Air temperature sensor wiring

just checked the value of my air temperature sensor which is perfectly fine. While working on this I noticed that one of the cables goes directly to something like a sensor on the fuel tube. (See picture).

It is connected in serial to the air temperature sensor.
Checking my handbook I see that the picture of the air sensor looks more like a relais and has three connectors. (see other pic)

Is the wiring correct going in serial with an other sensor?
What is, from the manual, the sensor (number 298)?


Are you sure, Robert - by wire colours?

Your picture/diagram doesn’t show it…

xj6 85 Sov Europe (UK/NZ)

Hello Frank
there are two things.

  1. Based on the physical wires, it is connected serial. I do assume it is a temperatur sensor for the fuel. Ever seen this?

  2. If I take the layout from the book and search for air temperatur sensor, then I see something having three pins.

And here comes my confusion. Just wanted to double check if the physical wiring is OK an ended up in watching on a diagram having something with three pins. And not mentioning a fuel temperatur sensor.

Does it explain my question?


Based on the diagram it could be the thermo switch which it is connected to but that has four pins.


This thermo/electric fuel temp sensor was used for a few years in certain markets. More common is the thermo/vacuum type.

Unfortunately not every build configuration is documented in every wiring schematic. I feel certain the info is out there to be found…with enough digging. Perhaps as simple as searching the archives under “Electric fuel temp sensor”.


Hello Doug
thanks for this. I was assuming that but like to double check if it is OK that way and also if I can check the fuel sensor. So I will dig for more information.

Just for the record. The fuel temp switch is in line with the air temp sensor. Here in the archive is the solutions:

Thanks to Paul and Doug

Sure, Rob - but does not clarify the 'why’s…:slight_smile:

The air temp sensor measure intake air temp - sent to the ECU - air temp sort of converts air volume to air weight. The correct mixture, proportion air/fuel, is decided by weight - which is otherwise not measured.

The fuel temp sensor, clamped to the rail, has the same purpose - but in previous set-ups it was not used this way. It was a sort of vacuum switch, connected in series (vacuum) between the fuel pressure regulator and manifold vacuum. In this set-up, at some high fuel temp, the switch opened the vacuum line to ambient air.

As the fuel pressure regulator maintains constant fuel pressure difference between rail and manifold, using manifold vacuum input to the regulator; opening to ambient air pressure increased rail fuel pressure increasing fuelling - greatly simplified very primitive. Operating at constant fuel pressure between rail and manifold greatly simplified ECU calculations - the ECU has no information of fuel pressure, it’s just programmed for constant pressure…:slight_smile:

Obviously, measuring both air and fuel temps, density, is a refinement - which seems to be the case here. But instead of separate inputs to routed to the ECU via the fuel temp sensor on one connection (I assume).

How this is done within the fuel temp sensor without revamping the ECU seems not described anywhere. There are no(?) testing procedures for the fuel temp sensor - and while a series connection (resistances) of the two is feasible - it would require some changes to some parts specs. And none are evident in manuals, though a plethora of different ECUs are evident, and may cover it.

Or the fuel temp sensor fitted may contain more subtle connections - given that the air temps sensor specs are unchanged…?

In short, it is a worth while refinement - but I miss a testing procedure to verify that components work to spec. Also, the pressure regulators still require vacuum connections to manifold - and I assume they are there?..:slight_smile:

xj6 85 Sov Europe (UK/NZ)

Hello Frank

thanks for the explanation. What I read in different threads and based on Pauls diagram it is a thermo switch not a thermo resistor. Means it is closed and at around 70°C it opens. With that it simulates cold air and the ECU will acct on this. Because it is in serial with the air temperature sensor. Probably a more primitive way for the fuel overheating issue.

If the 70° are the correct value, then the test procedure is simple. Must be closed at normal temperature. Put it in a pot with water, heat it up while having a thermometer in it. At 70° you should hear a click or measure it. Resistance should increase to infinite (open).

Once my car is back I will test it. It got a complete new painting and now it will get the final polish :slight_smile:


Have had thermal vacuum valve operation and testing on my website for maybe 15 years ?

Click on the blue link on the left of page “Thermal Vacuum valve EAC5086”

There were actually two variations of increasing fuel pressure. EAC 5086 was vacuum operated. Don’t have the part number readily available but the other switch was electrically operated…the one in series with the air temp sensor.

Some cars/rails had no bung on the rail, some had a bung only on the B bank side of the rail…for EAC 5086, some had a bung on the A bank side of the rail for the electric valve, some fuel rails had a bung on A and B banks for one of each switch.

SD Faircloth

will go through it as soon my car is back from final polishing.
Small world. I was ordering in November a injector revision kit from you :slight_smile: Robert

Indeed, Robert - though schematics imply it feeds into the throttle pot, thus informs the ECU…?

There are no vacuum hoses to the temp switch(?)
but the fuel pressure regulators are still vacuum connected to the manifold vacuum - just to dot the 'i’s…:slight_smile:

As you say; ‘primitive’ - though it depends on how
the ECU uses the information via the throttle pot. For all I know, the ECU just adds a like amount of fuel as the simpler original set-up. Which just increased fuel rail pressure with increasing manifold vacuum.

Thanks to you and Faircloth.

xj6 85 Sov Europe (UK/NZ)