XK 150 Help, Electrical Experts?

Hi all,

Not really a specific XK150 problem but any advice would be welcome.

My dashboard ignition light remains on for around 15-20 seconds on cold start up and around 5 seconds on warm start up. Once the light goes out everything appears fine.

Car is fitted with Alternator. I have swapped for a spare alternator and the problem remains.

Battery is good.

I have checked the charging rate and once the light goes out it seems fine at around 13.7v @ 2000rpm ish.

My electrical understanding is pretty limited so apart from a general wiring check I am at a loss.

May or may not be connected but the dashboard indicator (turn signal) light also is playing up. Instead of flashing it just goes bright/dim/bright/dim.

Any advice would be very welcome.


I would verify that all of the major ground connections are tight and not corroded. If you are checking the voltage with a meter right at the battery you could be bypassing the problem. In 1984 I sold a series V Sunbeam Alpine. Ten (10) years later, 1994, :flushed: the guy who bought it called and said it wouldn’t start, would I come look at it? I took a multimeter, checked the battery, got 12v. Turned the key - nada. I stared at it, looking for something obvious, then it hit me. BOOM! :joy: “Where did all the ground straps go?” I asked. “I took them off. Did I need them?”


Its fairly normal for older alternators to not extinguish the light for a few seconds,
especially at idle

What alternator and regulator do you have fitted ?

did you install larger diameter, newer main cables. Failure to do that could cause a slight voltage drop…do they feel too warm ?

If you have 13.7V at the battery terminals when charging, that is acceptable, but on the low side

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Internal or external regulator?

Oh, good Lord.

That… that is a double facepalmer.


thanks all for your help


did you install larger diameter, newer main cables. Failure to do that could cause a slight voltage drop…do they feel too warm ?

The alternator conversion was done 15 years ago (by Twyfords) and has been fine.
Last year I converted the 2 x 6v batteries to a single 12v red top and all was well.

Last last year the ammeter indicated over charging and when I checked at the battery I recorded 17v so I decided to renew the Alternator and bought a like for like model.

Shortly after this the problem began.

I also have a Triumph Vitesse with Alternator and swapped this into the XK. Problem remained so I assumed the Alternator was not the problem ??

I have had a good look at the wiring today and cannot see anything obviously wrong. The problem is once the light goes out all is operating fine.

I can start the warm engine, applying no throttle, and the light will remain on. I blip the throttle and it will go out and when it settles down to the original tick over the light remains off. Cold starts and the light stays on for 20 seconds despite revving the engine. :exploding_head:

I have converted to alternator on my XK140. I’m using the old (larger) genny pulley, so I need to blip the throttle to get the alt to charge and at about 600 rpm it wont charge and the light comes on. Normal. They are selling the replacement Lucas alternators with tiny pulleys to allow charging at low idle. I don’t trust what that tiny pulley might do at 5500 rpm.

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That does make sense but unfortunately Ive been using this same size pulley for 15 years without issue so don’t think that is my problem.

Hi Phil…without seeing your wireing diagram i can only take a stab at the problem…im assuming its just the alternator with the output directly to the ignition warning lamp. …so not much in the circuit (warning lamp relay etc)…so its either the alternator. …or the wireing…or the lamp or holder…so you have changed the alternator but that dosnt guarantee its a good one…have you replaced the bulb?..rig up a wire with a new lamp directly to the alternator would be the first easy step…Steve

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I converted my 150 to negative ground with an alternator rather than the big generator and have had no problems. I did however also convert all my light bulbs to LED’s so that might explain the difference. I also installed an after market new technology fuse box.

It is usually normal for the engine to need a bit of a rev, on start up to excite the alternator into action, light will then go out.

Thanks all for your further help.
I think the problem is in the wiring somewhere as two (relatively new) alternators exhibit the same issue.
The turn signal problem would point to an earthing problem somewhere?
Trouble is I don’t fully understand how it all works to be focused on checking specific areas! I think its a slow slog checking all related wiring hoping to find something.
Alternatively I could call in an Auto Electrician but where would the fun be in that?


Just remember " mechanical problems take minutes to diagnose and hours to fix, electrical problems take hours to diagnose and minutes to fix"


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Phil some points to think about
Alternators are reliable
They charge as a rule about 11/2 volts over battery voltage depends on regulator and temperature
Almost full out put is a available at idle ,low RPM but no cooling is available
A good system will run 14.1-14.5
Every connection will cause a voltage drop
You can test connections with a volt meter by placing it in parallel to a connection and a good connection will be less than .1 volts , a heavy load less than .3 learn to do it very useful
You can see that a few bad connections in series will cause significant voltage drops
I know people are saying that 13.8 is ok but in my experience it’s a sign of illness such as
Battery weak
Bad battery cable or connection
Bad diode in alternator , it will run but battery will be under charged and in electronic cars gremlins will show up
Use a good volt meter to test don’t rely on car gages
And not some hf giveaway . Spend 30$ and learn to do this voltage drop test it’s one of my best tricks
Sounds like you want to do it be methodical start with a tested alternator ( free at O Reillys) and good battery get a load tester 25$ Charge your battery to start

Then chase it down you can do it


As Jim said, a systematic approach to electrical fault finding is the way to go

If you are not in a hurry, start with the battery…how old is it ?

What is its voltage with terminals disconnected…should be 12.7V

let it sit a few days and measure it, it should not fall much day by day not below 12.5V and certainly not below 12.2

If in doubt, replace it, batteries sometimes only last 3 years,
even less if they are rarely used and allowed to discharge

If you are confident on the battery, move on to alternator.

It is acceptable to briefly disconnect items from the alt and see what it is putting out.
If the voltage suddenly rises to around 14.2, you have some fault

A failed diode could cause 13.8V, but you have changed alts, so that shouldnt be the case

As you said taking it to the auto electrician is not the fun way, and can be expensive,
(but they often nail the fault in minutes)

lots of good Youtubes on “automotive voltage drop”

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:rofl: :rofl: how very true

Tony, Jim, many thanks for the advice. I have yet to get back to it but will follow through methodically and report back.

If it gives you some comfort I have two cars that do the same - ignition light on until I blip past 1000rpm. The alternator light extinguishes and the whirr from the alternator changes as it charges the battery. Expect it’s reaching an excitation threshold or its the regulator doing its thing.
One car is a Jag that we’ve owned since 1987. I kept the alternator it came with going until two years ago. The new replacement alternator behaves the same way. FWIW.

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I fitted a non standard altenator to my fiat spider and I recollect that the ignition light wattage was very specific. Can’t remember what wattage was specified but if the wattage was wrong the altenator would not excite until it was revved up. The altenator manufacturer should have it in their specs.


To conclude and advise anybody else experiencing the same problem.
Methodically checked all the circuits all seemed fine.
Battery is a 12 month old Red Top and measures 12.7v. so all good.
When measured carefully alternator charge rate is 14.4v so all good.
I was totally at a loss so finally rang Twyford Moors who originally converted my car to an alternator in 2007! As per usual they are very helpful and confirmed it to be a common problem with fitting modern alternators as they need to be “excited” (blipping the throttle) before charging can commence and this can take sometimes take a while (15-20 secs in my case) As Bruce suggested earlier but I was just concerned it was taking so long.
Providing the light does not come back on as the revs settle back down all should be well.
So happy now looking forward to enjoying some summer trips in the old girl :blush:
Thanks again to all who helped :clap: :clap: