1981 Low-Level Fuel Light

[Hutch C.]

Question: When the key is turned on is the Low-Level Fuel Light illumunated as a means of verifying the bulb is working? Nothing in the owners manual or service manual addresses this. I know the circiut board (in the back of the instruments cluster) is OK since I replace both transistors and bench check it for operation.
Thanks!
Hutch C. hhutchison@ec.rr.com

 

[Fishman]

First... welcome to the Hutch! You should go to the new member forum and introduce yourself sometime.

Second... does the light eventually turn off (if yes, when)?

Brian

 

[theo]

No, the fuel light will only come on when the fuel gets below the preset level - I too wondered if there was a "bulb test" when you turn the ignition on, but the only time you'll see the light come on straight after you turn the key to run is when you already have less than the set amount of juice in the tank.
I had been thinking about trying to modify the board so it did "test" the bulb for you at key-on, but that's as far as i got - the thinking stage! Too much other stuff to do!

 

[cool80]

Hi, I have a question about low fuel light. Mine stays on all the time. Any idea what it could be?


cool80

 

[theo]

Provided your fuel gauge is actually working (ie proving that the sender unit is ok) then chances are you can fix it by replacing the two transistors on the circuit board. Seems this is quite a common problem......

 

[bill81vette]

welcome to CAC,Hutch C., sorry can't help you with you problem but enjoy the forums and be sure to check out the l-81 registry!!!!

 

[81 Corvette]

mine has only come on once. i was low on fuel. it never comes on at start up.

 

[garywcook]

For what it is worth while we're discussing this topic. Here's some additonal information you might not know about the low fuel light. It requires the proper calibration resistor on the rear of the fuel gauge. The fuel gauge may be more or less correct with the wrong resistor but the light won't come on. That typical of a resistor with a value that's too high.

The normal resistor color is red/orange (~85 ohms) or green (~90 ohms). I think RED is the proper value. I found a green one in mine. The light comes on at about 4 gallons remaining. I had an improper value there for a while which was light blue (~125 ohms). The light didn't come on even with an empty tank.

I ordered a new gauge for a 77 and it came with the RED resistor. That should reduce the level to about 2 gallons for the light to come on.

It's easy to mix up the resistors if you are replacing the rear flex circuit. Each gauge has one. I've never seen a document stating what is the proper value for each gauge. They show the resistor without a value. I've figued it out by trial an error and a little research into what other 77-82 gauge clusters look like.

 

[Daryllawman]

Hello and welcome to the Corvette Action Center.....

 

[theo]

Gary, what do these resistors look like? I've had my centre console out quite a few times on my 80 and never seen anything that looks like an additional resistor on the gauges...

The fuel light works now, but only because I built a new board for it! The point at which the light comes on can also be calibrated by the choice of certain resistors in the fuel-light PCB itself. By using a variable resistor for one of these, I've made mine adjustable.

 

[garywcook]

Gary, what do these resistors look like? I've had my centre console out quite a few times on my 80 and never seen anything that looks like an additional resistor on the gauges...

The first time I took mine apart I didn't recognize them as resistors and I'm an engineer! They don't look like a tradition resistor. I thought they were just insulators. There are a lot of older vehicles out that and I bet people have mixed them up over the years.

They are flat ceramic rectangles that connect between two posts. They are color coded for value. From what I've determined, here are the values:

Colors / Measured Values:
Light Blue - 125-126 ohms
Green - 90-91 ohms
Dark Blue - 83 ohms
Red or Orange - 85 ohms
There may be other colors but this is all I've seen so far.


Here's the results of changing or mixing up the resistors:
Volts - Less ohms = higher reading
Gas - Less ohms = lower reading
Temp - Less ohms = lower reading
Oil - Less ohms = higher reading

The Light Blue is used on the Volt gauge. If you use something with less ohms, the voltage reads higher. With one of the other colors here, I was seeing a reading of 18 volts.

Red is used on the temperature guage.

