Repairing a 3AW unit ?

[Durango2k]

It seems I found the problem why the ignition light does not work:

a) there is no oil pressure switch on the car, all I have is the pressure gauge and sender. And there is no wire for it, so I am pretty sure there never was a pressure switch.

b) the white wire on the bulb holder has zero Volts on the tip. The 3AW switches the earth connection to this bulb, via a BN cable, this is ok, I traced it from the bulb to the 3AW unit.

The white wire cannot be traced back to Fuse F6 where it should end. The wiring loom plan says there is a connection - but where is it ?

I think I should cut the wire behind the bulb holder, solder in a new one to F6 as shown in the manual ?

Then I opened the 3AW. Looks intact. Can I check anything inside it ? When I connect E to ground, and apply 12 V to either other connector, I get a hot coil inside within a second, or the other part becomes warm.

Here's a pic:






Carsten

[cactusman]

It is a sort of thermal relay. When the alternator is running the wire that goes from the alternator to the 3AW unit supplies about 8 volts AC . This warms up the wire inside and the heat then opens a bimetallic contact visible in your snap. one end of this contact is grounded and the other end goes to the ign light. So if the alternator fails the contact closes and on comes the light. The light doubles as an oil pressure warning as the oil pressure switch also connects from the bulb to ground and closes if the pressure fails. You should have a white live feed to one side of the bulb. From the other there is a brown/black to the oil pressure switch and a second brown/black to the WL terminal on the 3AW relay. The centre terminal of the 3AW goes to ground and the other marked AL connects to the AL terminal on the alternator using a brown/yellow wire. This is how the wiring diagram shows things. If you have not got one get a wiring diagram for your particular model. Hope this helps. Testing the 3AW would be by applying about 8 volts from the AL terminal and ground using either AC or DC and making sure there is no connection from WL to ground. If there is the unit is caput.

[Durango2k]

I do have the correct wiring manual- more then one actually :-)

I have now cleaned the contacts, and can thus test it as you said.

However- first of all I will feed a live 12 V from F6 to the bulb holder tip.

I thought that connecting the bimetal to 12 V would make me see it snap, but nothing- and I feared to grill it, so I stopped there.

Someone on a Landrover forum wrote, a standard relay can do the same job- but I believe the standard relay is in fact a digital switch at xxx volts, whilst the 3AW is somewhat analogue...means the bulb can glow, dim, whatever.

Carsten

[cactusman]

A standard relay wont work well as the alternator AL output is AC and car relays like DC which is why Lucas use a thermal relay as the heater in it works equally well on AC as DC. Given the crude nature of the device I would chuck it and replace with an electronic version. Readily available on the net. They presumably rectify the AC to DC and use this DC voltage to control a transistor that then actiivates the warning lamp. You could probably do a similar thing and rectify the AC to DC and then use a relay but I could not vouch for it.

[rolando38]

I got exactly the same problem yesterday also
when I dismantled the 3AW relay, the resistance was gone, so no option than change
and decided to order the "original" (?) part BMK1092J from B..TT

http://www.sngbarratt.com/ProductDetail ... f049321a38

[Heuer]

You would be better off installing one of these: viewtopic.php?t=8930

Simple two wire connection to earth and any ignition switched live plus it gives you information about under/over charging and battery condition. The repro 3AW are very prone to failure.

[Durango2k]

I measured the 3AW today. Has 16.2 Ohms and 0.2 Ohms, so no fault. So decided for now to put it back in service as in 1978 when the car went on the scrapyard.

To finally get the ignition light to work again, I cut the white wire, put in a new feed to the centre tip of thebulb holder, all problems vanished. Could not find the other side of it, so declared it for dead. Testing with the 3AW it's all ok.

Then built the wiring loom for the OD, using 2mm2 wire, and 16 Amp fuse and a relay.

So, everything, and I mean everything, works now in this car. Wheew !

Have to buy a few bulbs, and then fit the 2 large instruments and the dash top. Then seats, and then I can think about fitting a nightmare- the windshield.

Carsten

[cactusman]

Well done....the plan comes together....

[Durango2k]

..after almost 10 yrs, I started dismantling in Jan 2008....

Carsten