I'm having a hard time finding the correct coolant temp sensor for my 1994 XJS with the AJ6 engine. Everybody is saying "no longer
Available". Any alternate route? Thanks in advance.
Cheers,

 

The sensor for the dash gauge or the ECU? Gauge is showing NLA, but both versions for the ECU are available; part #9: https://www.jaguarclassicparts.com/u...hose-4-0-litre

 

It's actually the gauge sensor that I'm (was?) looking for, but I have not yet verified it bad. In fact, it may just not be the sensor after all. I just conducted a test via applying resistance values in place of the sensor. The values that it took for the gauge to bring the pointer to the three main spots on the gauge scale were quite odd, as follows:

COLD: Only 260 ohms
N Normal, in the middle of the scale: 80 ohms.
HOT at the HOT red mark, top of the scale: 32 ohms

Those resistance values are just too low because the typical values for coolant temp gauges at those spots are:
COLD: 2.2K ohms to 3K ohms
Middle: 650 ohms to 1K ohms
HOT: 236 ohms to 260 ohms

My original sensor reads 4.8K ohms at around 70 F degrees, which is not really that far from the typical readings I see for most cars in my search. However, the purchased sensor that I just received, which turned out to be a sensor for pre-face lift cars and with different thread, reads 680 ohms at 70 F. This is very different from my original sensor's 4.8 K ohms.

Based on the test resistance values that it took the gauge to bring the pointer to read cold, mid and hot, it would appear to me that no typical sensor will be able to generate those unusual resistance values, in which case this may have nothing to do with the sensor and I will have to look elsewhere to find the culprit. Is it, then, the gauge itself? Or, is the excessively low test resistance actually offsetting some existing abnormally high contact resistance somewhere in the coolant temp indicating circuit? Maybe it is those poor contacts that the back of the instrument cluster is so well known for?

Well, maybe it's time to pull that cluster off, for which I'd need some tips as to where specifically look for bad contacts affecting the coolant temp indication. Might as well look at that since the sensors are NLA, unless after reading this here somebody may come up with some other alternate routes to resolve this.

Cheers,

 

If the gauge is reading low then you have a bad connection somewhere, clean the contact at the sender then pull the cluster clean all the contacts on the connectors and grounds to the chassis.

 

You're absolutely right. I sat down the other night to try to understand why my dummy resistors test took such low resistance values in order to produce Cold, "N" and Hot readings on the gauge. This told me that there must've been "additional" (if phantom...) resistance in series to the sensor line, thus lying to the gauge and this could only be contributed by poor connections somewhere, which reminded me of how many times I've read in this forum about the poor connections behind the instrument cluster. In this case the amount of resistance presented by the bad connection(s) seems to be high enough for the sensor resistance drop not to be enough to move the pointer very far, if at all. Well, out comes the cluster.

Thank you, cheers,

 

I'm not familiar with the facelift cluster, but the pre-FL is easy to remove, about a 15min job if you've not done it before.

When you have the cluster out check the screws on the back that hold the gauges in, these also make electrical contact between the circuit board and the gauge, make sure there is no corrosion under the screw and that they are tight.

Check the spade connector that plugs into the sender and make sure it is a firm push on the sender, if it's loose then gently squeeze the connector while it is plugged into the sender to tighten the connection.

 

Facelift clusters are just as easy to come out, just a few screws and some trim pieces.

 

Much appreciated, guys.

Cheers