XK120 Dynamo / Generator, Voltage Regulator problem

Ok, now I’m baffled. I hooked up the analogue meter after testing it on the battery, marked 12v on the gauge with a black triangle, disconnected the digital meter, hooked up the analogue, started the car, increased revs to 1000, and now it’s reading 12v output. Checked the dash and the ignition light is now out … what the heck?

edit. Oops I need to reconnect the D and F terminal leads and start up again … could be the regulator after all. Stay tuned.

Ok. Still not charging. It’s the voltage regulator after all. Will be checking that out next, starting with the connections. A much less laboursome (and expensive) remedy proposition.

Thank you, Robert. I learned something about digital multimeters. Guess I’ll keep my old analogue meter around for awhile.

The only issue after this one is a leak out of the water pump spindle. I examined ithe pump many years ago and it’s in good shape but apparently the spindle seal is gone. Can’t find one listed at any of the usuals.

Regulator trouble is often just a bad connection, either at one of the two sets of contact points, or one of the riveted posts which you get to on the back. Muriatic acid (pool cleaner) cleans them. Some people solder all the posts.

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Nick Congratulations on finally getting to drive your labour of love. It looks fantastic.

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Thanks, Rob. I’ll do some diagnostics with the thing in situ and if no solution will extract it and check it out.

Thank you, Robin. This was the first time I’ve driven an XK120 and it was a delight. The engine was smoky after I did the rebuild so I was looking forward to getting the car on the road to give it some heavy throttle to seat the rings. The power for a post-war car is amazing. Love it.

Hi Nickolas,

Take a look at this if you haven’t got it: https://www.mg-cars.org.uk/imgytr/pdf/lucascourse5.pdf.

I’m also guessing it is dirty contacts in the control box, or poor connections - including poor ground/earth connections.


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Now that’s a comprehensive document, Clive! I’ll be digging into it this evening. Many thanks.

Just my opinion, but I wouldn’t get muriatic (hydrochloric) acid anywhere near a car. A “sealed” container of it will cause anything within 3 feet to rust.

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Here is a more concise version just covering testing: https://www.mg-cars.org.uk/imgytr/pdf/lucas.pdf.


Thanks, Clive. @Erica_Moss sent me the same document earlier. I’ll be an expert on voltage regulators when I’m done - alternatively completely confused :sunglasses:

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Sorry I thought it was obvious, clean the regulator in a small plastic tub of muriatic on a workbench outside. Rinse it of course. Worked for me with my irreplaceable 1938 Lucas regulator. Or you can use Copper-Brite.

You’ve got to rinse very throughly. Preferably with a mild base, like baking soda. Sorry for being a bit over-critical, but I like orthophosphoric acid for this purpose. Unlike HCl, which etches metal and leaves it ready to react with oxygen (rust), phosphoric acid creates a rust inhibiting barrier. IMHO.

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Right, rinse in baking soda, I think the package might have said that. Sure cleaned up the brass posts anyway.

Hi Nick,
First off congratulations on a job well done in putting together a beautiful car. You have shared your experiences in all phases of the restoration from body work to engine work and many other details. We all watched you start up your engine and shared the glee when you heard it fire for the first time in 30 years. You have now moved into the driving and enjoying phase of ownership and we all applaud you! Many people have benefited from your shared knowledge and experience, and many will continue to in the future, from your archived posts.

Now that the driving phase has begun I am particularly interested in your driving observations, the shifting, the handling, the braking, suspension, gas mileage, and anything else you care to share. Lots of XK Lovers posters have similar long term ongoing restorations and have never really driven their cars, your story is a great incentive to all.

Regarding your charging issue, many have already suggested the problem could be simply corroded points on the voltage regulator. They are notorious for that and there is a recommended procedure in one of the Lucas manuals on how to clean them without disrupting their adjustment. My brother recently went through his original 1954 regulator and discovered that and now it is working properly. Likewise, Rick Holland, an old time lister and true expert on all things 120, cleaned my NOS regulator before I installed it about 5 years ago. It operated fine but required an output adjustment to properly charge the battery, which is covered in the 120 Shop Manual.

