9 replies to this topic

[TerrA LX]

i think first you need the specs for the reading you are measuring then set the meter to that.

[Tiny]

TO test the sender pat, You will need to know what Ohms range it runs on.. Mine in the HQ was 0-30 Ohms.. so i set mine ot the lowest Ohm setting i have and touched one test lead to the output terminal.. the other to the base of the sender and then move the arm! she should read 0 at one extreme and 30 at the other ( or your readings of course)..

Move the arm INCREDIBLY slowly and see if you get any 0.00 readings.. that's an open circuit and can/will damage your guage! if its only a tiny little point then i woudlnt be too worried.. if there's many of them then it means the sender is pitted/damaged and will need to be repaired!

Most senders ive ever seen use less than 200 ohms.. so setting it to your 200 setting should be the correct setting to use!

(Long live auto range multimeters!! hehe!)

[rodomo]

Another way to test the fuel guage circuit:

Remove sender from tank.
With the key on put a test light between the wire at the sender and earth.
The guage (on the dash)should slowly rise to full, if so the guage and wiring are good.
Connect the sender back to the wire and hold it as if it were in the tank. Note the guage (on the dash)when the arm is down.
Guage should read empty.
Hold the arm 1/2 of its travel and note the guage.
Guage should read around 1/2
Hold the arm at the top of its travel.
Guess what? Guage should read full.
If the guage doesn't read at any sweep of the arm, the sender is no good.

The winding of fine wire on the sender varies the voltage and the guage reading as the arm moves up and down. What usually happens if the arm breaks the fine wire. These can be rewound.

Edited by rodomo, 27 August 2006 - 10:54 AM.

[Dangerous]

Yella, your multimeter has a number of different ranges, or sensitivities for measuring resitance. You need to select the range which is compatible with what you're trying to measure. The LX gauges work from 10 to 70 ohms, so the 0-200 ohm range on the meter would be fine. You can use the larger (less sensitive) ranges, but you'll lose accuracy in the readings. If the digital display reads "1", it's usually a sign that the resistance (or voltage, if measuring V) is outside the range selected, so you need to go up to the next range. Tiny has said 0 to 30 ohms, which is what an LC/LJ or HK up to HQ series gauge expects to see. LH/LX/UC gauges don't like seeing zero ohms at all, as it will overheat them very quickly. The 0-200 range is also useful for testing globes, earths and fuses, but eg for spark plug leads, you'd need to use the 0 to 20k range.

Rodo, your description of how to test the gauge doesn't make it clear that you also have to earth the body of the sender unit while conducting the test. I'd also add before removing the sender unit, try securely earthing out both the fuel tank, and then the sender unit and seeing if it makes a change in the gauge reading. It usually does for LH/LX series, as their fuel tanks are not earthed too well. It's a quick test, and may find the cause of the problem without having to remove the sender unit.

In general, a known good spare LH/LX fuel tank sender unit makes a good test sender for the temp and oil pressure gauges as well - the 'empty' position corresponds to cold/no oil pressure, and 'full tank' position corresponds to full scale on the temp and oil pressure gauges. Once again, the sender wire is connected to the electrical connector on the sender unit under test, and the body of the sender unit must be securely earthed.

A warning when using multimeters if you're not familiar with them. Measuring resistance and voltage is pretty straightforward and safe, but if you intend to measure current, you usually have to plug the red lead into a different socket, and select the amps scale on the dial. If you do ever measure current, MAKE SURE that you put the red lead back in the voltage/resistance socket before then measuring voltage or resistance. A multimeter when set up to measure current is a virtual dead short circuit (it has to be to allow current to flow through it), so if you then go and put the test leads across a voltage source while the meter is still set up to measure current, you'll short out that voltage, and most probably blow up the most expensive electrical item in the vicinity - and/or burn yourself in the process.

As I said, measuring voltage or resistance with a meter is pretty safe - you can make a mistake and get away with it, with nothing damaged. Measuring current is a bit more risky, and forgetting to set up the meter to measure voltage again WILL damage something.

[Toranavista]

Good thread. Plenty of useful info and a happy endiing to boot