I found Green on the Gas Gauge. I'm not sure this is correct for a 77. I think it should be Red. Less ohms will lower the full reading so I am guessing a Green might be for the 81-82 which has a larger tank. Green causes the low fuel light to come on with slightly over 4 gallons in the tank. I measured this. It should be something closer to 2. I think the Red will fix this.

 

[theo]

Well there you go! I'm an engineer too and I didn't recognise them either! I know exactly what you're talking about now.. good detective skills mate!
I'll be saving this page for future reference. I think you're right about the light coming on at about 2 gallons, I've read that before elsewhere. The resistors in the fuel light's own board are set so the light will come on when the voltage from the tank sender is about 2.4V

So i guess if you make any component changes (sender units etc) you can calibrate your whole gauge pack to a certain degree if you can get a handful of those resistors. I suppose they are ceramic because they are quite low resitance and as such carry a fairly high current....

Come to think of it I might have a pic of the back of my cluster (1980) - so perhaps I can help you by finding out the colours? To the best of my knowledge it's unadulterated and stock...
I'll get back to you.

*edit* that was easier than I thought - have a look at http://www.nottingham.ac.uk/~eaztl1/low_fuel_light_circuit.htm
it's a page I started but never finished about the building and calibrating an adjustable fuel light box. It clearly shows the resistor colours, specially the last photo...

 

[garywcook]

So you're in the UK?

I've moving to Tenerife in the Canary Islands next summer and my 77 Vette is going with me. I've been restoring it to get it in shape for the move. We need to chat later.

It's been a lot of detective work. I mixed them up when I disassembled the console. I could also see that somebody was in there before me. It has to be documented somewhere. I've been through the assembly manual and electrical troubleshooting manual but found nothing.

If you have a picture of yours, we can compare it to what I've found out already. I've found a few pictures of the rear of the clusters on eBay. You never know if they have been put back together properly or what year they are from. I found one supposedly from a 79. It has 83 ohm resistors on all gauge except the volts. That one is 125/126 ohms. Found another from an unknown year. It has 91 ohms on the gas gauge, 125/126 on the volts, and 85 on the other two. Orange and red seem to be the same value and they are very close to dark blue in value.

 

[theo]

If you have a picture of yours, we can compare it to what I've found out already.

Didn't the link above work?? That should take you to some pics of mine.


So, moving to Tenerife?? I would say you'll be spoilt with all that sun, but I guess being from California you get more than enough of that anyway!
Good work for bringing the vette with you, I think I'd be doing the same in your shoes.

 

[garywcook]

Duh. The link worked. Got distracted and didn't update my response.

The green resistor behind the fuel gauge is what I expected. The 1980 has a 24 gallon tank instead of a 17 gallon tank found in the 77 and earlier.

Light Blue on the volts matches every one I've seen. Voltage readings seem accurate with this value.

Yours has a couple of colors I haven't seen. Yellow for the temperature and
and Pink/Purple for the Oil Pressure.

Plenty of sun here in the Los Angeles area so I won't be gaining or loosing anything.

 

[theo]

Yeah, the 80 has a 24 gallon tank - and don't I know it! At uk petrol prices, it costs me nearly £90 to fill it up!

 

[Blitz]

Guage Calibration

I got my '81 as part of an estate in July... so I'm learning about the car as I go. The engine was replaced and only the essentials reconnected (no computer, no 'dummy lights'). When I got it, only the oil pressure and voltage worked. :eek

Water temp was my first priority. It 'comes alive' after the car warms up, but I still don't trust it. ;help I have a shop manual, but havent traced the locations for the sensors.


Is a IR temp gun the best way to calibrate these guages
?
My electric fan is operational when I get 12v to it, but I've not seen it come on -on its own. I thought it would come on w/ a thermo switch or when the AC was on - it doesn't come on w/ the AC. Is that correct?


The previous owner obviously loved the car - but if it required twisting two wires together - you can bet its screwed up now!

Your suggestions are appreciated ! TIA

Blitz