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Oil Pressure

Dazstap
Snr Member
Posts: 333
Joined: Tue Jan 19, 2016 5:51 pm

Oil Pressure

Postby Dazstap » Sat Jun 04, 2016 11:03 pm

Hello all,

This is more to see if anyone else has experienced this, and if so,what your findings were.

When starting from cold I get around 6 bar of oil pressure. This drops off to around 4 when the engine has warmed up a little. But once the engine is up to temperature it drops to 2. When sitting in traffic it even goes down to 0 and if on an incline the oil light flickers every once in a while.

I have checked the oil level and it is fine. There is no smoke under acceleration or when cold. So would I be right in (hoping) that it could be a faulty sensor/oil pressure switch. The car behaves perfectly otherwise. Well, apart from a few unrelated gremlins.

Many thanks.

Darren
Smiling Sovereign Owner

Starbuck
Snr Member
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Joined: Thu Mar 19, 2015 4:48 am
Location: Melbourne, Australia

Re: Oil Pressure

Postby Starbuck » Sat Jun 04, 2016 11:54 pm

Hi Darren,

The oil pressure sender on these cars (both 6 and V12) is a notoriously unreliable device and has caused more than a few 'heart in mouth' moments for owners over the years. The symptoms you describe are typical of a failed sender unit. To be 100% sure it is a good idea to have your local garage screw in an external gauge and double check there is no underlying oil pressure issue, but it will almost certainly be the sender.

Jaguar eventually gave up on the sender units and the factory replacement is now simply a switch and resistor that gives an all or nothing reading around the centre of the gauge. Many members (myself included) who wanted to keep the gauge working as it was originally intended, have used sender units that are available for earlier Jaguars and used an inline resistor to correct the reading for the '40. In my case I used a 1watt 49K ohm resistor on my V12 and this matches the pressures recorded on my external test gauge perfectly.

Good hunting,

Starbuck
'94 XJ12 (and other toys)

old del boy
Posts: 6874
Joined: Thu Jul 10, 2008 9:29 am
Location: Kent

Re: Oil Pressure

Postby old del boy » Sun Jun 05, 2016 12:01 am

As above really , i've never heard of an xj40 with genuine low oil pressure , except of course the lowering of pressure as the oil reaches working temperature ,

Derek..
'93 4.0L Daimler, solent blue,
'93 4.0L Daimler, Westminster,
'85 2600 Rover sd1 vdp oporto.

abercanadian
Posts: 1500
Joined: Tue Oct 16, 2012 6:50 am
Location: Vancouver Island

Re: Oil Pressure

Postby abercanadian » Sun Jun 05, 2016 1:08 am

I fitted an aftermarket oil pressure sender and ran a few tests with a variable resistor. It turned out I needed to make up an inline 75ohm resistor to calibrate it for the 4 litre engine.

- the later switch/idiot light type sender needs a 150ohm inline resistor to centre the gauge.


Larry
94 XJ40 VDP vin 701729
Oyster Metallic/Doeskin on Aero Alloys
240,000 km/149,000 Miles
Daily Driver

94 XJ40 SOVEREIGN vin 703632
Diamond Blue Metallic/Barley
111,000 km/69,000 Miles
Parts Car

nlcrane
Spain Rep
Posts: 7540
Joined: Sun Feb 07, 2010 7:36 pm
Location: Spain

Re: Oil Pressure

Postby nlcrane » Sun Jun 05, 2016 1:36 pm

As an (ex) process control engineer I hate all or nothing (two state) measurements (switches) there is absolutely nothing like an analogue measurement (live, 0 – 100 %) with all the deviations in between when used with an essential variable such as oil pressure. But as Del boy says these motors of ours are bulletproof as far as oil pressure is concerned and anyway we are all far too conscientious in our care of our respective motors to ever allow a manually taken low oil (level) to slip through our fingers.
ImageImageImage
1994 Sovereign Gold 4.0

Area rep. SPAIN
espana.xj40@googlemail.com

Starbuck
Snr Member
Posts: 665
Joined: Thu Mar 19, 2015 4:48 am
Location: Melbourne, Australia

Re: Oil Pressure

Postby Starbuck » Sun Jun 05, 2016 2:42 pm

Hi Nicrane, Derek and Larry,

You are spot on about the 'sixes' being bullet-proof as regards oil pressure - not only do they tend to run really good oil pressure, the bottom end in general seems to be very long-lived if looked after with regular oil and filter changes. I know of '40s with over 400,000ks with a couple of reconditioned heads during that distance that still use no oil!

