1985 XJ6 3.4 why three oil pressure switches?

The XJ6 3.4 engine I am about to install into my 1968 Mk2 has what looks like three oil pressure switches plus an oil pressure sensor for the gauge, why three oil pressure switches?

and does anyone know if the XJ6 and Mk2 oil pressure sensor for the gauge are the same?

One oil pressure switch is used to operate a solenoid to place a vacuum on the float bowls to prevent ‘run-on’ when the ignition is switched OFF. (it is the one that is wired electrically ‘backward’ to the normal switch)

The other switch goes to the light on the INSTPK and the other is a transducer for the gauge.

There is yet another switch. I have no idea why there are 3. My 73 4.2 had the two you mention (plus the OP gauge sender) but not three. I fitted an oil temp transducer into one of those 1/2-20 passages opposite each main, but all three in the photo here look like OP switches, none oil temp. PO could have added one for some reason, I suppose.

**
One for oil pressure warning light - switching off the light above 5(?) psi. One for the anti-run on valve - which applies manifold vacuum (from a still turning engine) to the carbs when the ign switch is turned ‘off’. Grounded with oil pressure present - and only used with carbed engines. And one for the fuel safety relay - grounded with oil pressure present. Again only with carbed engines.

That said; not all three were used/fitted on all cars - the ‘run-on’ was only used on carb types, with the ‘run-on’ problems…

Frank
xj6 85 Sov Europe (UK/NZ)
**

1 Like

The Mk2 will have a 60, XJ S3 a 100 psi gauge so the sender is different.

I can look for a parts catalog entry.

Don’t worry about looking the switch up, but could you look up the part number for the oil gallery blanking plugs that go along that oil gallery, not the larger one behind the flywheel, I must download the XJ6 and Mk2 parts books but they look like large files!
So would using an Mk2 sender in an XJ6 boost the reading :wink:

Mine is a carb engine, was a five speed manual, I am using the overdrive Mk2 box, sounds like a typical British Leyland solution, instead of modifying the loom to use one switch they just kept adding switches and sub looms, ended up with a rats nest of wiring!

C2352, 1/2” 20tpi for the plugs in the drillings to the bearings.
I don’t know if it would boost the reading but I would be tempted if it did!

Pat,

don’t worry … it’s all part of the magic!

Oil pressure switch 1 has an earth connection via black wire and triggers the oil pressure warning lamp.

Oil pressure switch 2 has an earth connection via black wire as well and triggers the solenoid anti run-on valve.

Oil pressure transmitter 3 has an earth connector via black wire as well and supplies the oil pressure guage.

Using two OP switches sounds unnecessary, but the idiot light sender is pt. # C.42200, while the anti-run on switch is pt. # C.39370 (= AAU3886). So I guess there should be some difference.

While the carbed SII cars used the anti-run on devices only for 4.2 litre cars destined for CDN/J/USA, they used them for all 3.4 litre engines from 8A.5099 on.

Good luck with your transplant

Jochen

75 XJ6L 4.2 auto (UK spec)

Is it really a black line? The block is the ground.
I‘m sure the run on switch has a lower rating. If it always shut off at light-pressures it wouldn’t work that well.

**
Sure is, Jochen - the idiot light switch is grounded with oil pressure below 5(?) psi - it is really a sensor. The anti-run on switch is grounded with oil pressure, and the fuel safety switch ditto - and they are likely identical; plain on/off switches. But they perform different functions with different connections/feeds…

Ie, the four different switches/sensors requires different wire connections to different places…

Frank
xj6 85 Sov Europe (UK/NZ)
**