Ok, that seems to have worked, so I’ll try and describe how the circuit works first. By the way, if any of what I say is obvious to you, then I’m sorry, but I suppose it’s better to have too much info in this case, and save having to ask questions later! But I must admit, I haven’t had a great deal of practical experience with electronics before, and fixing my board was the first project of this kind I’d done, so if anyone’s got any “corrections” then I won’t mind being put right. I found out the following from contact with other forum members, (cheers - you know who you are!) and it’s not hard to grasp. If much of this info is more than you need, then I hope others will get something from it too….
So you’ll notice the board has got three terminals on it. One is the 12v feed that becomes live when the key is in run. Then there’s a connection to the fuel sender, this just jumps onto the back of the same feed that tells the gauge what to read. Then finally there’s an earth/0V.
I’ve put question marks by R1 and R2 in the drawing, ‘cos I can’t remember the exact values (remember my board was completely missing, and I made one from scratch – I got the resistor values from a website that is no longer working, unfortunately) The design I’ve shown is exactly the same in layout as the GM one, just not certain about those two resistance values – they’re not far off though. If when checking your board you need to find out which one’s which, there are loads of sites on the web that let you de-code the colour bands on resistors, into their resistance values.
How the transistors work:
With the NPN transistor (Q1), current can flow from the collector to the emitter only when current is also flowing from the base to emitter.
It’s the opposite case with the PNP (Q2) – Current can flow from the emitter to the collector, only when current also flows from emitter to base.
The emitter to base current (PNP) and the base to emitter current (NPN) depends on the voltage difference between base and emitter – it’s a good analogy to think of the voltage as fluid pressure, and the current as fluid flow. The flow will be in the direction of the pressure drop.
As the tank sender moves from full to empty the voltage at the sender terminal will change from about 6V (full) down to under 1V (empty).
The key to the circuit is the value of the two resistors, R1 and R2. The size of R1 and R2 as I’ve drawn them means that, with 12v at the input terminal, there will be 3 volts at the middle of R1 and R2, i.e at the base of Q1. (R1 is three-quarters of the total R1+R2 resistance, so the voltage drop across it will be three quarters of the supply, ie 9V, then a final 3V drop across R2)
So we’ve got 3V at the base of Q1 and a voltage at the emitter that varies between about 1 and 6V, depending on how full your tank is. The current can’t flow from emitter to base at all, even though you might have a higher voltage on the emitter side – that’s the way the transistor works. It can only pass current from base to emitter – but this current can only start to flow from base to emitter when the emitter voltage drops below that 3 volts. In fact, I believe the minimum voltage drop for the transistor to start working is 0.6V, so you actually need 2.4 volts or lower at the sender terminal to allow current to start flowing from base to emitter. As you get lower on juice and the sender voltage dips below 2.4 volts, current can start to flow from base to emitter on Q1, which also means that current can now flow from collector to emitter. So if you look at the PNP, Q2, you can see that that this gives current a path from emitter to base - which switches Q2 and allows current from emitter to collector. And it’s that emitter to collector current which lights the bulb. The bigger the difference between Q1’s base and emitter voltages, the more current can begin to flow through R3, and hence the more current that can flow from emitter to collector in Q1, lighting the bulb progressively brighter. So your light “fades in”. At a certain point the transistors will switch on completely and thereafter light will be fully lit.
The diode is there to limit the voltage into the rest of the circuit and protect the components – since the car’s voltage will rise to 13 or 14 when the alternator’s charging, anything over 12 volts will go across the diode to ground. R3’s purpose is just to limit the current through Q2 and keep the transistor within it’s rated current so it doesn’t fry. The 47uf capacitor is to damp out any voltage fluctuations from the fuel sloshing around in the tank, so the light doesn’t flicker on and off when the fuel is getting close to the turn on level.
With that lecture out of the way, let’s see why your light is constantly on. Does the gauge read ok? If so we can assume the sender’s ok. If the gauge is always reading empty, then the board just “thinks” the tank is empty. But I assume your gauge is good, or you would have mentioned it. Since it’s the easiest thing to do first you might want to pull the board out and check all the tracks and components for obvious damage. You can get to it easily if you take the glovebox inner out.
Unfortunately it might be time to pull the console out to check that bloody awful flexible circuit board – I spent ages getting all the breaks fixed in mine. I’ve attached a circuit diagram of the gauge cluster, so you can see the right wire colours (from an 80 – hopefully still the same)
With the cluster out, as you look at the back of the fuel gauge, the bottom middle terminal is earth, the top right is 12v, and the top left is the sender voltage. Check the continuity of all the tracks with a multimeter. Try to hook the cluster back up to the main plug, while it rests on the shifter console. With the light circuit board removed check for 12v, earth and sender voltage at the ends of the tracks up at the bulb hole. If that’s good, then at least you’ve isolated the problem to the board itself. I’m a bit confused, since you’d expect the light to NOT come on if there was a break somewhere, not exactly sure what can cause permanent lighting! It will only cost pennies to replace the transistors in the board, perhaps the PNP is passing collector current when it shouldn’t be – I honestly don’t know if this is a likelihood, not having had much experience with what happens when transistors die!
The ones I used to build my board were:
PNP, BC327, 620milliwatt rating
NPN, BC338, 625 milliwatt rating.
Since I’m in the uk, you may end up having to get different ones of a similar rating, but they’re a common type, so that won’t be a problem…
So that’s it for the moment – make sure the right voltages / earth is up at the bulb hole, check the board tracks for damage, replace any of the components using the diagram.
Once again, I don’t know how much you’ve done with electronics, so I’m sorry if any of the above comes across as insulting your intelligence – I assure you it’s not the intention.
I hope you get it sorted OK, keep me posted!