The V12 is a very different beast though and is notorious for running very low oil pressure (particularly at hot idle) so I would never recommend running a 12 without a working oil pressure gauge - particularly on the 40/81 where there is no separate circuit and switch for the 'idiot light'. A 6.0 is streets ahead of a 5.3 when new, with around double the hot oil pressure at idle due to running tighter bearing clearances, but neither are exactly world-beaters. In a road car the V12 seems to survive ok despite its low oil pressure at idle, but it is important to make sure the needle heads smartly upwards as soon as the engine revs rise above 1000 rpm and it is for that reason I would always recommend running a working gauge in a 12.

I'm not sure if Darren's car is a six or a 12, but hopefully there is enough info in all of our responses to cover his needs either way.

Best,

Starbuck
'94 XJ12 (and other toys)

Dazstap
Snr Member
Posts: 333
Joined: Tue Jan 19, 2016 5:51 pm

Re: Oil Pressure

Postby Dazstap » Fri Jun 10, 2016 11:04 pm

Thank you to everyone that replied, you put my mind at ease. I shall keep a very close eye on the oil level, but won't panic when the oil light flickers in traffic!
Smiling Sovereign Owner

leo_denmark
Posts: 767
Joined: Sun Sep 11, 2011 6:32 pm
Location: Denmark

Re: Oil Pressure

Postby leo_denmark » Sun Jul 02, 2017 6:49 pm

I have now replaced my old, faulty sender with a C46272 from David Manners. This is supposedly a proper oil pressure measuring device used at older V12 and XK engines and not the on/off thing used for late XJ40 and X300.

I ended up with 90 ohm in series to get the same 6 bar oil pressure cold as I had before the change. I see a small pressure increase when revving it, but not much. I allowed the engine to warm up in idle, and the pressure dropped a bit to around 5.5 bar. I would have expected it to go lower, but a real drive will show if I get same readings as before.
My old sender would drop to zero at pressure below 3.5 bar, and that would not happen after quite some miles when RPM drops

With no resistor in series the needle moved out of the scale.
220 ohm in series resulted in 1 bar indicated at cold idle
Sender unit resistance at 0 bar: 330 ohm, dropping when pressure increases. Hard to do a proper test using a handheld compressed air gun...

I will update after having driven the car.
BR Leo Jensen - Denmark
Image
XJ40 3.2 LHD 1992 (VIN: ...660341). Owned since Nov 9th 2005
XJ-S V12 coupe 1977 (SW53309BW). Owned since April 28th 2016. http://www.xj-s.dk/home/
Citroën 2CV 1975. Owned since Sep 1st 1984
Board member of Jaguar Club of Denmark

nlcrane
Spain Rep
Posts: 7540
Joined: Sun Feb 07, 2010 7:36 pm
Location: Spain

Re: Oil Pressure

Postby nlcrane » Sun Jul 02, 2017 8:27 pm

There are a few variables at play here.
Number 1….The FSD (Full Scale Deflection) of the milliampere or millivolt meter in the instrument pod, in other words the quantity of mA or mV to drive the indicator from 0 – 100 % of scale.
Number 2….Is the range of the actual pressure transmitter, in other words the change in resistance it undergoes when subjected to actual pressures of 0 – 8 Barg. (The range on the dash).
Number 3….The value of resistance (Called a shunt) needed to calibrate / accommodate the current available from the pressure transmitter to the current needed by the meter (FSD) this resistor can either be in series or parallel depending on the meter used.
The function of the resistor is to “sink” any excessive current by bypassing the meter (parallel) or limiting the current flowing to the meter (series)
It is very hard to understand why a company like Jaguar did not get this right at the start and that later a "Jerry built" patch would be needed in an attempt to resolve it.
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1994 Sovereign Gold 4.0

Area rep. SPAIN
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M-e-l-l-o-w
Posts: 1413
Joined: Tue Jun 29, 2010 4:20 pm

Re: Oil Pressure

Postby M-e-l-l-o-w » Tue Jul 04, 2017 3:45 pm

The older XK oil pressure sender with a resistor is the way to go. I did this on my 40 soon after I got the car. I cannot recall the value of the resistor I used.

From what I have been lead to believe it was not Jaguar that messed up with these sender units it was their supplier who got it wrong and they stopped trading! But this could be wrong???

Mellow
2004 (Facelift 05-MY) 4.2 Jaguar S-Type R in Black.

Morocco Red / Magnolia 4.0 Sovereign Auto Vin.708xxx.Insurance write off on Friday 13th March 2015.


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