Working through some problems on my '89

Hello all and I hope everyone had a wonderful holiday season.

I’m currently working through 3 problems that have recently popped up …

  1. Engine won’t reach operating temperature
  2. Security (remote fob) system stopped working
  3. A/C refrigerant leak

I’ll deal with each one in a separate post so let’s start with #1

Just got back after a 30 minute highway run and the engine coolant temperature gauge never got above app 70 deg C (158 F). Now even though the outside temp was around 40F this still can’t be right. Pulled the car in the garage and checked all over the radiator hoses and thermostat housing with my IR heat gun. Sure enough none of the readings were over 80 deg C.

So it’s got to be the thermostat stuck in the open position … Right ? Just to be sure I’ve ordered the correct gen-u-wine Jaguar thermostat (about $55) so we shall see.

   *****  BIG FAVOR  *****

I’m wondering if anyone with a multimeter would please do a reading on their temperature sensor located on the thermostat housing (the one with the single wire) for me. With the meter set to ohms (horseshoe symbol), just pull off the wire connector and place one of the multimeter probes on the top of the sensor and the other probe anywhere on the lower body of the sensor and read the result. If you could do this with the coolant cold and once again after the engine has reached operating temperature I would REALLY appreciate it.

OK, Grooveman, here is what I found with my 1994 4.0L (and it appears all XJ40s use the same sender). With a cold engine, not operated for a few days, 49° F ambient air temp, 1.495 kohms. With the engine up to operating temp (needle on temp gauge stationary, straight up on the ‘N’ mark on my analog gauge) I am getting 0.000 ohms.

Just for grins I checked the temp sensor (the 2 wire one that feeds the ECU) with the engine fully warm and got 375 ohms, which would seem to be a bit out of spec based on this published data:

14F 9200 ohms
32F 5900 ohms
50F 3700 ohms
68F 2500 ohms
86F 1700 ohms
104F 1180 ohms
122F 840 ohms
140F 600 ohms
158F 435 ohms
176F 325 ohms
193F 250 ohms
212F 190 ohms

I didn’t think to check the sensor when the engine was cold.

Mike … Thanks so much for the information !

First of all I assume that the single wire sensor only provides information to the temperature gauge and the dual wire sensor is what the ECU uses. Is that right ?

I’m glad that you included the temp-ohms chart. I have the same chart and was mistakenly trying to match that up with the single wire sensor readings rather than the dual wire.sensor.

I’ll recheck the cold and warm temp readings on both sensors tomorrow.

That is my understanding. BTW, a quick check shows several vendors list the one-wire sender (DAC2583) at a cost of $5 - 12 for a URO aftermarket unit.

For cool coolant temps, replace the thermostat.
For the fob, replace battery and go through the pairing procedure again.

  1. ALRIGHT ALRIGHT … the Jaguar thermostat fixed my problem of the engine not getting up to temperature. The reason I make a point of a “genuine” Jaguar unit is because I’ve previously replaced the thermostat 3 or 4 times over the years with aftermarket ones and always had that same problem. So it was exciting to see the system finally working properly and definitely worth the extra money.

  2. About a month ago the key fob stopped working. Very inconvenient having to lock and unlock the car with a key and also not have any security system.
    — I had the dealer install the security system in the early 90s. My car is an '89 which would have come with the IR system (with the little receiver dome in the overhead panel). But the dealer installed the later RF system and patched it into the existing central door lock system.
    ---- So the question is the fob not working or is the problem with the receiver ? What is the operating frequency, I believe through my research that it’s 318 mghz which of course is an oddball freq (I think garage doors use that freq ?) Used Jaguar remotes for these years cost around … are you ready for this … $200.
    ----- So how hard would it be to use a modern security system and just figure out the wiring (which should all be right there at the old receiver) ? The most expensive aftermarket systems with ALL the bells and whistles are less than $200. Simple systems that I’d like are ridiculously inexpensive, most less than $50.

Anyone have any experience with something like this ?

Grooveman, circling back to your initial post about this issue:

1989 XJ40 central locking?

Did you locate the fuse you mentioned in that post? And I have not been to a local auto parts store to see if they still have the fob tester, have you?

If it comes to swapping out the entire system I believe armed (pun intended) with the wiring diagrams it would certainly be doable. Here is an example from one of the ‘modern’ systems for sale on eBay:

^ I fitted one of those Lanbo ones to one of my cars that I don’t have a working IR fob and receiver and it was simplicity itself…and dirt cheap…about £15, iirc. Might actually have been cheaper…

I elected to wire it to flash the indicators, which was the hardest part of the installation. I used the indicator wires coming out of the hazard warning light switch.

On most cars the central locking works by either front door lock making a connection to Gnd when the lock unit is set to lock, or unlock, to make the central lock unit (relay) function.
One channel for lock, one for unlock. Diagram C.

Wire the new RF unit to +12v and Gnd, feed either +12v, or Gnd (as required to match the system) into the Orange and Orange/Black wires, connect the Yellow and Yellow/Black to the central locking feed, tap into them…job done.

I see your XJ has a console lock/unlock switch, which would be an easy place to tap into…no removing door cards, assuming the central locking unit isn’t easy to access.

The units come with two fobs but you can pair more to them.

Mike …

Here’s the security module located all the way to the left under the dash, fairly easy to get to …

I called around 4 or 5 car parts stores and no one had a fob tester. Even if they did I believe it will only tell you if is transmitting a signal, not the frequency.

The entire key fob thing is very murky in my mind it’s been so long ago that I’ve dealt with it. The key fob I have is called a Linear “ladybug”.

Screenshot 2024-02-03 at 10.19.45 AM
Believe it or not this remote is for garage doors and operates at 310 mghz.with 8 dip switches. The really weird thing is years ago I purchased the exact same remote and programed in the correct code … and it didn’t work !

So to keep the old system I need to pull out the controller today and see if it has the operating freq. and if in fact it even has dip switches. All very head scratching.

It’s certainly looking like the after market system may be the way to go. When the dealer installed the system they spliced several wires into the central looking module harness so I hoping that wiring goes directly to the security module and doesn’t go through any of the processors.

Is there a part number on the module? Is the single black wire an antenna? Are there really only two other wires going into the module?