I wanting to confirm the coil I am presently using on my late 120 FHC is working correctly.
From previous posts I can see the primary resistance should be about 3.2 - 3.7 for the standard coil and 2.6 for the SA-12 sports. The secondary resistance being 7.08K and 10K respectively.
Is there anything to be learnt by measuring the voltage from the coil to the distributor when the car is running? as I measure only 6 volts? I’ll compare this to my Mark V’s voltage.
Does anyone have a diagram of the coil and what and where to take measurements.
Regards, Graham.
Hi Graham, if the car is running the voltage from the coil to the distributor on the primary side varies in time during the ignition cycle. There is a fast, about 200-300 volt, swing when points are acting to make the high voltage, this is due in part to the condenser in the distributor. In addition, there is a low, more steady voltage near 12 V or 0 V depending on when the points are open or closed. If the dwell angle is around 30 degrees, then an average voltage or around 6 volts might be seen on a DC voltage multimeter. Really what is needed for this measurement is an oscilloscope for more detailed time-dependent information. The dwell angle is the equivalent terminology of this measurement you mention. And neither the multimeter nor the dwell meter will pick up the fast 200-300 V swing.
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Thanks Roger.
I raised this question due to investigate hard starting (on occasions not at all) when the engine is hot after standing for some time after being driven. Very difficult to ascertain if it is fuel issues or electrical.
I have already replaced the coil with another and made no difference, checked leads, replaced rotor. Haven’t tried condenser.
It may be down to finding a hot starting procedure that works. Im sure the starter motor is taking all the power leaving a weak spark.
I’ll endeavour to check the spark whilst cranking.
I’m also now carrying a spare set of plugs as I have found they are often fouled up. I no longer start the car using the starting carb as our climate doesn’t really require it.
Regards, Graham
Hello Graham, and here is some further answer to your questions. Attached is Section 3 Coil Ignition of the Lucas Overseas Technical Correspondence Course.
Section 3 Coil Ignition.pdf (2.1 MB)
Just took the car for a run around town.
Mid trip started to miss a bet and by the time it got back it was running badly.
Idles but missing on acceleration. Ill let it cool overnight and have a look at the plugs which I am sure will be fouled.
Almost exactly the problem I was having, Graham. Except it wasn’t so much starting from hot, but all starting. It would run very badly with a lot of surging for 20-30 secs after a hot start, but that was cured by fitting 6mm heat spacers between carbs and manifold. Jaguar found this out for themselves in the early days of the XK120.
But general starting was poor (though I had very low compression and later a blown head gasket, before I had the engine rebuilt). After that, things improved, but I always had the feeling that the engine wouldn’t fire when cranking, only doing so as I took my finger off the starter button. A new WOSP high-torque starter, and two new batteries didn’t seem to help, and I was starting to suspect I might have wired something up a bit wrong on the instrument panel, though I didn’t see how I could have. Connections at the starter solenoid seemed OK, and, more because I couldn’t think of anything else to do, I tightened all the connections at the fuses on the regulator and fuse box, as well as cleaning the contacts of the fuses themselves. Bingo! I think the problem’s sorted!
One thing I have noticed - I’m only measuring about 10½ volts at the coil with the ignition on, and this stays the same when cranking. Should I have a full 12 volts there…??
Chris
This is an absolutely brilliant manual on ignition put out by Lucas, Roger - thanks so much! I’ve printed it off and it will be a valuable reference. It’s written in a way even I can understand! And, it dates from the era of our cars, when positive earth was the norm, so it’s even more relevant. I was interested to see that it was copied from an original that obviously found its way to the Burnham military camp near Christchurch, NZ. I went to school in Christchurch, and indeed my father went through that camp in WW2…
Hi all.
Thanks for your replies.
No fuel filter. It has new batteries.
Chris I have 12.3 volts at the input to my coil with the ignition on so sounds like you may have an issue there. What is the voltage, in comparison, at your starter solenoid?
Mine does the same. It makes no indication it will fire or not. It’s like a toss of a coin if it will start. No reason it shouldn’t start immediately as it has fuel and supposedly spark present.
Graham,
I’m going to check the voltage at various points tomorrow - at the starting solenoid, the A and A1 and 3A terminals on the voltage regulator, etc. I thought it should be full voltage at the SW terminal on the coil, and in fact have read just that in the Lucas manual Roger has recently posted. It could simply be that the batteries are a bit down, as the car hasn’t been given a decent run to charge them up properly, just a lot of starting and backing in and out of the garage and quite a bit of idling…
Well, I did a few checks today. There is near enough 12V at the starter solenoid, and at the A1 (2nd from left terminal - comes from the solenoid) on the control box, BUT only 10½V at the A terminal (first on the left) and the A3 terminal (far right - which feeds the coil). The coil of course still has 10½ volts. So, somewhere between the A and A1 terminals I’m losing voltage. I’ve checked and cleaned the connections, with no improvement. Is there some sort of problem in the control box, and would a new one cure this…?
I’m open to suggestions from any kind soul who knows about these things!
And I began to investigate mine after removing all plugs and finding a variety of fouled ones.
While I was using a colour-tune plug I checked the spark. Nice and strong at time but at other times hardly a spark at all. You could hear the change in the engine and I could see it also affecting the fuel mixture.
Basically I’m starting from scratch. I removed the coil, rotor and condenser and am going to replace each in the hope it is one of these. Also looking for any shorting issues in the distributor but it appears ok.
What I didn’t confirm was that the voltage to the coil remained at 12 volts and I also don’t have an intermittent issue like yours Chris.
Does anyone have make / model of proven good replacements for the items I mentioned above?
Regards, Graham