Oil/ignition light explanation

[paulsco]

I have just had a great deal of fun trying to prime my engine for oil pressure.

Anyway to cut a long story short, I hadn’t suspected the mechanical oil pressure gauge because the separate oil/ignition light was not going out ether.
I eventually discovered that the oil pressure pipe had ruptured, squirting oil all over the wiring behind the dash

I have fitted a Denso type small alternator with a wire to the ignition/oil light and also connected the brown/black wire from the oil switch to the circuit. It is a long time ago now, so I cannot remember the exact wiring details.

My car is a November 65 FHC and was never fitted with a 3AW, but I still had assumed that the ignition light was also the oil light; with hindsight very naïve.

Now it is clear that if the ignition light comes on, it (as an oil light) is unlikely to go off until the alternator is producing charge, which will not happen when trying to prime the oil pressure!!.

Could someone please explain in what way the light is it supposed to act as an oil light and if it does, I assume it is not an ignition light as well?

Thanks,

Paul

[mtnjag]

If it’s the same as my 62, and it may not be, it’s only an ignition light in that it lights when you turn the key on. The wire doesn’t go to an ignition/charging circuit source it goes to an oil pressure sensor/ switch below the dist on the oil galley. When you start the car, oil pressure come on and the switch turns the light out. Breaks ground I think. Has nothing to do with charging. Later cars used the 3aw or direct to alternator I think but someone else can confirm how that works. I converted to an alternator and mine now goes to the alternator.

[mgcjag]

Hi Paul just to try to help i went through the wireing diagrams in the big green Jag service manual....others im sure will jump in with additional info.....First wireing diagram for 3.8 cars shows an ignition warning lamp comming straight fron the controle box....so just ignition........The first diagram in the 4.2 section(it does give chassis numbers).. has a 3aw which operates an (ignition warning lamp) so this is controled by the alternator....connected in parallel to this lamp is an oil pressure switch. .....the lamp is lit till oil pressure builds up or until the 3aw operates.......The later S1/ S2 diagrams just have the lamp operated by the 3aw.....
So as per your findings the duel purpose lamp will remain lit untill both the pressure switch and 3aw operate...however under normal operation with the car running the lamp will be lite by a loss of oil pressure or a loss of charge......hope this helps....Steve

[angelw]

Hello Steve,
I have a 1966, S1 2+2 in work for restoration. It has three Oil Pressure Senders; one for the Oil Pressure Gauge, one presumably for the Oil Pressure Warning Light and one presumably for the Ignition Light. It only has a light for the Ignition and no separate Oil Pressure Warning Light. There is definitely no evidence via the original wiring that it ever had a 3AW.

I can see the logic (at a stretch) in the 3AW and Oil Pressure Sender driving the one warning light. But to have two separate Oil Pressure Senders to drive the same Warning Light for Oil Pressure and Ignition, which must be the case with this car, just doesn't make sense. If the Oil Pressure is non existent before the engine is started, then the Ignition Light is on. If the Oil Pressure were to drop when the engine is running, then the Ignition/Oil Pressure Warning Light comes on; both scenarios could be accomplished with the one Oil Pressure Sender.

Regards,

Bill

[mgcjag]

Hi Bill.....yours could possibly be aftermarket addition.....im just thinking one sender for low and one for high?..havent see a Jag diagram that covers 3 senders...but could be......Steve

[paulsco]

Thank you all for your help.

I’m fairly certain (original wiring now renewed) that the 3AW was never fitted and I think when I was doing the wiring I checked the chassis number to make sure.

Just to confirm; I have a mechanical oil pressure gauge connected to the point at the top of the oil filter where the electric sender would normally be and a separate oil pressure sender in one of the holes in the oil galley.

So… the light is both an ignition light and an oil pressure light?

When testing, I tried disconnecting the brown/black wire on the pressure sender (engine not running but ignition on) and the light stayed on. I earthed the wire and the light stayed on??

This clearly is not extinguishing the light no matter what.

I suppose it is possible I have wired it incorrectly, but am I to assume that the idea is once the alternator is charging and turning off the ignition light, then if the oil pressure fails it will come back on again? If this is how it works, I suppose you have to guess which problem you have… until the oil pressure gauge drops to nothing??
Seems very odd

Paul

[mtnjag]

in the black Bentley book and an old Haynes manual there is a wiring diag for 3.8 cars with no 3aw, next diag is on 4.2 models from referenced chassis numbers that shows both the “ign warning light pressure sw & warning light” and a “3aw ign w/l unit” circuit both going to the same circuit. Next to the 3aw circuit it says “(Fitted on later cars)” so it would appear there was a transition on the referenced cars from the pressure sw to the 3aw rather than both at same time. The next diag for following referenced chassis shows only the 3aw.

