Low Fuel warning circuit board--

[cesully]

Anyone know how I can get ahold of the Low Fuel warning light printed circuit board that fits in behind the guages and it also hold the bulb? Mine stays ON even with a full tank of gas.

thanks for any input.

Sully

 

[Mr.Cool]

Ecklers, Riks , Mid America they all sell the printed circuit

 

[cesully]

Thanks MC, my Eckler's C3 mag is not showing it. I'll try the others...

Sully

 

[Mr.Cool]

It's on page 66 of my Eklers catalog #203. Try looking up part #25067 at there website.

 

[theo]

I think you'll find those suppliers will do the main flexible board OK, but that little add-on fuel light board is no longer available.

I'm fairly sure about that, since i found mine was competely missing when i bought my vette. I think the only way to go is a second hand board (don't ask me where from, sorry!) Or to repair the one that's there. I built my own new one from scratch, cost me a matter of pennies. I'd never done an electronics project before, so it's not too taxing - I found a website giving details on how to do it. If you like I could outline the procedure for you.....
(Or try and dig up the web address)

To be honest, if you still have the board, it would probably be better to repair it, as this would naturally be quicker than starting from scratch!

I'd be glad to help you out.

 

[dmrodco]

Let us know if you find that sight ......

 

[theo]

Yeah, I thought I'd get in quick with a reply to this question while it was near the top - but it's gonna be the weekend before i get the chance to post any more info on it - busy with work at the moment.
When the time comes that I can take a while to use the PC for "non-work" stuff, I'll sort you guys out. (Don't you hate the way work gets in the way of good honest slacking off!)

 

[cesully]

Good call- Theo

Theo,
That is why I have not been able to find it in the mags...
I have been pondering fixing the cir brd myself, however its been a long time since I was fixing componant level. I would appreciated any help/assistance you can provide...
Look forward to hearing from you...

Regards,

Sully

 

[cesully]

MrCool....

I looked up that P/N in Ecklers and at $119 I'm glad my guage printed circuit is ok.

As Theo pointed out, I believe my problem is with the small circuit board attached to the bulb that twists onto the "guage" printed circuit you mentioned in your reply- MC.

Hopefully we'll get an answer from Theo on this as I suspect that a lot of C3 owners have this problem.

Sully

 

[theo]

No worries mate,
I don't think it will prove to be a problem for you, if I could do it with no prior electronics experience, then anyone can.

Hey, it might even be a dodgy connection!
If you don't mind waiting til the weekend when i can let my ar$e touch the ground for two minutes, then we'll sort it, and yes, I bet a lot of folks have this problem...

Seems the electrical gremlins are the problem with the "fully loaded", later c3s - twenty + years is a long life for electrical/electronic stuff, i guess. Someone ought to write an article on things like fuel light/delay wipers/rear defog and all these things that are getting past their sell-by date!

 

[Mr.Cool]

That's interesting when I got my 82 there was no bulb or circuit board in place for the low fuel. But now that I think about it I found something that looked like that lying in the rear compartment all busted up.

 

[cesully]

take your time theo-- we'll talk this w/e...

MC- I'll bet that that busted up thing was you low fuel waring bulb/circuit board... I sure there are some around at a swap meet.

as for me, I have a 1978 I recently bought and I remembered back 10 yrs ago my 1979 low fuel light would come on at a certain point. I checked my 1978 and realized that the board was there but no bulb. Replaced bulb and its stays on all the time.

Hopefully with theo's help- this will be temporary...

see ya,

Sully

 

[theo]

Right, here's a diagram of the circuit layout first of all, so you can see what i'm on about - If I manage to get it to attach ok, then I'll go on to try and describe how it works.

Here goes.....

Right had a look at my notes, and the values of R1 and R2 should be doubled - they should be 3M and 1M, respectively.

 

[theo]

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!

 

[cesully]

theo... a man of your words..

Thanks a lot theo,

This is very valuable info... can't wait to get started...

I WILL keep you posted...

thanks again

Sully

 

[cesully]

theo,
no insult was taken for the detailed instructions. I has been a long time as I said since getting into componant level repair. Everything is digital, solidstate IC chips. What you've described is a lost art. Glad there is still folks like you out there.

Anyway I believe my fuel guage IS working properly. When I start the car the warning light is OFF and after a few seconds it turns ON and stays that way. I came to a similar conclusion you did after reading your post that MAYBE voltage is leaking through the PNP and illuminated the bulb because the transistor is so old it's just not working properly.

I think I'm not going to chance replacing that PNP and just replace both x-istors and make sure the resistors are working and replace if need be as I do not want to get everything back and have another componant fail later on...

thanks much,

Sully

 

[theo]

Hey Sully,

So you've done this sort of thing before? Well I expect you had to smile when being told how the transistors worked!!

I feel like a bit of a young upstart now!

But I'm glad to help, mate - When I found that schematic and tried to build my own board, I was so frustrated trying to get it to work.... Without the help of other people from the forums I'd still be swearing at it. So it's good to be able to share what I've learned. (By the way, the reason I had such a nightmare with my home-made board was that I'd wired the NPN the wrong way round - It only took me three weeks of cussing to figure that one out!)

So do you think a fault in the PNP is the likely reason? Looking at the circuit layout it seems to be the only logical explanation, being as current HAS to go thru here to get to the bulb.......

Might as well replace as much as poss - it's not going to break the bank, is it?! Might be interesting to do one component at a time to see if it's down to individual or several bits?

Apparently, you can replace both transistors with "Darlington" transistors, and get the board to switch on the light instantly, rather than bleeding it on slowly, as standard. This might be an idea if the dimly glowing light gets on your nerves.

Anyway, good luck with it. My next project is to fathom out the intermittent wiper controller, and see if I can fix that. Might take a little more swearing than the fuel light did!