Wiring the 3AW

[Wicker]

Greetings,

This is a first time post for me. I have read all posts relative to this topic and I have yet to clearly understand the relationship between the 3AW relay, the alternator, oil pressure switch and the ignition warning lamp. This car is a 1965 E-Type FHC LHD VIN 1E31318 New looms were purchased from SNG Barrett

This car has the ?3AW? relay near the battery of the left side of the car. There is an oil pressure switch on the right side of the engine block. Is this the proper setup for this car? There is a brown/black wire coming out of the loom near the 3AW and the other end of that wire goes to the ignition warning lamp and ties in with a white wire also coming to the lamp. There is no wire coming from the loom to connect to the oil pressure switch. There is no brown/yellow wire coming from the alternator loom to the AL terminal for the alternator and 3AW. What would be the proper way to wire this car? Eliminate the 3aw? Eliminate the oil pressure switch?

Your guidance would be most helpful.

[chrisvine]

This is has a good explanation of the alternator, 4TR and 3AW circuit:

http://www.coolcatcorp.com/faqs/Lucasalternators2.html

[Heuer]

You don't need both the 3AW and the oil pressure switch, just one or the other. The first 4.2 cars had an oil pressure switch to activate the ignition warning. Someone thought it would be clever to use it because if there was oil pressure the engine must be turning. They reasoned that if the fan belt broke the ammeter would deflect hard right and that would give the driver an indication there was a problem. Then it was decided that this was probably not the best idea Jaguar ever had so installed the 3AW relay. This worked as follows - once the alternator was energised and producing sufficient voltage the AW terminal was earthed (inside the alternator) which in turn operated the 3AW relay which in turn changed the normally closed earth contact which illuminated the light to 'open' so extinguishing the warning light. If the alternator stopped generating the earth would be removed from terminal AW which would mean the 3AW relay de-energised and earthed the warning light causing it to glow.

So if you want to use the oil pressure switch (easiest option) simply make up a brown/black extension to go from the brown/black out of the harness and route it around the base of the picture frame and along to the pressure switch. Connect and you should have a warm glowing bulb and sense of satisfaction!

[Wicker]

Thank you

Are you suggesting that I eliminate the 3AW relay all together and not bother with the brown/yellow to AL at the alternator? Just connect the oil pressure switch to the ignition lamp by way of the brown/black wire that's already in the harness. Will that hook up alone turn the ignition lamp off once oil pressure achieved?

[ALAN COCHRANE]

Wicker

Late reply I know but the answer is yes to the question.
You get the added bonus of doing away with the fragile 3AW relay.

I have the same set-up on my car thanks to a later 4.2 wiring loom being fitted.
This took me a lot of head scratching to work out what was going on but I think it's a better option than the 3AW set-up.

I've been doing some trawling on other forums and one has stated that the failure of the 3AW will mean the alternator field excitation being interrupted and no output as a result.

For the life of me I can't see how this can be the case. Perhaps some of our more learned Forumites can enlighten me.

This is the diagram referred to in the post:-

http://roverp5.proboards.com/thread/871 ... ning-relay

It looks identical to the E-Type setup.

Cheers

Alan