Fuel Light Sporadic + Electrical Woes

[Ludigdrums]

Hi all...first time post here. Just got my 81 White Corvette coupe on July 13th and I'm thrilled. I love the car. Already put in a starter motor and a CD/Speakers and that's about it. In putting in the Stereo, I noticed that the car has had some crazy wiring things done to it.

Just to add a little quick history...I think the car was once stolen as it has a state issued VIN #. So, I don't really know what's what as far as that goes.

So, anyhow, for a while my power door locks only locked and wouldn't unlock. After I did the stereo and put things back a little nicer than they were, the locks started unlocking. I was thrilled. To make an already too long story short (sorry everyone), I ended up blowing fuses left and right. I finally tore everything apart, and have solved that problem. I have found 2 grounds so far that weren't hooked up to anything and just hanging out. Plus, I fixed the AC by merely plugging in the electrical connector to the compressor. The windshield washer fluid motor doesn't seem to be working, but I have yet to really try and diagnose that one yet.

As you can see, there have been a lot of weird electrical oddities.

Now to the fuel light. Everything was working fine until I took it for a test run after I fixed the fuses from blowing. I accidentally got a little air on a road that suddenly dips and shortly afterwards I noticed the fuel light come on. I have nearly a full tank of gas. So, now it comes on and goes off randomly and don't really know where to start to try and diagose it. Plus, I was wondering if it in anyway is connected to the ECM as far as restricting performance when it thinks there's not much fuel left.

I thank you in advance for reading all this (I know it's wordy) and I'm looking forward to your responses! THANKS!

- Jeremy
'81 Corvette - New Baby
'87 Monte Carlo LS - Old (But still loved) Baby

 

[Scooter76]

Welcome to the forum. I have'nt been here real long myself. I deffinantly know what you are talking about , doing all those "little" things to your car right after you get it. I just went through that stage with my 76. I've had it about 2 months now & love it. You will find that the people on this forum are very friendly & helpfull (wish I could say that about the other forum that I am on for my F-body.) Anyway, sorry I can't help you with your problem except look for a loose wire. Just wanted to welcome you onboard.

 

[mfendley]

The low fuel light is a little circuit board with the bulb attached that fits directly into the circuit board for all your center guages. I'm guessing that your little airborn fling loosened this circuit, and you need to twist it back into place.

 

[theo]

Pull out the glove box inner, and you should be able to reach round and get at the circuit board for the fuel light. The little board and bulb holder twist out together. Does your guage dip to empty when the light randomly comes on? If so it might not be the circuit board itself - instead it might be being "tricked" into thinking the tank is low, by a problem further back in the sending circuit. How long does it stay on for, when it comes on? I've done quite a bit of work resurrecting my 1980's fuel light, so if you have any more Q's, I'll try and help!

 

[21again]

I've done quite a bit of work resurrecting my 1980's fuel light, so if you have any more Q's, I'll try and help!

My problem is I can't get the light to come on! I've pulled out the circuit board and it looks fine, and also replaced the bulb. I then removed the guage cluster and inspected the large "main" plastic circuit board to be sure there's continunity from the wiring harness, and that's OK too. I've even gone so far as emptying the tank, but it just won't light up. The service manual (wiring diag.) does a bad job in describing the electrical layout of this circuit. Can you explain how the small circuit board knows when to turn on the light? The gas guage is working just fine. :crazy

There is "an online schematic of the C3 low fuel circuit" link in the L81 Vette Registry "Tech Tips" section, but it's a dead link. Anyone have this saved or printed out?

 

[theo]

Oh yeah, it seems that link has died! I've had that one saved in my bookmarks for ages - I wonder where its got to?! Lost in cyberspace.
Not to worry, as i've got quite a lot of other info on the fuel light (i think i've got a printout, maybe an electronic copy, of that very schematic)
Give me a day or two and I'll get you what I have. I've been meaning to host the photos I took when i fixed mine. I found a good design for an adjustable circuit (ie you can set the turn on point of the light) courtesy of Dale - useful fella! It's at http://www.corvettefaq.com/fuellight/
If you havn't already seen it, the corvettefaq site is about the most useful C3 resource i've found to date......
The design above is slighly different to the stock one, but does essentially the same thing. I'll get it together and host all my info, so you can decide what's wrong with yours...

