Peder,
One other consideration for you as to cause of the problem: since you have decent charge voltage, maybe the faint red light is cross-talk from one of your other lamps. Check for loose bulb holders if you have not already done so. I have seen this happen.
i don’t know and haven’t looked at the Mk10 wiring diagram as i have not got my hands on one yet.
but the dark Lord Lucas and any thing British. the ignition bulb is usually not earthed. one side is battery and the other side is alternator. key on and the power runs from battery to alt which is acting as an earth. motor running and the alt is pumping out power to the battery so both sides have the same current = no flow so no glow. so why. ok alt voltage low, Battery is soft and always pulling current even though voltage is good ( load test) or a diode dead or out of place meaning low current from the alt. and a really weird one is diode’s do voodoo stuff and you get you DC good but also so a little bit of AC down the line. no balancing load for the AC so a bit of a glow.
I still think you guys are looking down the wrong path. The pressure switch which controls the ignition light is NC “normally closed” providing the earth connection to the light when the ignition is turned on. Once the engine is started the circuit is broken and the light goes off. If the pressure switch is faulty it can provide an earth leak which makes the ignition light flicker… This is what mine was doing. I replaced the pressure switch and solved . You can easily test by removing the wire to the pressure switch and the light should go out, if it still flickers then there is another crossover or something … teh pressure switch is located on the side of the block under the carbs and beside the oil filter…
See below wiring diagram I found on line somewhere for reference for mk10 be its the same on 420g…
Hi, this is interesting, as I have never heard of this before, and dont know what the “pressure switch” is ?
Is it the oil pressure sender ?..(that is shown separately on the attached circuit diagram)
My 420G has no “idiot light” OP sender (originally), although I have fitted one
That leaves me wondering what the “pressure switch” is ?
Its an oil pressure switch and different to the sender unit for the oil pressure gauge. It just breaks the circuit once it sees oil pressure thus switching the ignition light off … Its screwed into block next to the oil filter unit … see below
Interesting
My 420G with original engine does not have one
I added one which goes into the oil gallery plug holes, but it only operates as an OP idiot light, its not connected to any other part of the circuit
- it does show in the circuit diagram for 420G though, so that one is a bit of a mystery.
worth checking for sure, little things like that will stump you !
could it be the instrument voltage stabiliser?
Where is that located? What does it look like?
I don’t know anything about an instrument voltage stabilizer, but you might check to see if your car has the 3AW Warning Light Control Unit that was used on other later model 60s Jaguars. My 68 e-type has it. My Mk10 wiring diagram doesn’t show it, but we both know that the later cars may not follow the book. A faulty unit can cause the problem you describe. It looks like this:
this is the description in my e-type service manual
The part seems to be obsolete, but this UK outfit sells a facsimile:
https://www.autoelectricsupplies.co.uk/product/1209/category/259
I had an alternator go out on my mk2 last week that started with a weakly flickering lamp. It later fluctuated from occasionally full red, to flicker, then not lit. When I checked for charge voltage, it was first normal, but dropped intermittently while I watched. You might watch your charge voltage for awhile and see if it holds at the same 13.8 as you found before. Your alternator may be failing. If the charge voltage stays steady even with the flicker and your car’s voltmeter works, I’d probably just drive it. I’ve seen it written that it’s not uncommon to have an unexplained flicker.
I agree with Rons post
I have only ever taken my 420G to an auto electrician once (as I am a qualified technician)
Because my charging system did not work, and I couldnt figure it
He was a friend of mine and vastly experienced in Lucas electrics
He ended up just swearing and telling me to get rid of all that junk and put in a “Universal” Lucas 70A, (they are internally regulated) and thats what I did, cost me about $200
2 wires, looks original, the old junk is still there, 43amps is very marginal for an A/C car
I’ve had this on a Holden in my youth. Ammeter indicating charge but the warning light (within same instrument), remaining softly lit. It turned out to be a dry joint on a single diode in the rectifier pack. Resweated the joint and the alternator came good, light out. Paul.
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Back in the day (1963 - 1985) when the ignition light was faintly on in our 1963 3.8 MK2 it was an indication that the voltage regulator was on its “way out”.
Hi Peder excuse the delay in reply.
The voltage regulator is a small metal covered device in the back of the fold down centre instrument panel. It’s about an inch by 3/4 inch attached to the metal panel. Another possibility is one of the phases in the alternator is dodgy. One of my cars had this problem and I kept driving it because it was still charging adequately so to avoid the nuisance of seeing the feint light always there I removed the bulb! Later was told by an auto elec that one of the phases was gone.
Hope this helps
Regards
Peter
Thanks Peter,
My alternator is a 4 yr old Bosch unit, 70Ah. Same as in2 other Jaguars. But my faint red kight when driving might have started after I, accidentallt, left the ignition onfor about 45 min, 2-3 yes ago. This coukd have damaged one of the 3 diodes…?
But why does the faint red light only come on when I put the instrument lights on… That is a mystery!!
Remember that I mentioned cross-talk from another other lights. If it only happens with the panel lights (not other loads), it’s most likely a light leak.
Suspect this is the issue. Same outcome as my earlier post regarding the “dry” diode joint. One phase not available dips the voltage output. The regulator might be able to recover as engine RPM increases. Would be visible using an oscilloscope. The ripple at the top of the voltage trace would be deformed as affected phase would not be available to “sum” up properly. Paul.