Hi,

I'm new to the Volvo world. I've been fairly active working on my 87 VW Westy but definitely have some things to learn. I recently purchased a 92 740, in really good shape, 122K. With the help of this board, a good friend, and fcpgroton, it's running well. One problem I'm trying to chase down is FI error code 2-4-1, which indicates possible EGR issues (and illuminates the check engine light). The vehicle is running well, so the initial thought was the temperature sender was malfunctioning (Since it is only reporting, not controlling?). The Bentley (for the 240 -- seems like that and this board are all one needs) suggests impedance testing the temp sender. I tested the installed unit by decoupling the bail connector and using my multimeter to test. I'm a bit confused with the results and I think this has to do with several things (one of which may be my misunderstanding of my multimeter). I tested the unit about 15 minutes after I turned the vehicle off. My meter is selected to the setting which measures Ohms, and was in the 200k position. I got a reading of 58. As the temperature cooled, additional readings showed progressively higher values (62, 68.5, 72, 75...) I stopped there. I also have a new temp sender. The reading is 177. This unit has not been installed. The Bentley defines valid ranges of 500-1000 ohms. So, my first question is, how to I interpret my MM readings and is the existing temp sender faulty? Secondly, is the above logic sound? I'd hate to install a new (~180usd) unit if the installed unit is fine.

Thanks in advance

I'm a bit confused with the results and I think this has to do with several things (one of which may be my misunderstanding of my multimeter).

Howdy Klaus,

On a 3 and 1/2 digit meter there are four digits shown on the meter, the last digit being resolved to the next higher integer. For any range, the decimal point will shift left as the measurement decreases. That is pretty much common sense. However, the first position has no leading decimal so the very lowest reading you can get is 0.000. If your meter only shows three digits on the display, the resolution will be 2 and 1/2 digits. Maximum resolution would be 0.00. Some meters suppress leading zeros, some do not. If your meter does, it will do so only when the following digits are whole numbers. That is to say, there will never be a decimal in the left most position.

When probing a circuit or device remember to not touch the metal end of the probe. On live circuits you can be shocked so it is a safety consideration. But on resistance measurements you can throw your readings way off by touching the probe end. When testing a descrete device (such as the temperature sensor you're dealing with) always unplug the device from the circuit. If you back probe a connector in circuit you will be reading the sensor and everything else connected to it.

Always "zero" a meter before measuring resistance. This isn't as critical as it was back when the Simpson 260 (analog meter) was THE meter to have, but it is still good practice. Touch the leads together and watch the display on the DVOM. Does it read zero? Good. If not, press the REL button make it read zero. If the meter reading is way too high or won't stabilize while you hold the probes together, suspect your test leads to be faulty.

To read Ohms on your meter, watch the decimal position relative to the range. At the 200k range, the whole digit readings (with the decimal suppressed) indicate up to 200K Ohms. Say you read 66; that is 66K Ohms. If the reading had been 6.6, that would be 6,600 Ohms. 0.66 indicates 660 Ohms.

At lower readings, move the selector to the next lower range to get a more accurate reading. In the example of 6.6 on the 200K range, moving to the 20K range would cause the display to change to something like 6.575 (6,575 Ohms).

Decrease the range you are using as appropriate until you have an whole integer leftmost in the display. That gives you maximum resolution.

The rightmost digit can always be assumed to be rounded up. Say the actual value of the device being tested is 205.7 Ohms; too high for the 200 range but right at the bottom of the 2K range. The best you can resolve that reading would be 0.206 (206 Ohms).

If the display flashes '0' repeatedly, the range you are using is too low, move up until you have digits showing. I don't know what your maximum range is on your meter. I will assume it is 20M Ohm. In automotive circuits, any reading above 20M (flashing '0' at that range) can be considered an open circuit.

(I have used one cheap meter made in the Far East that did not follow the flashing zeros convention; it read the maximum value of the range and stayed there. That is to say, on the 2K range it read a solidly displayed 1.999. Moving to the 20K range showed the actual value to be much higher than 2,000 Ohms.)

If you use "Auto Range", pay attention to the messages or indicators on the display. Some meters will display the range as tiny digits in one corner or another and some meters have an arrow pointing to the range selector buttons.

The accuracy of the $20 meters available to the DIY at the auto parts chain stores can be considered ball park only. Accurate meters cost more than 20 times $20. But absolute accuracy is not necessary for the things we will be using the meter to do.

The readings you got for the installed sensor are way out and progressing in the wrong direction for a cooling PTC thermistor. It is basically open. You can make that call because the readings are 60 to 75 times the maximum value of the device. Ensure you are reading the meter correctly while measuring the device, then call it dead.

As for the 177 you read on the new sensor; assuming you read 177 Ohms, that may be correct for a cold sensor. Install it in the circuit, bring the operating temperature up to normal, and read it again. If the value falls somewhere between 500-1000 Ohms at operating temperature, you can call it good and move on to the next problem.


--
Mr. Shannon DeWolfe -- (I've taken to using Mr. because my name tends to mislead folks on the WWW. I am a 52 year old fat man