this:
>>>>>>
>>>>>>> https://www.imagebam.com/view/ME4MPQ7
>>>>>>
>>>>>> Uh-oh :) I don't think that should be there :)
>>>>>>
>>>>>> The wiring harness sheath had melted itself together, so after
>>>>>> cutting if off and removing the base plate, the ignition module
>>>>>> looked like this:
>>>>>>
>>>>>>> https://www.imagebam.com/view/ME4MPQ9
>>>>>>
>>>>>> Fucking nasty. Massive overheat by the looks, but this thing was
>>>>>> wired *exactly* as MSD's instruction book said it needed to be.
>>>>>> The melted brown wire was actually jammed in between the capacitor
>>>>>> and the remaining resistor where it met up with the other three
>>>>>> wires and was fed out through the sheath in the lower part of the
>>>>>> housing.
>>>>>>
>>>>>> And on the other side, we had this:
>>>>>>
>>>>>>> https://www.imagebam.com/view/ME4MPQB
>>>>>>
>>>>>> Just fuck. Massive fail.
>>>>>>
>>>>>> As I said, this is a new distributor that the previous owner paid
>>>>>> over a thousand bucks for, it's done just over 2000 miles and it
>>>>>> has been installed *precisely* as instructed buy the company who
>>>>>> made it. Pretty piss fucking poor if you ask me, and a *perfect*
>>>>>> example of how paying high prices for "brand names" isn't all that
>>>>>> great an idea these days.
>>>>>>
>>>>>> The solution is to buy a new replacement ignition module which
>>>>>> comes as a "kit" which includes the module itself, the base plate,
>>>>>> the advance plate and the magnetic pick up. But given that they're
>>>>>> 300 bucks Australian and and are 6 weeks away in the US, I'm not
>>>>>> all that keen to wait that long let alone to pay that kind of
>>>>>> money only to have the exact same thing that fucked up in the
>>>>>> first place.
>>>>>>
>>>>>> So we move to plan B, which is to try using a remote ignition module.
>>>>>>
>>>>>> Given that the triggering mechanism in the distributor is a basic
>>>>>> single pole reluctor it should work with *any* generic module like
>>>>>> a Bosch 021 or a GM HEI, and given that I have both of those here
>>>>>> and nothing else to lose my plan is to gut the distributor and
>>>>>> just leave the reluctor and pickup in place and feed those into a
>>>>>> 4 pin HEI module which I can mount in an enclosure on the
>>>>>> firewall. I can't see why it wouldn't work, and I'll know by this
>>>>>> time tomorrow if it does. If it does it will be a good cheap home
>>>>>> brew fix that will hopefully help other MSD owners avoid a massive
>>>>>> repair bill.
>>>>>>
>>>>>> In the mean time, if anyone's thinking of purchasing automotive
>>>>>> ignition products any time soon my recommendation would be to stay
>>>>>> the *fuck* away from anything offered by MSD.
>>>>>
>>>>> A bit of a disaster! We have a destroyed MOSFET, an exploded
>>>>> capacitor, a burnt wire and a resistor that has got so hot that it
>>>>> has melted its solder connections.
>>>>>
>>>>>
>>>>> The capacitor that has exploded is interesting, without a circuit
>>>>> I'd have to guess that it performs the same function as the
>>>>> condensor (an ancient name for a capacitor) in a Kettering ignition
>>>>> system i.e. absorbing the back EMF from the primary of the coil
>>>>> when the points open or in this case when the MOSFET switches off.
>>>>> If that were the case then the MOSFET would get the whole back EMF
>>>>> and self destruct. The resistor that fell out could possibly be a
>>>>> ballast resistor, if the MOSFET failed short circuit then 12v would
>>>>> be constantly across the coil primary and the resistor in series
>>>>> and the resistor would be dissipating rather more power than it was
>>>>> designed for (remember the days when it was recommended not to
>>>>> leave the ignition on for long periods with the engine stopped to
>>>>> avoid overheating the ballast resistor). The wire is only burned in
>>>>> on spot, probably because it was in contact with the resistor.
>>>>>
>>>>> The upshot is that my guess is that the capacitor was underrated
>>>>> and its demise lead to all of this.
>>>>>
>>>>
>>>> There are a few components that need to be checked if a repair is
>>>> attempted. Since he reported erratic ignition stumble prior to
>>>> failure I know *exactly* where I would start. The repair is trivial,
>>>> anyone with basic electronics knowledge and soldering skill can fix
>>>> it for a few dollars.
>>>
>>> Obviously the MOSFET and capacitor would need to be replaced, the
>>> capacitor with a higher working voltage item. R23 could probably just
>>> be soldered back in (after checking it) and the orange wire replaced.
>>>
>>> Then work from there, possibly being suspicious of the coil.
>>
>> A reasonable assumption however a Zener diode is prone to failure on
>> certain MSD distributors the symptom of which might be commonly noted
>> as an "erratic ignition stumble" prior to MOSFET failure. The Zener
>> diode has an important (and fairly typical, I'm sure you can work out)
>> role in this design and also must be replaced. The issue is not of
>> cheapness of components or "China" but of lousy design by MSD.
>
> Knowing nothing about MSD distributors other than those 3 pictures, you
> may well be right.
>

It a common fault. The Zener is used to regulate the voltage and when it
fails the rest follows as seen here.

I expect Google to be pretty sluggish with the fraud busy Googling what
a Zener diode looks like. Maybe I should send him a few, then he'll know :-)