Jaguar Repair: temperature gauge problem, metal probes, negative probe

Question
QUESTION: Hi Howard,

Following up on your email from earlier in the week. I got a mutli-meter and will try taking some readings. I'm not that technical and so would appreciate some guidance from you in how to take them.
When taking temperature readings at the top of radiator and at sensors, is it simply a case of placing the two metal probes (touching together) against the rail (beside sensors) and against the radiator? If so, I can only get a reading of 25 degrees celcius. I must be doing it wrong.
Also, I'm not fully sure that I have found the temp. sending unit. On top of the rail there are three small wires that plug into what look like three separate hexagonal bolts which are fixed to the rail. Is the centre one the sending unit? and How do you take the ohms reading of the unit using the meter?
Finally, when taking a voltage reading between engine and battery, is it simply a case of placing the negative probe end against the (-) battery post and placing the positive probe end against any part of the engine? Sorry if there are too many questions. Your help is much appreciated.
Frank

ANSWER: Frank, You have a multi meter that can read temperature? Cool!!! I never seen one.
All the sending units on the rail (4 of them) have two terminals on each except one sending unit and it only has one terminal with one wire on that terminal. That is the gauge sending unit. The factory usually puts it in the center of the rail but don't count on that as someone may have placed it in the wrong hole.

The method of testing is to set your multi meter on ohms and connect the two probes together and adjust the needle to 0. Then place one probe on a clean part of the rail or on the brass case of the sending unit and the other probe to the terminal (with the wire off) Write down the readings (if you have a ohm meter section on the meter that requires you to set the scale, set it to X 1K so you can read 1 K ohms (engine cold) Then set the scale to read X 1 ohm and warm the engine up to operating temp and read the meter and it should read about 100 ohms. If you have a digital meter it may not have adjustable settings and do it automatically. Read the instruction sheet that came with your meter. Let me know if your instruction sheet says anything about checking temperature.

Let me know,
Howard


---------- FOLLOW-UP ----------

QUESTION: Hi Howards,
To follow up on the last few emails. Firstly there is a ground strap from temp. rail to engine. The volt reading between the neg (-) terminal and engine is minus (-) 12 milli volts with engine running. With engine switched off the reading was zero. This sound ok?

Next I took the Ohms and temperature readings with the meter as follows; (Not sure if the engine had to be running or not so I took both)
With engine cold and not running the Ohm reading at temp. sending unit was 1.3 k Ohms. With engine hot and not running the reading was 120 Ohm.
With engine running and cold, the reading was 1.2k ohms (but varied) and with engine hot and running the reading was 117 Ohms. This sound ok?
Finally temperature. Yes, the meter has a temp. probe. You simple plug it in and place it against object that you want to measure temp. off. When placed against the temp. rail (betside the sensors) I got approx. 70 degress max. Similiar or slightly less at top of radiator. I took the temp. sending unit out and placed probe in liquid (with engine turned off) and it read 80 degrees max. Not sure if I'm doing this part right. Can you advise and do these figues sound ok? If engine is running cold, any ideas why? Thanks again.
Frank

Answer
Hi Frank,
The ground sounds ok and the ohms reading is a little higher than mine but not that far off. The only other possible thing I can think of for the low temp is that the thermostat is a cold thermostat. Is the AC working on this car? Does the heating system work ok on this car? All of the ser. III cars I have worked on ran very close to 90 deg C. and the rail (which carry all of the sending units ) should read close to the upper radiator readings.
Have you had this car tested for HC/CO? When the readings are low as you have indicated the sending unit for the injection system will also see this low temp and make the ECU lengthen the pulse duration on the injectors and thus give you a rich mixture. What is your outside air temp in your area?
Howard