 

[Ludigdrums]

Wow, lot's of great replies so far...thanks!

As far as how long the light stays on is random it seems. Maybe a minute or two, sometimes less. I've also seen it on occasion come on half way or so, like dimly.

Thanks again for the info. I have yet to take another look at it but will shortly!

 

[theo]

The fact that it comes on for a while might be helpful in diagnosing it...

It works directly off the back of the fuel gauge - so when working properly it should only come on when the gauge hits E. Yes, that sounds obvious, but what it means is that next time it comes on, check what your fuel guage is doing - if the gauge has dropped too, then the problem is likely to be further back, i.e the sender. In other words, your light circuit board is working fine, it just "thinks" it should come on, because the gauge has dropped (because of some other problem) and has told the bulb to light!

But if when your light is randomly coming on, the guage stays in the same place, this might suggest the problem is in the circuit board for the light bulb.
Does that make any sense?!

 

[Ludigdrums]

Yes, exactly. The fuel gauge stays put. I have a feeling it's in the circuit because the other day I had everything apart to fix some other electrical problems, and when putting it back together, the connector didn't seat totally properly for the center dash. It ended up bending some of the contacts and almost ripping one off. I put them back to where they were supposed to be and it seems all the guages are working fine. I'd really like to not have to spend over $100 to replace that circuit board. My Monte Carlo has that same stupid circuit board stuff, and it deteriorates after time. Anyhow, What's still confusing is that if the fuel gauge is working correctly why should the light mess up. Maybe, like you said, in the circuit board for the light. But how does the light get the info from the fuel guage?! Could it be from that main circuit board?

 

[theo]

The add-on circuit senses the voltage from the sender. The fuel guage has 12v, 0v(earth) and a variable voltage (6 to 0, i think) from the sender unit. So you'll see three contacts in the back of it. As the fuel level drops the sender voltage drops.
As you've had the cluster out already, I would bite the bullet and pull it out again, as its easier to have everything in front of you. Those flexi circuits are a right swine. If you take it out again, I'd really spend some time fixing up as much as you can now, so that you can put it back and forget it (put in new back light bulbs etc). Pulling the plug out the back really messes up the contacts doesn't it?! Crap design and expensive to replace. I broke and cracked quite a few of those contacts before now, and although it was very fiddly, i managed to resolder them back together, and reinforce the cracks with more solder. You can try sticking them down with double sided tape, but evn with that, i'm wary about taking the plug in and out too much.
As far as the fuel light goes, check the three contacts up around the bulb holder. As you look at the hole from the back, the contact on the left is 12v if i remember rightly. Then the one in the middle at the top is the variable voltage from the sender, then the right hand one is earth. You can double check (my memory is far from perfect!) since the 12v one will follow it's way around the circuit to other places, and go back to the copper contact that corresponds to a coloured wire in the multiplug (cant remember what colour) The earth track will also snake around the ciruit, but go back to a contact that corresponds to a black wire in the plug. The variable voltage track only goes from bulb to sender to plug, in that order. check the continuity of these tracks with a multimeter. If they are broken, you can solder a thin wire across them to bridge any breaks, but be careful not to melt the backing of the circuit, since this insulates everything from the metal cluster. If you do, you may have to pull the whole flexi board off and put insulating tape over it.
If the track for the variable voltage is breaking past the fuel guage, then you can see how the fuel guage might work fine, but now and again, the add-on curcuit is being deprived of that same voltage signal, so it senses 0volts and puts the light on. So check carefully inthe track past where its screwed into the back of the guage. I assume the earth and 12v are ok, other wise the light would never work at all.
If you get all this sorted, and it's still coming on, then the problem is most likely in the electronics, and you will need to get the transistor(s) replaced. Not a tricky job, just as long as you get suitable ones. But at at least you can do that with the cluster back in, thru the glove box. Like i say, get everything done you can possibly do with the cluster while its out, 'cos every time you unplug that multiplug from the back, it's one step closer to shelling out for a new flexi board!