Where is the AW3 flasher relay located on the4,2 Mk10?

It is not above the TR4 near the washer bottle in the front left corner, like on the 420G.

Hi, the AW3 in left front corner of a 420G is not a flasher relay, just looking like one. It is controling the ignition warning light. If you look for the flasher relay i believe it is located behind the instrument panel but I can check it when I return home today.

agree with previous answer…the indicator relay is behind the drop down instrument panel

Oh, yes, that’s right, but istn’t it involved when the red ign light runs at 1/3 power? Seem to temember that on my 430G.
And mow I have the same problem on the 4.2 Mk10😰

The ignition warning relay was introduced with the 420G and perhaps on the last Mk X4,2 acc to manuals so you probably do not need to search for the relay. We need more info:
1.Have you ever seen the light working ok previously?
2. Is the ign light glimmering or full light?
3. What voltage do you have when measuring over the battery reving the engine slightly?

It is a ‘3AW,’, and here’s a link about its operation.

If you go to Coolcats website he has a page detailed to diagnosing the entire charge system on the 4.2MKX/ 420G, (which is identical to same period E-types)…it is a very good resource



bear in mind the original 3AW may have been replaced years ago,
so not looking the same, or located where it should be.

My solution was to remove the the Lucas 11 alternator 43amp, and replace it with a 70A internally regulated Lucas “Universal/Ford/International/Tractor/Jaguar” unit

therefore bypassing the 3AW, and 4TR

these parts are available via Mike, and another company in the UK
Finding an auto electrician that can diagnose the issue will be difficult

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The 3AW warning light assimilator was used in all cars which were equipped with Lucas 11AC alternators. Internally, it’s just a stretch of nichrome wire, powered by an AC feed from one phase of the alternator stator. When the alternator is working, the wire will heat up, causing a bimetallic switch arm to release. This turns off the dash light. It’s not really meant to detect problems in the charging system, but rather to let you know that the key is on, but the engine isn’t running. If it’s bad, the one and only thing that won’t work is the red indicator light in the dash.

There are many problems that can cause the 3AW to fail. Aside from age and vibration, overvoltage is the primary cause. A bad voltage regulator is the first suspect. Another problem is sloppy wiring, bad brushes or a bad alternator relay, which can create an inductive surge by de-powering the rotor. Because the 3AW is always almost always ruined by something else failing, the symptom is often mis-identified as the problem. But bad alternators and regulators cause 3AW failures, never the other way around.


The modern Bosch generator gives 13.8 volts. The red ignition light is on all the time, at about 25% of fullmlight power. Does not vary with rpm.
How do I solve that?

Just to clarify: the red ign light is on at full power with the key turned but the engine not yet started.
Could it be that the TR4, which is still connected, is causing the light to be on at 25% power? The modern Bosch generator has an internal regulator.

take a reading with a multimeter at the battery after it has sat overnight.

It should be read above 12.5 to 12.7V.

With engine running at at least 2000rpm, read the + battery terminal. It should be near 13.8V, if it is, your issue is probably not serious,

If voltage at the battery terminal is not at least 13.2V, your battery wont charge properly.

follow the fault finding procedure on the links I posted before

The connection to a modern Bosch is really simple and you should not have any other voltage relays connected as you have one built in the Bosch alternator. 12v is coming from ignition switch when ign is switched on and goes to ign warning light (about 2W bulb). From other side of ign warning light a cable goes straight to separate terminal on alternator. If connected as above you probably have some issue with your alternator.