Fuel Issue - AED Under Bonnet Test

Hi, my problem began a week ago. Went to start & no go, fuel pouring out the rear carby overflow pipe.
Gentle tap with small hammer & away she went.
Next day no start & fuel pouring out again, another tap & no good.
Figured I need new float seats, so replaced both “ball” type seats with the tapered “pointy viton " style seats (apparently the only style available now)
Adjusted float levels with the 7/16” drill bit gauge & there was a marked difference in levels between the two types of seats.nearly an 1/8"!
Re-assembled and still no start.
I now suspect the AED auto choke switch.
My question is, how do you test the electrical operation of the switch from under the hood so you hear it “click”
My vague recollection is to disconnect the battery, disconnect the otter switch live wire & reconnect the battery.& …??

My electrical knowledge is poor, so any help appreciated.


The ASC is activated by the Otter switch providing an earth path

therefore you should have 12V coming into the ASC, and earthing the other wire to the block should make the internal solenoid on the ASC “click” on

If in doubt, test with a mutimeter, and run some wires if needed

Dont drop any small ASC parts in the engine bay…they can be very hard to find

There is 2 wires , live and earth , the earth is the one that goes to the water rail , get a wire , place one end on a good earth , and the other end to the AED earth on the water rail , ignition on , the solenoid should click each time you earth it .

Thanks Ian & Tony for the prompt replies.
I’ll give it go in the morning & report back.
Cheers Pierre

Hi Ian, just as an aside, what brand are those silver steel wheels on your white MK 2 ?

If you look at the posts I wrote several weeks ago, you will see I faced the same problem. In my opinion, the reason your car may have started one day & not the next, is this: Sometimes, as in my particular case, the solenoid may operate intermittently until it finally fails completely. I replaced the toggle switch under the dash , and the car started up with the ‘hiss’ clearly audible. The very next day, no hiss, difficult start. Ordered a new solenoid from SNG Barratt, hooked it up, and device has performed flawlessly ever since. As noted in the previous post, I installed the old (used) one 31 years ago, so I really cannot complain.

Thank you for your posts on the AED and Otter switch. I tested the one AED and two Otter switches that I have yesterday and they all worked great despite have been out of use for 45 years or so. :slight_smile:
I will do some additional tests before installing them when I get the 3.4L engine from my 1957 MK VIII back from the machine shop at the end of May and the carburetors hopefully shortly afterwards.


OK, here’s where I’m at.
I ran a wire from the water rail earth wire to earth & turned on ignition as suggested but no “click”.
Ran the same wire to positive side of battery & it “clicked” & while connected tried to start but no go.
Sorry MRCHB, not sure what toggle switch your referring to? Is that a manual choke switch you have installed?

So do I need a new solenoid?
I hate electrics!


You can temporarily push down the plunger on the AED (use a stick as fingers are a tight fit) whilst someone else starts the engine and vacuum should keep things working until the engine is warm. A new solenoid is in the works.


Thanks Gerard, more testing tomorrow.

Finally got back to the MK 2 today.
Took the solenoid out out of the Aux Carb & just hooked the terminals up to the battery & it does operate the disc valve, so the solenoid should be OK.
When it’s in the car it’s not working, & does not fire up, so assume this must be a power feed issue?

do you have a multimeter or test light?..check for the presence of 12V at the solenoid

It is shown to be fused by No8 in the circuit diagram of my car, a MKX, so check the relevant fuse

another method is to run a temporary wire from battery + to the solenoid direct, see if she starts right up

Have a fire extinguisher handy in your assistants’ capable hands, just in case!


Tony, I put a test light to the solenoid to check for 12V - NIL response.
Looked at the 50A fuse carefully as suggested - looked fine.
Checked Brake lights & Fuel Gauge & indicators - neither working.
Ah, must be the fuse.
Took 50A fuse out, its looked good, no burn marks, no rattles, nothing loose.
OK. Let’s just put a new fuse in & see what happens.
Surprise, Surprise, everything comes to life!!! :blush:
These non Lucas fuses leave something to be desired!
I might change out the 25A fuse as well as they were both made by the same supplier.

Appreciate your advice Tony, I’ll fire her up & take it for a run tomorrow weather permitting.

Gerard, your suggestion noted for next time. :face_with_hand_over_mouth:

Let you know how the startup & run goes guys


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Often times, fuses which ‘look’ ok are not. Glad you isolated the cause to the fuse and saved yourself the time, aggravation, and expense of obtaining a new solenoid.

Guys, 1st start was OK & the Aux Carb worked fine.
Kept trying to stall after the Aux Carb cut out, but once up to temperature was fine.
Went for a 20km run with a couple of stops & re-starts along the way & all good.
thanks for your assistance

This has been the situation with my 3.8S for decades. As soon as the Otter switch controlling the aux carb senses some engine temperature it turns off the aux carb. My work around is to have a secondary switch under the dash to turn on the aux carb momentarily when coming to a stop sign or traffic signal and turn it off again once the car is moving. The design deficiency with the aux carb is that it is either on or off, there is no progressive fazing out ability as the engine gains operating temperature.

You can adjust the Otter switch by bending the contacts. Heat water in pan to desired temp and put ohm meter across contacts.
Pat H

Yes, I have thought of this but am reluctant to even attempt to try to remove the small hex head screws that hold it in after decades of being in place. Anyone had problems trying to get them out? Where are you located?

John, can you provide some wiring details how you have this 2nd switch set-up & connected please.

Pat, my live & earth wires to the auz carb just sit loosely on top of the connectors & the bakelite lid screws down over that.
Not sure how bending the contacts makes a difference? more info please