Fuel Gauge/Sender Variants

[CJH]

What zener's are coming? 5.6's?

Yep - 5.6V. 'Out for Delivery' today.

[CJH]

Could this be that whilst the part number for the gauge didn't change, its calibration did, using the 2 holes in the rear of the gauge you can alter the endpoints of the needles sweep.

Yes, that's very interesting. But it does raise the possibility that mixing up the gauges and senders (e.g. buying a gauge from a late van and using it in an early van) will mean that it's not accurate without re-calibrating.

So for a warning light to be useful it needs to come on when the sender says the tank is nearly empty, regardless of what the gauge indicates, and in turn that means some adjustment of the warning lamp circuit is required, and maybe a bit of trial and error.

[CJH]

simplifying:

VW1301 value / 2 + 10 = ohms

Ohms * 2 - 20 = VW 1301

That's useful. Why on earth would VW introduce 'Resistance Units' when Ohms are a universally recognised unit of resistance. Would it be to force dealerships to buy the VW1301 tool perhaps?

[bigbadbob76]

I've been thinking about the soft led turn on issue.
This is a far better circuit and should give a more positive transition from off to on, as well as reducing the circuit's effect on the gauge reading.
I've ordered fets and zeners to give it a try.
You've got me hooked Chris.

E D I T- use a 5.6V zener, the fet need a couple of volts to turn on.




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[CJH]

I've ordered fets and zeners to give it a try.
You've got me hooked Chris.

Marvellous, that sounds promising. I looked up the 2N7000. Wikipedia says "The 2N7000 is a widely available and popular part, often recommended as useful and common components to have around for hobbyist use". This hobbyist, however, had never heard of them. Much as I enjoy experimenting, it'll be much more efficient for you to perfect this!

[bigbadbob76]

No probs Chris.
It'll be easier for me to play around with it than try and do it by remote.

[Cobra88]

Hi Guys

Just been looking in Bentley manual

doesn't say Ohms but says VW1301 tool value
pre 85 55 Full 560 Empty
Post 85 50 Full 320 Empty

Might be of use /might not

Rick

[bigbadbob76]

Yeah, cheers Cobra, I've seen those and their converted ohms values somewhere on "the samba" as a quote from Bentley.
I'm still waiting for my components so I'm hanging back for now.
At least I have a spare late gauge now to work with.

[bigbadbob76]

Ok Chris. I got the fets and zeners today.
Had a play with the circuit.
Here's my late gauge with 150 ohm "sender" resistor and no led circuit.



And here's a circuit that works.



The response is still a bit soft but that's no bad thing as you can set it so it starts to come on at say 7.5v and gets brighter as the need for fuel gets more urgent.
The circuit doesn't affect the gauge reading at all.
I've shown the led powered from 10v so that the led brightness doesn't change with ign volts, if you put the lights on for example, but field tests will prove if that's nessesary.

I'll see if I can knock up a little veroboard that will mount on the back of the fuel gauge screws and then I can do some field tests and see if damping is needed.


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[CJH]

Great stuff. I've also received an order of 2N7000s (and some 6V8 Zeners), so I might have a try with this too. I don't have a 47k variable resistor - could I put a 20k variable in series with some fixed value resistors to achieve the same thing? How much of the 47k range are you using?

[bigbadbob76]

I've just done a quick check and you can use the ign+ to drive the LED.
There's no difference in brightness/trigger point between 10v led supply and 15V led supply.
The gauge has a 10V supply anyway so that's fine for that side.

I'd suggest you use a 50k 10 turn trim pot to set the trigger point.
You might find you need quite a bit of the range for early and late senders.
I can only test with my late sender when I get the circuit hooked up to my dash and had it set close to the middle of the range.
Have a play and see how you get on anyway.

[CJH]

Thanks - I will do.

[bigbadbob76]

Here's my "back of the gauge" version.
Pictures speak for themselves.











The final fit has a couple of capacitors across the output of the pot for damping and ignition suppression.






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[CJH]

That looks good - I agree that a gradually brightening LED might be a good thing, provided it doesn't come on too early and is bright when things are getting critical.

Do you know what value your variable resistor is at, and based on that, can you hazard a guess as to where it would be for an early sender? The highest variable resistor I have is 20k, and I'd like to have a stab at copying your circuit. Can you perhaps show where the capacitors are on the diagram, and what values you've used?

[bigbadbob76]

Don't worry about the capacitors yet, They're just experimental.

Vr1 (50K) is set at approx 20K above ground when using my late gauge at 7.5V.
I havn't checked it with my sender yet so can't speculate on where it would sit in a real situation or with an early sender.
But 7.5v triggers the circuit so wherever 7.5v is on your gauge is where it will come on, give or take a chunk of volts depending on where you set VR1.

I'm playing with a Mk3 version using a MC3423 chip that will give even more positive response and delayed triggering to reduce bounce.
No promises on that though.