Stubborn misfire

[JonB]

Hi

I'm having some problems with an annoying misfire. When I reassembled the engine I took the braided earth off the coil bracket and capacitor fixing and used it to provide an alternative ground path from the chassis to the engine. The bracket and capacitor (assumed to be for radio suppression) got a new ground wire that runs direct to the chassis bolt.

My car has what looks like an Aldon LU142A ignition module in the distributor. It uses a hall effect sensor to detect the rotor arm position. This is connected to the + and - side of the coil respectively. The coil appears to be a gold anodised unit, like the Lucas DLB105 sport high output coil. It has no ballast resistor fitted, which jibes with what is said here: https://www.autoelectricalspares.co.uk/ ... -100-p.asp - the DLB110 is for Lotuses but there's a comment saying use the DLB105 if no ballast resistor is in place.

During diagnosis of the misfire I have checked timing, carb balance and idle mixture. It's all spot on. I noticed some arcing between the coil output (HT plug) and the + / - terminals, only visible in the dark. I know it shouldn't do this and I have cleaned the top of the coil and the rubber cap for the HT lead, to little avail. Also tried to put extra insulation around the cap, by way of a dry section of water hose, slit along its length and slid over the HT connection to the coil, such that the rubber is between the HT plug and the coil terminals. Still have the misfire.

It all worked fine before I took it apart. What else can I check?

I have suspicions:

• There may be a crack in the coil's insulated neck, where the HT lead plugs in. Can't see one. Have attempted to fit the plug with silicone inside its rubber cup, then a spray with WD40 to displace any water but it hasn't helped.
• It might need a ballast resistor? Although it didn't have one before and ran fine.
• Removing the braid has affected it in some alchemical mystical way, despite an alternative earth being put in there.

Does anyone have any suggestions as to how to further diagnose it? I'm not keen on shelling out on a new coil, HT leads, dizzy etc unless I know there is a fault in the parts being replaced. It's expensive doing this with trial and error.

[2cams70]

Try a single complete new HT lead - only the one from the coil to the distributor. It's often the first to fail because it works 4 times harder than the spark plug leads. If there's an internal break in the carbon conductor (often not visible to the eye) the spark energy may try to take the path of least resistance which may be between the coil tower and the low tension terminals.

[bitsobrits]

To assist a remote diagnosis it would be helpful to know:

Does the misfire occur at all rpms?
Under all load conditions?
At all engine temperatures?
Affected by cornering or other driving dynamics?

[alanr]

You have arcing/hairline crack between the coil HT and -+ve and you definitely need to replace the coil. As others have said this is a breakdown in the carbon conductor in the top of the coil.
This may or may not be your total misfire problem sorted but it will certainly cause you issues and the car will not start or run correctly until you have done this.

Alan.

[jimj]

Definitely fit whichever coil Aldon recommend which, almost certainly, won`t need a ballast resistor. It won`t be expensive. Personally I`d replace the HT leads too.
Jim

[JonB]

OK, it is as I thought. I'm going to borrow a coil from someone and see if it sorts the problem out.

Regarding ballast resistors (I will do another search on this) - where would it be fitted normally? I have two wires connected to my coil positive (not including the ignitor +ve). One is the tacho sense wire (white) and there is another which I believe comes from the solenoid. The wiring diagram shows both, but there's a round thing drawn in the side of the coil that the white wire goes to:

WY (white/yellow) from solenoid. W (white) from tachometer. Both are connected to the coil +ve terminal. B (Black), coil -ve to ignitor -ve (effectively, the points in a non breakerless setup). Not shown is the red wire that powers the ignitor. It's connected to the +ve connector on the coil.

Is that supposed to be representing the ballast resistor?

[Mazzini]

The Aldon ignition module in my S3 failed after about 18/24 months use. The car developed a misfire that got worse and worse. I replaced it and the same thing happened 18 months later. Maybe I was just unlucky, but I went over to a 123ignition distributor and haven't had an issue since.

[mbell]

Is that supposed to be representing the ballast resistor?

Yes. From factory the coil power feed would connect to the resistor. The wire from the solenoid direct to the coil, this supplies power to coil when the car is cranked.

[JonB]

Thanks, mbell - shame it doesn't show the other end of the resistor (or label it explicitly).

Mazzini, I suspected this might be the problem but as you can imagine I am reluctant to revert to points, or spend ?400 on a 123 dizzy. I can't think that it would be any more reliable than a module or points, what with all the sophisticated Bluetooth electronics it has. I'm surprised they last at all, being mounted on the engine the whole thing must get quite hot. Have you experimented with the advance curve at all?

[alanr]

Ensure you get the correct coil.
Not just any old coil will do!....A 9V coil is used with a the ballast resistor. A 12v coil is not used with a ballast resistor. The wrong one will give you problems.
The small round thing that you have pointed to in your photo is the ballast resistor so you currently have a 9v coil.
I do not know if this is the correct coil for your Aldon ignition module.
Personally I will always stick to a simple straightforward points ignition system on a road car.

Alan.

[JonB]

Hi Alan

There's no ballast resistor on my car that I can see, and it appears to be fitted with a Lucas DLB105 high output unit which doesn't need a ballast resistor. I was asking to be sure.

What about the earth braid? Where is it attached to on the engine? As I noted above, I removed and reused it as an additional engine ground, but I did put an earth line in from the braid mount point on the coil bracket to the adjacent chassis bolt.

[alanr]

The main earth braid on the engine is usually between the engine block and the chassis on the n/s engine mount on +2. I would be surprised if this the issue. If you had a bad connection here you would have loads of other problems with the speed of starter turning the engine over etc. The only other ignition earth braid is between the distributor internal baseplate and the side of the distributor.

First step if it were me I would obtain a new and correct coil for your Aldon ignition. You clearly have arcing between the HT and the -+ side so the coil has to be replaced regardless of any other issue you may have. As I said previously I do not know if this correct coil is a 9v coil that needs a ballast resistor or a 12v coil. I suggest you either look on the Aldon website or give them a call.

Alan.

[JonB]

Agreed. VinceReynard is going to lend me a coil to test this hypothesis. Arcing could possibly be caused by a defective HT cap too, though I couldn't see any cracks or other deformities, and it's all been cleaned with brake cleaner. I'm sure my coil is a Lucas DLB105 (12v without ballast) which is OK for the Ignitor (needs to be at least 1.5 ohm impedance).

But. I'm also worried the Ignitor is playing up. They are over ?100 to replace.. if necessary I will revert to points I think!

[pauljones]

John.

I fitted Aldon 2 and flame thrower coil plus leads before going efi. One thing that can be said is check plug gaps. A lot of guys I know have issues with miss fire and arching because they open the plug gaps further than they should. This leads to damage further back because the energy finds it's easy to earth else where.

[oldelanman]

Arcing could possibly be caused by a defective HT cap too, though I couldn't see any cracks or other deformities, and it's all been cleaned with brake cleaner

May also be worth checking that the grub screws securing the HT leads in the distributor cap aren't corroded....

#2 plug lead