[mgcjag]

Hi Paul.....yes you last sentance is how it would work if wired as shown in the diagram....however we dont know how you have wired yours.....do you have a wireing diagram....Steve

[mtnjag]

Interesting, so on those cars factory was both the 3aw and the oil pressure sw wired together with the two circuits effected the light independently? So you could have oil pressure drop with good charging and the light would come on?

[mgcjag]

See diagram top center

[mtnjag]

Yep, that's the same one I'm looking at but I was thinking that when it said "later cars" it meant the oil pressure sw circuit was eliminated and was replaced by the 3aw circuit in later cars. I don't have a car of that vintage to look at and I was just postulating on that assumption so I could be wrong. Interesting.

[mgcjag]

Hi Layne.....note the chassis numbers...the next page in the service manual has the next diagram for later cars with just the 3aw and ignition lamp ...no oil pressure switch...Steve

[mtnjag]

Hi Steve,
Yep, saw that too, appears to be a misinterpretation by me. Not an EE so don’t know how that would work and my 3.8 already has enough wires to deal with! Give me mechanicals, at least I can see how they work.
Layne

[paulsco]

Hi Steve,

I have both the Jaguar service manual and the Haynes Manual, which both show the two different wiring diagrams for the Series One 4.2, but both have the 3AW in the circuit.(I know some don't)

The first one however does not show the oil light in the circuit, but the second one does.
I may be misunderstanding them, but my chassis number does not appear to be represented in ether.

The Series Two just has the ignition light and the 3.8 has another different wiring diagram and with the dynamo only a ignition light, not both.

The 3AW is I suppose not relevant to any of this really, as it is just the mechanism for the original alternator to work the ignition light.

With my new alternator I have simply run the wire from the alternator to the bulb and the other side the white wire to the fuse and joined the brown/black to the same point as the alternator wire as in the photo of the wiring diagram you posted.

So as we have established it would appear that mine has both functions.

Thank you all for your input and help,

Paul

[paulsco]

Sorry guys; I notice that much of what I have said had already been discussed

Paul

[mgcjag]

Hi Paul..at the bottom of the diagram i posted above the note says the diagram is for cars upto 1E354582.so should include yours?.....Steve

[angelw]

Steve Wrote:

...yours could possibly be aftermarket addition.....im just thinking one sender for low and one for high?..havent see a Jag diagram that covers 3 senders...but could be.

Hello Steve,
All looks quite original; three Oil Pressure Senders on the RH side of the Block. The owner is rewiring the car and I give assistance when he gets stuck. The old loom had been removed and the new one roughly in place when he asked the question regarding the three Oil Pressure Senders.

The old loom has three wires coming out of a very original looking, cotton braided covered loom, which correspond to the position of the three Oil Pressure Senders. Immediately I suggested that one would be for the Gauge, one for an Oil Pressure Warning Light, and one for the Ignition Light; that would have made sense. However, on inspection, there was no separate Oil Pressure Warning Light.

Regards,

Bill

[paulsco]

Yes, sorry Steve; somehow I completely missed the obvious "to" on the diagram

Bill, the ignition light does not have a sender (at all) or wire on the side of the engine near the filter?

Unless the 2+2 is different, you should only have two wires down there, a brown with black tracer (oil light sender) and a white with brown tracer for the sender for the oil gauge.

What colour is the other wire you are referring to?

Can you post a picture?

Paul

[angelw]

Paul Wrote:

Bill, the ignition light does not have a sender (at all) or wire on the side of the engine near the filter?

Hello Paul,
Following are pictures of the three Oil Pressure Senders on the RH Side of Block. I'll have to wait for the owner to bring the old loom back in to take pictures of that, but there are three wires, each corresponding in position to the three Oil Pressure Senders respectively. Each of these wires emerge from a very original looking, braided cotton covered wiring loom.

Regards,

Bill



Above Oil Filter



Aft of Oil Filter



Aft of Engine Mount

[paulsco]

Interesting??

The one above the filter is the sender for the electric oil pressure gauge, (white/brown wire).

One of the others, I would guess the one nearest the filter would be the one for the ignition/oil light Brown/black) (pressure only works the oil light, or more specifically lack of pressure).

The other one is not original as far as I know; could it be for a separate oil light, i.e. one that works independently of the ignition light??

Be interesting to know what colour wires are attached.

Just thought; one is not for oil temperature is it?

Regards,

Paul