AED - Friend or foe - can you help

I have owned a Daimler DS420 1981 for some time now (about 10yrs) and the car has always been hard to start and runs rich. Countless mechanical services and plug changes, (by a long time jag trained mechanic) plus always need to warm up the car for long periods of time. I changed mechanics a few years ago and he asked why the AED was put on back to front? No wonder you have choke and running issues. He corrected, and the start up changed enormously.
The car has been sitting for awhile and at rego time the car wouldn’t start for a number of reasons. Carbs were completely serviced as they were clogged with fuel gum etc. new serviceable parts installed etc. The car however, was still not running correctly. Reluctantly, he suggested the AED will need servicing. The unit was pulled apart, new diaphragm ordered as other was hard and dried out. He has mentioned there are parts missing and asked why, which I just shrugged.
Looking at the following link;

See parts 21,22 and 23.
Can anyone advise why these would be missing? If essential, where can I obtain, as these are not serviceable parts. I’m sure he want wants to install just to see if it improves start up and decreases the number of time the plugs are changed.

Can’t open the link, Mark…

One question; is the car running as it should when hot? The AED is only supposed to be active during start and warm-up. And if AED parts are missing; why/how did it function after the mechanic ‘reversed’ the AED? The AED is rather complex to ensure perfect operation - but but missing/malfunctioning parts may have minor effects depending on ‘which’.

…‘always need to warm warm up for a long period of time’ - does that refer to engine temp, or that it would not perform properly during warm-up?

xj6 85 Sov Europe (UK/NZ)

Replacement AED units are available from Burlen in the UK (and possibly others also). They are expensive and out of stock at the moment. It sounds like someone has been in there before trying to sort out a problem and that your unit is no longer complete/correct.

Instead of replacement there are conversion kits available that do away with the troublesome bi-metallic strip actuation. I have that conversion on my 4.2 and it works well if correctly set up.

Your mechanic should be able to isolate the AED from the carburettor fuel/air supply to see if hot running on the carbs alone is OK. That would help diagnosis.

What happens to the plugs that they need changing so often?


Thanks for your reply Frank. The car always needed the choke on for an excessive amount of time. If you back off the choke to early, the engine would die, it was a fine balancing act. After placing the AED corectly, the choke could be backed off more quickly. An amazing change. I have no doubt its a complex piece of kit. Most people lve spoken to that have worked on cars, threw them away and installed a new unit. The old ones were not repaired. Anyways, it could well be the unit only needed a new diaphragm and the missing are missing for a reason.

Thanks Frankie for your comments. I’ll keep the conversion kit idea in mind and discuss with the M. My understanding is the plugs blackened from the choke staying on to long and the adjustment needed to keep the car running with a more rich setting. If the truth be known, the first M didnt understand the AED or didnt want to be bothered with it.

That your plugs are sooty suggests that either the AED isn’t shutting off or the carbs are incorrectly adjusted. The first step is to find out which.

I’m a bit puzzled by your reply to Frank Andersen which suggests you have manual control over the AED; or have I got that wrong?



first of all, congratulations to such a long-time serving and unmolested car! Then again, your tale leaves a string of mysteries behind:

Who put on the AED the wrong way - how would that work anyhow? How on earth do you “lose parts” from inside the AED (can’t open your link either)? Finally, if the AED is still on the car, how come you talk about “the car always needed the choke on for an excessive amount of time”, as there is no perceptible indicator when the bimetallic strip in the AED fully shuts of the aux carb? Frankie’s reply indicates he understands your comment just the way I do.

Maybe, you help us out with a description of your setup: I was under the impression that the DS420 in 1981 had the same engine setup as the XJ6 SIII and am surprised you’re talking about carbs in the first place. Maybe I’m just ignorant and the Daimler limos used the SII setup.

If this is the case, then what exactly do you mean by “needed the choke”? No stock XJ SI or II car had a simple “choke” fattening the mixture and part-opening the throttle on the main carbs. Instead, they all had either an Automatic Starting Device (ASD) or an Automatic Enrichment Device (AED) added upon the main carbs during cold start and cold running.

In particular, the AED used on SII cars consists of an auxiliary carburetter sitting between the (main) carbs and feeding a richer mixture into the intakes. This auxiliary carburetter is located in the lower part of the unit, while the upper part, hidden behind a black bakelite cover, houses the control of this aux carb, in particular a bimetallic strip operating the air and the fuel valve of the aux carb.

This AED, to function properly, needs hot air to make the bimetallic strip shut off properly when the engine has reached operation temperature. Hot air is guided from underneath the heat shield covering the exhaust manifold around the rear end of the engin in a foam insulated tube. Once the foam insulation has deteriorated the AED won’t work any more. The bimetallic strip has a tendency to fatigue over time. All in all, the control part of the aux part is mostly considered costly to replace every couple of years and cumbersome for classic car driving habits…

Most owners have replaced the control part of the AED with a manually operated camshaft replacing the bimetallic strip. This camshaft is sold as a kit by most parts suppliers, as Frankie mentioned. If mounted correctly it will solve most of your problems.

Good luck


75 XJ6L 4.2 auto (UK spec)

DS420 were always carburetted.

