I have been using an OBDII Bluetooth device, and watching temp/MAF/intake temps. It is interesting. Only limited data comes from our early OBD-compliant ECUs. Some of the data is weird which I suspect is a mismatch between 1996 Jag electronics and 2018 software. For example, as I have posted elsewhere, the MAF reading in my XJR max’s out at about 2.25g/sec around 5000rpm at full throttle (eg: 150km/h in 3rd gear manual, full throttle) on the OBD app (Auto Doctor pro, iPhone version). The XJR’s MAF max is theoretically 284 g/sec. go figure....

Anyway, to the title of the thread. Assuming the accuracy of the OBD temp readings and comparing with the dashboard temp gauge which I have often read referred to as a dummy gauge, I have noted the following:
As the car warms up, it moves through the blue region at the cold end of the gauge between 51-54/55’C, then in 1’C increments up to the lower left hand edge of “N”, then sits rock steady on that edge of N from 60’C to about 93’C . So it is ‘dummy’ between 60-93’C as far as I can tell.
However in my car, the presence of an OBD device seems to mess with the dashboard temp gauge a bit. The last 3/4 mile of my drive home, I ascend about 120 vertical meters, and it is hot summer. Several times my car has been fine, sitting on 93’C, but several times it has also heated up to 100’C by the time I get home, in which case the gauge goes beyond N. This has never happened prior to fitting the OBD reader. So there must be some sort of interaction between the ECU and OBD device that causes this, or else I need a new radiator...

At some point I will try it in my 3.2 Daimler which is sitting forlornly under a tree wondering why it hasn’t been for a blast for 3 weeks.

Any thoughts?

 

Mine was a dummy gauge from 70ºC to at least 95ºC




Having struggled with OBD II weirdness on my X300/XJR, I'd say both seem plausible. However, if there's an interaction I would expect it to be consistent, not occasional.

OTOH, in my XJR tribulations many things happened that I wouldn't have expected

Cheers
DD

 

There are 2 temp sensor sensors on the AJ16 engine next to each other above the water pump . the single wire sensor runs the gauge and the 2 wire thermistor runs the ECU . When I ran the ELM327 the thermistor ran smooth up to the operating point

You can double check the readings against a infrared sensor that are cheap now to know if you have a real problem with the fluid temp or if it's a indication problem .

I had a temp sensor code as I received the car and went away after I cleaned the connector

You can remove the sensor and calibrate it with ice water 0 c and boiling water 100 c

The MAF should be around 4.5 to 5 at idle . Andy mentioned that ELM327's have a problem reading this PID . I hand calculated the same figure at full throttle at the ECU limited 5950 RPM

The oil temp is the dummy indicator and can be modified .

See page 77

http://www.jagrepair.com/images/Trai...20-%202000.pdf

 

I think you meant to say 'oil pressure'

As I recall the very earliest X300s had a smart oil pressure gauge.

The coolant temp gauge is only semi-dummy

The service literature says ".....the temperature gauge is designed to give a fixed reading over a wide range of coolant temperatures". I've never known or discovered at what point the needle is supposed to start swing over towards the "H"

Cheers
DD

 

Yes I was incorrect on the oil pressure . Mine is a 96 # 755xxx and both the needle in the instrument cluster for that sensor and the ECU ( observed by the ELM327 ) sensors run smoothly through the range of temps . Never saw mine go to max

 

Roughly at 40C it will start moving up towards N and it will start going towards H from just below 110C.

 

When the thermostat stuck closed on our X308, the temperature gauge suddenly went from N to H in only a minute or two, with no prior warning. After I let the engine cool down and checked the coolant level (fine), I decided to (carefully) test the temperature gauge. I connected the ELM327 I keep in the glove box and watched the coolant temperature rise to 239.6F/115C before the needle moved off of N and began to rise toward H. In my book, that makes the temperature gauge virtually worthless.

Our member White XKR, a.k.a. The Jag Wrangler, offers a kit for the X100/X308, called RealGauge, that eliminates the "dead-center" of the gauge and causes it to read in a continuous analog to actual coolant temperature. I was the lucky winner of a kit offered in a giveaway on this forum, and I intend to install it as soon as work/life permit. As far as I know, he hasn't developed a kit for the X300, but perhaps if sufficient interest was expressed he could be persuaded to do so, if it's technically possible.

RealGauge XK8/XJ8 by TheJagWrangler

Cheers,

Don

 

The only way to keep an accurate eye on this engine is aftermarket gauges, VDO, Smith, Auto Meter, etc. The only question is where to mount them inside the cabin without making it look like a airplane cockpit.

 

I have been reading this thread with interest after noticing a similar behaviour of my temp gauge on my 95 X300. I first started the investigation after interestingly both my fans kicked in after I switched the ignition off after a short drive home in moderate heat. This has happened a couple of times now and I have been keeping an eye on the reported engine temperate with my OBDII reader.
Generally, when it happens the engine temp is reported as about 80 to 100C, Interesting though if I turn the ignition back on, while it's doing the self-test the gauge appears to give a momentary accurate indication with the needle sitting just below the H. After the self-test it drops back down to just below the N as usual.
I suppose the real question is at what temperature are the fans supposed to start running? At what temperature would you consider the engine to be over heading? Is the reported behaviour a fault or just a characteristic of our Jag?