Busy trying to get a 1975 XJ6 running and have hit a small wall: I have 12 volts at the coil but am only getting an orange spark at the plug leads. I've cleaned the rotor arm and the distributor looks OK, wires are correct, and I'm not sure what else I should be looking at as the problem.
The battery is a year old and is probably around 50% charged (it's recharging now). Could I still be looking at a dud coil?

 

If the spark is still orange after the battery has charged I'd be inclined to replace the coil, yes.

Do you have a volt-ohm meter to check the primary and secondary resistance of the coil?

Cheers
DD

 

Thankyou Doug! Yes I do, I measured the voltage but not the resistance. I will do so before I replace the coil - I have a new one in the garage anyway.

 

The coil reads 0.0 between the contacts and the secondary resistance reads 7.6 ohms. Oddly, the brand new one I have reads 0.0 (and 9.3).
I'm not convinced everything is wired correctly - the PO had attached the 12v power to the negative post of the coil, I'm wondering if that damaged it. Also, the distributor has one red and one black wire which are attached to the coil +- respectively and it's not clear what function the resistor block has in the current setup .

I'm off to study wiring diagrams again.

 

The resistor is a ballast resistor used where a 6v coil is fitted. When the car is turning over on the starter it is effectively shorted out to provide a higher voltage to the coil to compensate for the drop in battery voltage due to the starter current. In normal running mode it is in circuit and reduces the voltage to near 6v to avoid burning the coil out.

If your replacement coil is a 6v one put the resistor back in the circuit. If it's a 12v one leave it out.

Check the resistor is not burnt out first and check the wiring is all intact and in the correct places.

 

Thanks for the help. I've attached a photo of the dark space where the wiring is located.
I'm going to take the resistor out of the circuit and try it again. I just have to figure out all what's connected to it.

The ballast is the one on the left on front of the coil then? To its right is another component that has connectors but had no wires on it when I got the car so have no idea what it is. Any idea ?

 

bypassed the ballast resistor, and the problem persists. I tried the new coil and the spark is still orange. Replace the HT lead? Otherwise I think there must be a loose connection somewhere.

 

Two thoughts:


1. A 50% battery isn't anywhere good enough!!


2. The HT wires have detiorated carbon cores!!! High resistance. Poor spark. If you have a known, good HT wire available. Use the VOM to establish a base line. Compare to the wires on the sick car.


Points and condenser ignition or electronic?


Carl

 

Ah! spotted the problem....your HV lead is missing!!

But seriously, Yes that's the ballast resistor.
The other component, though I can't make it out clearly, is probably a noise suppression capacitor. Reduces noise to your radio. If it's not connected

Poor/ incorrectly set contact points would also produce a weak spark or a weak condenser/ capacitor.

If yours is, as 1975 should be, a contact points system then the following link is useful, I hope I'm not insulting your knowledge.
The Classic Mechanic: Points/Condenser Ignition Explained

If it's been converted to a contactless ignition system then see if you can make the brand out and post on here.

To take the leads out of the equation if you stick a wire or even a screwdriver in the top of the coil direct to the HV connection and run it near an earth point it should give a healthy spark. Obviously check for fuel leaks first and don't hold the screwdriver or wire!! I would probably have done this test first as it quickly divides the problem between HV and LV side + coil.

All this assumes an adequate battery supply.

 

I don't think the distributor is the problem, although it's clearly the wrong one for the car ! It's not a points type but is a Lucas.. it has a pickup module and only two wires, red and black. Might be from a later xj, but - shouldn't that type use an amplifier?? I don't see one on the car..

Anyway, a new HT lead makes no difference, spark is still poor. I'm wondering now if the voltage is dropping too low to create enough spark?
I will have to try running a wire directly to the coil from the battery..

 

For most electronic type ignitions there should be an amplifier somewhere (rarely it's in the distributor itself). Is there a brand name on the distributor?

Did you try running the ignition lead directly form the coil and seeing if the spark is any better?

Have you checked the main negative strap to the engine is secure?

 

You suppose that is a Pertronix unit in the Lucas housing. And, that
it's amp is contained therein.


The red suggests 12v goes here. And black suggest the - post on the coil?


Google up Pertronix for pictures and compare.


And, yes, jumping 12v to coil + has been a favorite diagnostic
tool for many of my years.


But, the fact you get a wimp spark indicates it is possibly OK, but, with a flaw somewhere. Not enough v+ from a weak battery?
starter drag on a marginal battery can do that.


Carl


Carl

 

Thanks Carl, you know I'd never considered that as a possibility. I have a new pertronix unit for comparison but the wires and connectors look identical, I will have to open the unit and see if the innards are the same.

As for the weak battery, it's certainly possible and I could always try jumpleads from my land rover . I have a new battery I can use but it's at my storage locker.
Thanks!

 

It also looks like the *brand new* flamethrower coil I got from Amazon with my pertronix distributor is a dud: getting 0 ohms primary resistance and 9.4 secondary. If both coils are dead, that would likely explain the problem. But in that case wouldn't I have no spark at all?

 

What are you measuring the coils with? Is it a digital voltmeter? These coils should be in the range of 0.5>2 ohms primary and 7>10 k ohms secondary. If you are using an analog meter that low primary resistance can look like 0 ohms. Im assuming the secondary are OK and this is just a typo or scale misread if you are getting similar readings twice.

 

It's a quite new digital voltmeter. It reads 0.0 on both coils, which confused me initially because one is a pertronix unit just out of its box... A mechanic friend is dropping by in the morning so if I've done something wrong I'll soon hear about it !

 

good friends never miss an opportunity :-)