Apologies if I didn’t make clear the setup.
However, I’m still wanting to source the flap, the spindle and the spring.
The car has twin carbs, with an AED. The AED is controlled by a cam/choke, so it can be shut off when hot, the bi metallic strip setup is long gone (before my time). The foam insulation also long gone. Not until now, when servicing the unit ( initially to replace the diaphragm) it wasn’t realised the flap parts were missing.
How the flap parts got lost, also before my time - unknown. Why they were removed, another mystery. There doesn’t seem to be any information of why the flap should be removed when upgrading from the bimetallic strip. Will adding the flap make a difference? I’d like to think so, but unknown.
I’ll come back and let the forum know, when I source and add the flap :slight_smile:

Like Jochen, I’m bewildered, Mark. Unless the AED has a manual conversion, you cannot tell whether the ‘choke’ is on or off.Otherwise, the AED is fully automatic - no manual inputs are necessary or provided for…

An AED has, crudely, to failure modes; either is does not engage - and the engine runs lean and misbehaves cold. Or it is constantly engaged - and the engine starts misbehaving as it warms up, running too fat. Or anything between the two extremes - but a not uncommon cause of the latter malfunction is clogged hot air pick-up, due to lack of attention…

However, a failed thermostat may delay warm-up - requiring choke for a longer time…

As an aside; converting the AED to manual operation requires that relevant parts of it is functional…

xj6 85 Sov Euirope (UK/NZ)

At a slight tangent, do you know a source of replacement insulation for the exhaust manifold heat transfer pipe that would withstand the temperatures adjacent to the block? Am looking to redo this on mine.

Rock wool, glass fiber, automotive heat shielding… whatever shape you need?

This now begins to make sense. The intake pipe ducts hot air from the exhaust manifold to the AED (that is why it is insulated). The hot air acts on the bi-metallic strip causing the air and fuel valves to open/close thus altering the mixture for cold start. You have the cable and cams conversion so the bimetallic strip has been removed and the temperature of the AED inlet air is immaterial. What is still important is the volume of intake air. I suspect the spring flap was removed in an ill-advised attempt to increase the volume of air for some reason. It is important your M checks the inlet pipe is clear (there is a gauze filter at the manifold end which can become clogged). Correct running can be wrecked by even a small obstruction.

Because you have the cam conversion, the AED is now a manual enrichment device and there is NO automation. Its set-up needs to be in accordance with the instructions of the conversion kit supplier.



I have a friend who’s retired now and in his old premises there a quite a few old AEDs lying about that got replaced back in the day because of the troubles with them.

I have one under my bench that I could take out the parts you are looking for and send them to you for the cost of the postage.

Having said that, if you have the HS8 carbs, I would recommend you convert the carbs to a proper manual choke that lowers the main jets with a normal choke cable,

I did this to my own car and it put and end to a lot of problems.


Waaaay back in the day I worked on a Dodge that wasn’t running right. It came with a shutter that closed the air intake to the air filter while opening an intake from a duct bringing air from around the exhaust manifold. The shutter was actuated by a bimetallic coil recessed into its own little chamber atop the exhaust manifold. I found that the shutter wasn’t moving, it was continuing to feed hot air into the engine no matter how hot the engine was. Took the linkage all apart, made sure everything moved freely, still the shutter wouldn’t open when hot. Come to find out the problem was the bimetallic coil itself: It looked fine, but in reality it was no longer bimetallic; one side had rusted away. It was just a coil, and as such it didn’t move when heated.

Of course, this being a Dodge, I just went down to the auto parts store and bought a new one. Problem solved.

Thanks, Mark,

for the clarification. Unfortunately, I still cannot figure out which flap is missing in your setup, but, as Frankie mentioned, it is essential for troublefree operation to install the manual conversion kit exactly as advised by the original instructions. This relates in particular to the gaps at the cams. We do have the original instructions - there seems to be only one original producer of those kits - somewhere in the archives. Find them and pass them on to your M. He should be able to set your car straight.

Thanks, David, for setting me straight! - I’m still surprised to hear all DS420 were carbed. My impression was that from 1988 on cats were mandatory in Europe as well. So how could Daimler limos survive with carbed engines until 1992 (again I think;-)?



75 XJ6L 4.2 auto (UK spec)

This is a quote from Thorley’s Original Jaguar XJ.

It implies that some 6 cylinder cars had a manual choke from the factory.

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Blockquote That’s very interesting and something I’ve never seen in Jaguar parts catalogues or Jaguar repair manuals. Do you know what models/dates Nigel Thorley was referring to? I assume it was the very early cars.

I think we need to see some photos of the OPs setup before we can give any further advice!


Quote was made in conjunction with S1 V12 manual choke , so assume it related to S1 6’s, but early S2 V12s also had carbs, so may also, or only, relate to S2 cars.

You’re right Frankie; increasing only air would lean out the mixture - defeating the purpose of ‘choke’ action. In practice; there really is no choke at all, it is just an air leak, and the engine will not perform adequately during warm-up. But when hot and the choke pushed fully in; the carbs alone should run the hot engine as they are intended to do…

xj6 85 Sov Europe (UK/NZ)