Once again Congratulations!

Tom Brady

If it’s the same as a Mk7 pump it’s just a regular seal, measure it & you should be able to get one from a seal company.

Thank you, Tom. It’s been a long haul. I was a much younger man when I acquired this car as a semi basket case and I had dreams of getting it back on the road in a year. (Let me change that description to much younger and naive man). I grossly underestimated both the work involved and the free time I would have to do it. Within weeks after purchase the company I worked for began increasing my responsibilities and demands on my time, transferred me (and the XK120 in moving vans) from Northern Ontario to Montreal to Northern British Columbia then to Southern Ontario. I figure from the time I first trailered the car home almost 31 years ago to it being transported to this current location 16 years ago it covered 6,200 miles, none of it under its own power. I’ve so far driven it exactly once, for ten miles according to the odometer and only up to 50 mph, so I’ve still got a lot of impressions pending … once I get past this fettling stage. I’m hoping to get it out for a second and much longer run in a week or so, depending on how long it takes to sort out the water pump and voltage regulator.

First impressions compared to the E-type. The unassisted drum brakes are more than adequate though they understandably take greater effort to slow and stop the car. The XK’s e-brake, however, is much better than the E’s. The recirculating ball steering is fairly precise though quite heavy at parking speeds and not as tight and nimble as the E’s rack and pinion. Driving position more cramped. Cornering not as graceful, as might be expected comparing a live axle rear end with leaf springs and lever shocks v coil over telescoping shock IRS. The engine starts easier, though I’ll want to install a cutoff switch for the hisser. Throttle actuation is smoother and more effortless then the E’s. Throttle response itself is just as good and, though not quite as snappy as the 4.2, the 3.4 puts out a surprising amount of power. Clutch effort is heavier but not strenuous considering it’s mechanical. Shifting the Moss box more quickly and smoothly will be a learning experience. I had a hard time finding first at a couple of stop signs till remembering the recommended neutral-second-first routine but did ok 2-3-4-3-2 up and down shifting (slowly) without double clutching.

Overall it is a delight. It’s a much different car but really, really fun to drive. It has been a long time since I was this thrilled.

There are a few different XK120 water pump versions, Randall, maybe the same for the Mk7 given the identical power plant. I’m rather thinking the more expeditious move will be to purchase and install a compatible aftermarket unit then rebuild the original at my leisure. From what I’m reading, though, the spindle seal is the achilles heel of the OEM pumps and they need periodic replacement while the aftermarket units have addressed the issue. It’s a matter of sorting out which one to buy - if price point is an indicator then there are multiple choices from the usuals ranging from around $300 to over a thousand.



It’s really nice reading how you see both the XK120 and the E-type.

I concur: adding that the ride and straight line stability of the E-type is almost modern, a really good car… and let us not forget fresh-air heating, over-drive (in some cases), which I really miss… and yet the XK120 has something vintage to it that, would you fit inside, is really impossible to resist, it’s just a wonderful car, where the E-type, for me, is well… great.

Also the 4.2 is almost brutish, the best of both worlds being perhaps the 3.8, but I haven’t driven one since 2005 or so.

Frankly, going to Spain on holiday now, I guess that I will be driving my bother’s SI 4.2 FHC, and no matter how much fun I will have, I will miss driving the XK for a while… plus its glorious pre-war, post-war identity.




Nick before you go in the direction of a seperate switch, once you are comfortable that the engine will run without the hisser, try a quick off/on of the ignition switch.that used to turn off the hisser in my 3.8 ‘S’.

Now that would be convenient. I’ll give it a go once I get things sorted. The hisser seems to work well. The engine fires right up but revs high for a few minutes before the hisser cuts out, long after it’s really needed.