400SE Fuel Guage Fault Fixing
Discussion
I have found that the fuel guage in the dash is the cause of my problems.
After looking at the sender unit I found out that the fuel guage wire is black/green.
Reaching behind the centre console on the driver's side, I pulled out some loom and there is a very helpful loom plug and socket there. Disconnecting it disconnects all the centre console guages. Aftere eliminating the plug and socket as the cause of the problem (well you can always hope...) I found the same black/green wire going into it. An ohmmeter between this and solid black reads the fuel sender resistance.
So - now I can at least see how much fuel is in there (roughly) with my DVM.
The next problem is getting a new guage. I don't spose anyone has a spare one - working or not? This is not the stock VDO guage but the one with TVR on the bezel. How easy is it to dismantle them and fix any internal problems? (I'm good at electronics, down to 0402 SMT...) Are there any equivalent guages I can use?
After looking at the sender unit I found out that the fuel guage wire is black/green.
Reaching behind the centre console on the driver's side, I pulled out some loom and there is a very helpful loom plug and socket there. Disconnecting it disconnects all the centre console guages. Aftere eliminating the plug and socket as the cause of the problem (well you can always hope...) I found the same black/green wire going into it. An ohmmeter between this and solid black reads the fuel sender resistance.
So - now I can at least see how much fuel is in there (roughly) with my DVM.
The next problem is getting a new guage. I don't spose anyone has a spare one - working or not? This is not the stock VDO guage but the one with TVR on the bezel. How easy is it to dismantle them and fix any internal problems? (I'm good at electronics, down to 0402 SMT...) Are there any equivalent guages I can use?
I'm thinking - the obvious internal electronics for the fuel guage is a resistor from +ve to the sender connection, and a meter from the sender to earth, plus maybe a big capacitor to damp the meter. As the guage is dead, and doesn't supply volts to the sender, I would imagine that the internal supply resistor has gone open circuit. So - I probably need to add a resistor from +ve to the sender connection, which I can do externally, actually I can do it at the multiplug I mentioned before. So - I might not even have to open up the dash!
An alternative might be that there's a constant current reference in the meter, but I think a largish resistor, maybe 10k, will do the trick for now.
-----
Nope - tried that, no dice. I'll just use me DVM temporarily and calibrate it by sticking in a gallon at a time.
Presumably the crimping can be undone gently with a screwdriver, or maybe it would be better to cut it off with a small grinder, and stick it back on afterwards.
An alternative might be that there's a constant current reference in the meter, but I think a largish resistor, maybe 10k, will do the trick for now.
-----
Nope - tried that, no dice. I'll just use me DVM temporarily and calibrate it by sticking in a gallon at a time.
Presumably the crimping can be undone gently with a screwdriver, or maybe it would be better to cut it off with a small grinder, and stick it back on afterwards.
Edited by adam quantrill on Friday 21st July 11:49
adam quantrill said:
I'm thinking - the obvious internal electronics for the fuel guage is a resistor from +ve to the sender connection, and a meter from the sender to earth, plus maybe a big capacitor to damp the meter. As the guage is dead, and doesn't supply volts to the sender, I would imagine that the internal supply resistor has gone open circuit. So - I probably need to add a resistor from +ve to the sender connection, which I can do externally, actually I can do it at the multiplug I mentioned before. So - I might not even have to open up the dash!
An alternative might be that there's a constant current reference in the meter, but I think a largish resistor, maybe 10k, will do the trick for now.
-----
Nope - tried that, no dice. I'll just use me DVM temporarily and calibrate it by sticking in a gallon at a time.
Presumably the crimping can be undone gently with a screwdriver, or maybe it would be better to cut it off with a small grinder, and stick it back on afterwards.
An alternative might be that there's a constant current reference in the meter, but I think a largish resistor, maybe 10k, will do the trick for now.
-----
Nope - tried that, no dice. I'll just use me DVM temporarily and calibrate it by sticking in a gallon at a time.
Presumably the crimping can be undone gently with a screwdriver, or maybe it would be better to cut it off with a small grinder, and stick it back on afterwards.
Edited by adam quantrill on Friday 21st July 11:49
Adam, it is possible to get the crimping undone and put it back. I sprayed all the bezels on my gauges matt black. Can't remeber how they come off but I don't remember it being a problem.
Edited by SEvans on Friday 21st July 13:20
Well - I did an experiment today while filling up. I had the ohmmeter on the sender unit and stuck in 4l at a time.
Starting from pretty empty:
0l = 200 ohms
4l = 168 ohms
8l = 145
12l = 128
.
.
24l = 90
.
32l - 46 ohms
I missed a couple of readings because the sender went a bit crazy and wouln't settle (maybe a duff sender as well) but the general trend is to a lower resistance as the tank fills up. At least I know what to look for over the next few weeks before I get around to fixing the guage...
Starting from pretty empty:
0l = 200 ohms
4l = 168 ohms
8l = 145
12l = 128
.
.
24l = 90
.
32l - 46 ohms
I missed a couple of readings because the sender went a bit crazy and wouln't settle (maybe a duff sender as well) but the general trend is to a lower resistance as the tank fills up. At least I know what to look for over the next few weeks before I get around to fixing the guage...
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