Getting some dodgy readings on my fuel gauge...standard stuff, and already replaced the Volt Reg and Fuel Gauge. Been doing more testing testing today, connected the Red Multi Meter wire to brown sender wire , and Black multi meter wire to Ground, to test resistance of the Sender Here is my question. With ignition on - resistance is 34.3 ohms With ignition off - resistance is 36.5 ohms. How would this happen? Does it mean there is some kind of short somewhere else on the bus? Thanks for your help in advance. Jim
This means that somewhere in the loop there is an imperfect ground - probably the point at the front of the bus where you connected the meter is not perfectly grounded. Currents returning from the front of the bus to the rear are probably causing a small voltage drop. A digital ohm meter passes a current through the circuit under test from the test leads, and measures the voltage across the leads. It uses Ohms law - volts=current x resistance. On the 200 ohm range it reads 200 millivolts and tries to source one milliamp of current. It has to assume it is the only source of current. So if there is current flowing from another source somewhere in the circuit you are testing, there will be a voltage developed and your meter will be bamboozled. Ignore the reading with the ignition on. Switch the meter to volts and you will most likely see something on the 200 millivolt range. That is due to current flow in the earth producing a voltage drop. The voltage is about 2.5 millivolts ( the 2.5 ohms difference ..)
Another question now. Just popped out again, this time resistance was "1", which I assume is maximum (broken circuit)? So I moved to different earths and also adjusted connection to brown, but all the same. THEN .... get this, I switch the interior light on, cos it's getting dark, to see if I'd connected something wrong. And ohms returned on the meter! Turned the light off, and ohms went down by 10-20...tried a few times, it jumped up and down by 20 ohms when switching the light on. Is my bus haunted? Does this change things?
All this tells you is that you either have a real open circuit or the circuit has a voltage of more than 0.2 volts across the test points. If turning on the interior light alters the meter reading then it might be you are actually measuring between two points connected by different routes to the battery . And its just showing that one route has a slightly different current induced voltage drop. Basically if its giving you trouble, disconnect the batteri(ies), isolate solar cells before you make measurements to be sure your meter is the only source of current in the circuit you are testing. The brown wire for instance is a battery feed.. The ohm meter doesnt directly read resistance, it reads voltage. In general trying to measure resistance on circuits with power on around them is a waste of time.
Mike - latest update. I disconnected both the Starter Battery and Leisure Battery. The multimeter read "1", so assume that means open circuit. Bummer. I then reconnected SB, got a reading of 60 ohms, but then disconnected the SB again and got a reading of 79 ohms, and slowly increasing. About 1 ohm per 2-3 minutes. After about 10 mins or so, it's at 82 ohms. Thoughts?
If you left the fuel gauge sender wire connected to the fuel gauge, disconnected the battery and measured the resistance, there will be a circuit back through the voltage regulator and through whatever is still powered up in the bus directly from the unswitched battery feed, like the door courtesy light. Things like the stereo memory circuit will have capacitors in them that charge up with the current from the meter so your 1 milliamp from the meter will charge up those capacitors, so the voltage across the probes will increase with time. One way to tell whether this kind of thing is happening is to swap the position of the red and black probes. If you are dealing with just a resistor, the resistance will read the same. External voltages, capacitors, semiconductors all will produce asymmetrical readings. The fuel gauge sender does seem like it has failed...
Many thanks Mike. The brown wire is not actually connected to anything, I removed it from the gauge, so should be "clean". I agree with your last point Interestingly, overnight it has settled on 111 ohms. The tank is almost empty, but I know that is far too high (should be around 75-85 empty I believe). I'm going to watch the multimeter when I fill up later, see what happens.
If you do replace the sender, don't chuck out the old one if it's an original VDO. Once my bus is on the road I'm going to start refurbing old senders and need a few to practise on
It is possible that the wiper is not contacting the track in the sender over its entire range, so for some fill up levels it will not read . The other thing is that if the sender has already been replaced then the 111 ohms may actually be just showing a correct reading for an aftermarket sender. . I dont know what resistance mine has at empty but it definitely reads empty with about 60 miles left, and enters the last quarter with about 160 miles left ; if a full tank is about 260 miles...
Interesting yesterday Mike, I filled up whilst watching the multimeter. 1 - I put 53 litres in, so the tank had more left in it than I thought, it must have had about 10 litres (roughly 2 gallons) 2 - After driving down to the garage, the weird "ignition has been turned on, so I will now show a different reading" kicked in, and the reading was now 43 ohms. Now, by my calculations, that's actually not far out from the 2-3 gallons left in the tank ... I RECKON THIS IS A FLUKE 3 - That said, the multimeter didn't register any change at all until I had added 35ish litres (i.e. the tank now contained 45 litres/10 gallons); and it suddenly started showing readings as follows (some approximations to make reading the info easier), 45 litres (10 gallons) 27 ohms 50 litres (11 gallons) 25 ohms 55 litres (12 gallons) 23 ohms 60 litres (13 gallons) 20 ohms 65 litres (14 gallons) 18 ohms So, from 45 litres (10 gallons) onwards, looks like the sender works. Ish. But, my general conclusion here is that the sender is completely fubar'ed and needs replacing. As you said. I've been putting off cutting an access whole for ages, and wish I'd sone it all when I had the interior out last year. Ho hum. Jim
Its likely whoever had the bus in the past never filled it up much but drove it a lot and wore out the bottom part of the track.. Anyway thanks for the calibration on the top of the tank.. That helps with my slow time analog fuel gauge replacement project using a micro stepper motor and a microcontroller.
I've created a spreadsheet thats uses the ohms readings on a curve graph, and an equation of the exponential line to recreate an accurate Fuel Gauge reading from a given ohms. If you're interested.
Sure. I can't seem to upload it to here, but try this link. Type in the OHMS reading on the Fuel Gauge tab, and it works out the gauge reading. The other tab has the workings, if you're interested. Not saying it's spot on, but seems OK to me. https://1drv.ms/x/s!Al4ZTFLXLKvTgrtdkqP9bzjXCO2H2g Jim
Thanks for that, I am just making a little prototype of a fuel gauge using a modern GM style dashboard stepper motor and a microcontroller. I think the motor easily fits in the space under the scale in the existing fuel gauge.
I am using an STM32. At the moment its one of the STM32F103 Blue Pill boards, which is quite big , but cheap. I am also using ChibiOS in it, doesnt need it for this application, but it makes using the controller emulate a USB serial port with setup and debug commands a lot simpler..
