Ignition Coil swap on 1994 4.0

Car refused to start yesterday morning. First time in 20 years! Anyway, popping a spare spark plug into one of the leads showed a reddish purple spark and I had a new ignition coil in my “in case of emergency” kit in the boot so set to swapping it out.

The ROM shows two leads to each LT post on the coil - on the positive side sure enough I had two.
On the negative side I only had one, but I did have small black wire connected to one of the bolts holding the clamp to the inner wing. Any other 4.0 owners confirm that’s the same?

Anyway, I wired the new one exactly the way the old one was and car started up (after a little cranking because it was a a bit flooded) and is driving great with the coil wired that way. Just thought the difference in the ROM was interesting.

BTW the old coil measured 22ohms on the primary and 7.1 kohms on the secondary when cold and probably worse when warm.

Just for a record of comparison, what does the replacement coil measure at the same points - Tex Terry, II - 1991 XJS V12 Classic Coupe, 1986 XJS V12 Coupe - sent 11/17/2020 1723hrs. EST USA.

Slightly different coil won’t make much difference if the external dimensions aresimilar and the size of terminals is the same. AJ6 engine is prepared for that (AJ16 obviously not…). Some peopke are changing coils to bigger. Bigger coil usually means stronger spark. Stronger spark means more effective ignition. Usually in 2stroke engines I’m afraid…

what does the replacement coil measure at the same points?

I should have measured that at the time, but was in a hurry to get going and get to work… Now that I have measured it I’m more confused!

Old Coil: Primary = 22ohms Secondary = 7.1kohms
New Coil: Primary = 0.7 ohms, Secondary = 7.2kohms.

The specs in the ROM call for Primary = 0.5ohms plus/minus 10% and Secondary 6k ohms plus/minus 10%

So, the new coil (bought from popular jaguar specialist in UK using the correct Jaguar part number) is already out of spec, though clearly still works.

Hello Jamesken - prior to taken the measurements, did you touch both ends of the meter leads to each other so as to determine the meter’s lead resistance(this is a normal procedure prior to doing any ohms test) - this is generally 0.3 ohm to 0.4 ohm - using this information, subtracted from your test results, would give a more accurate result of the coil tests - therefore, your primary result “could” be possibly closer to specs - Tex Terry, II - 1991 XJS V12 Classic Coupe, 1986 XJS V12 Coupe - sent 11/20/2020 1311hrs. EST USA.

Your old coil was clearly FUBAR.

By chance would you happen to know the part number of the coil that you used?

Well I don’t know about completely FUBAR but it was 26 years old so no point putting it back in just because I forgot to use my multimeter correctly!

Hi A.J, I ordered it from Jaguar Classic Parts using PN DBC1140. It came in it’s Bosch box, not in a Jaguar box, but there is no marking on the box. I didn’t think to look for any original Bosch part number before fitting it and if there is any marking on the coil itself it must be on the part I can’t see.

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