In the good old days when I hung around the XK8 forum, there was a lot of unhappiness surrounding the temperature gauge design. As I understood it, the engineers at Jaguar had decided that the average Jag owner would be unduly discomfitted by a gauge needle that wandered around a bit as the thermostat opened and closed in the course of its job, and it was agreed that the solution was to make the gauge a sort of 'Idiot Light' which sat still at mid-range, and then suddenly pinned at the upper limit just as the red light came on, and the engine self-destructed. Is this a bit of an exageration, or is the gauge really just a small stepper motor with 3 positions? And, if the latter is the case, why does the needle move smoothly through the bottom half of its range as the engine comes up to temperature, then freeze at mid range, and then (in the case of an overheat) jump to the upper limit? I can watch the temperature of my 'B' wander around a bit as I drive (with its old-school resistance-based temp sender) and the Vantage gauge seems to behave in the same way, but is the XJ8 setup just as deceiving as the XK's?

 

I would say it kind of is the same. The self destruct part might be exaggerated just a bit, but to prove your point I keep a scanner plugged in to watch coolant temps when I am doing test pulls and I have seen the "dummy" gauge be right at the middle line from 180F all the way up to 235F -240ish then start to shoot up above the middle line.

Having the scanner plugged in allowed me to start looking for a good spot to pull over before it was too late. If all I had was the factory gauge, I might be installing my 3rd engine right now.

 

Thanks, JJ

To take this a step further, then, I'm assuming that you have your scanner plugged into the OBD port, and it follows that the ECM must be receiving 'real-time' information from a coolant temperature sensor capable of accurately distinguishing the range between 'normal' and 'cooked' in increments. Then it would seem that the 'gauge' in the instrument cluster is 'dumbing down' that information on purpose??!! This gets even stranger than I had feared.

 

Exactly.

I know I had read on a thread a long time ago that someone installed a more accurate gauge (I think it was on an XK) for the same reasons. Ended up being very effective. For me, I keep that scanner plugged in most of the time and monitor a few things, coolant temp included.

 

I'm guessing also that, as these cars age, the possibility of a catastrophic hose failure (like that damn lower rad hose) increases, and whereas a slow leak at a clamp, or a circulation pump decline would give us a slow temperature rise, a blow-out leak would spike the engine temperature very quickly and we might not be attentive enough to spot it in time. Yikes!

 

The oil pressure gauge in my 2nd-gen Miata is like that... two positions. It comes up when the pressure is sufficient, falls when there's no pressure. You never actually know how much pressure there actually is, and I hate it! For years in my cars so equipped, I've used oil pressure, not coolant temperature, to determine when the car is warm enough for "thrashing," as the pressure is high when it's cold, and remains slightly high for a bit even after you get a normal coolant temp reading.

By the way, the coolant temp gauge on the Miata is similarly dampened, and the word on the forum over there is, if your gauge climbs, find a time machine and shut the car off 20 minutes ago.

 

Do not rely on the instrument cluster gauge to determine coolant temperature. The gauge indication doesn't change between 96 C and 128 C, which is too late to avoid an overheating situation.

Monitor the coolant temperature using a smart phone and the Torque app to connect to the PCM through the OBD port using a Bluetooth interface.

 

Those numbers seem high. Fail safe is usually around 120C (250F)

 

An inexpensive Autool on E bay as I recall 30 some bucks plugged into the OBD port and mounted on the dash will give you accurate temp readings and a multitude of other reading from fuel trims to codes and everything in between.

 

Have been thinking off & on, mostly off, about getting a couple for my cars.

The numbers projected on your windshield, would be a handy option.

 

I have the Jagwrangler Realgauge on my X308, and wish one was made for the X350. I have it set so normal temp (180°F) is one tick mark above half-way. You can see it fluctuating slightly, rising a bit when stopped, dropping back down a bit when at speed, just like a gauge should behave. It also has the piezo alarm set to go off when temp exceeds 220°F (which thankfully has never happened).

 

The Engine Coolant Temperature Sensor connects directly to the Engine Control Module, which then passes along the signal to the Instrument Cluster via the CAN.

I haven't measured an X350, but on an overheating X308 I worked on, the temperature gauge needle remained at the middle position until the coolant reached 239F/112C, then suddenly moved upward toward the hot end of the scale.