[Sean's Jaguar XJS Tech Pages]
XJS Air Temperature Sensor

Last modified 2005 APR 07 04:40:05 GMT

[Fax from Jaguar]

This is a page from a later Jaguar service manual (probably for dealer repair facilities - note the test box for the ECU) which was faxed to me by a Customer Relationship Representative at Jaguar Cars North America in response to an inquiry about errata for the multi-volume Jaguar Service Manuals, specifically WRT the Air Temperature sensor on the HE being different than that on the Pre-HE, while only the Pre-HE test specs were listed in the Service Manuals (and with absolutely no mention that there are differences in the sensors).

The test procedure shown is for testing it at the ECU (definatley a good idea, provided you have the test box, since this measures the signal actually arriving at the ECU - if it differs from that which is measured directly at the sensor, you have a wiring problem), though one will generally test at the component. The values given in this fax are for the HE version.

So, here's the table for either sensor - the HE from this FAX, and the Pre-HE from the Jaguar Service Manual (p19-21 in my edition). Note that values in bold were computed by myself using the Steinhart-Hart equation with derived coefficients for the two thermocouples, because the Pre-HE lists values for each 10'C from -10 to +50, while the HE (in the above fax) uses different intervals. The HE values provided by Jaguar are a range (+/- 100 ohm), where it can probably be assumed that the middle value is considered a typical reading (having computed the coefficients, I can say for example that the 2400-2600 range for 20'C only reflect a +/- 1'C range -- most likely limiting factors in the methodologies used to TEST your thermocouple will prevent you from measuring the air temperature that accurratley). The Pre-HE values from the Jaguar ROM show a single static value - DO NOT expect that your readings will fall spot-on with this value - just somewhere reasonably NEAR it, possibly +/- 5-10%.

IMO, what you'll mostly be looking for is a linear pattern that closely resembles the values given here - if you do, chances are that if your temperature measurement is slightly different than the reading the resistance value would indicate, it is because your temperature gauge (not the termocouple itself, but the gauge you're using to determine the temperature external to it) isn't fast enough reacting, or has difficulty reading air temperature. For my own tests, I used an infra-red non-contact temperature reading device, aimed down the air horn, which was connected to a Fluke 123 Scopemetre. I got values which were remarkably similar to the figures indicated for given temperatures (having computed out a complete resistance-to-temp table using the Steinhart-Hart equation).

Temp 'C/'F Pre-HE HE
-10/14 960 9100-9300
0/32 640 5700-5900
10/50 435 3764
20/68 300 2400-2600
30/86 210 1698
40/104 150 1100-1300
50/122 180 833
60/140 158 500-700

The HE figures comprise Lucas as well as Marelli ignitions (both 5.3L and 6.0L). This can be easily confirmed by calling your dealership parts counter -- the part number for the HE Air temp sensor (Lucas AND Marelli, including 6.0L), is EAC2863, which they sell for US$60.78. The Pre-HE has a part number of C42167, which is apparently NLA from Jaguar Cars.

BTW, these figures demonstrate that the HE Air Temperature sensor has virtually the same coefficients as the Coolant sensor (computing their temperature/resistance charts show less than a 0.5'C difference across the range).

As I get some time, I hope to collect up some other material I have on the Air Temperature sensor (including if possible, a photo of the pre-HE alongside an HE sensor) and post it here.


Sean B. Straw
Post Box 751224
Petaluma, CA 94975-1224 USA

EMail to: Sean.Straw+Jaguar@mail.professional.org

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