Panhandleslim65

When I get down to half a tank the light starts coming On and Off. I used a timer and it's On 13 seconds then Off 13 seconds then back On 13. It does this as long as power is on. Is there something on the low fuel circuit board that could be doing this? I haven't had time or warm weather to pull panel to check for solder joints.
It just started about 2 weeks ago. A month and half ago I pulled the instrument panel trim to install a new radio. Could there be something in the trim where the connector attaches that I could have damaged or could it be the sending unit in the tank. Any other ideas that some of you have had the same issue?

j_martin

All that's in the circuit is the low fuel lamp, a wire going to the fuel tank to a low fuel switch inside the tank that grounds it. Power is supplied inside the message center to the lamp.

Best guess is that the switch is fuddled in the tank. Second is that there is an intermittent wiring fault to ground.

There are no fancy electronics involved at all, unless there's some in the fuel tank.

I'm assuming your fuel gauge is fairly normal. The low fuel switch is independent of it, and should close at around the 6 gallon level.

Panhandleslim65

Fuel gauge reads normal as always. I have looked at the wiring diagram in the factory shop manual and the circuit looks like there may be a relay on the back side of the low fuel circuit board. When I get a chance I will pull panel and check connections on board before I pull tank and check there. I haven't been able to locate a sending unit in case that's where the problem is at.

Panhandleslim65

Now that it is down to 1/4 tank on the guage, the light stays off. Will see what happens when it gets down to 1/8 tank. The guage has never been accurate. 100 miles on each of the first 3 quarters and then 200-250 on the last and still have 4 gal remaining.
According to the wiring diagram there isn't a seperate sensor. It's built into the sending unit. The top 2 connections are for the guage (one to ground and one to the guage) and the bottom is for the low fuel light. Apparently when the level gets down to a certain point there is something on the circuit board that is triggered by the resistance change at that point or there is part of the wiper on the arm that closes the circuit at the low point on the sweep.
I have found the part number and the sender is available for about $55. PN 52017992.

Panhandleslim65

I pulled the panel last night. There is a circuit board that turns the fuel light on. It's what the FSM calls a relay. Attached are pictures of it. I found the PN for it too and they are still available if the sender doesn't fix the problem. Also is a picture of part of the sender circuit from the tank to the connetor for the message center. I forgot to get a picture of the rest of the circuit. Terminal E on the sender is the sense for the fuel light.

j_martin

I'd examine each of those solder connections, both top and bottom, with high magnification. I personally use a lab microscope. If you see a ring around the soldered pin, it is a cold solder connection. Re-solder it, preferably with a good small soldering pencil and 63/37 eutectic rosin (or resin) core solder.

If you wind up replacing it, please send me the old one. I'd like to play with it. No doubt I could figure it out and fix it in short order. Setting up a bench test jig would be easy.

Panhandleslim65

I looked at it last night with a 10x magnifierwith a bright light and couldn't see any cracks. It's either sensing a bogus resistance from the sender or the IC in the corner has a problem. I looked up the numbers on the chips and they don't cross reference to anything I could find online. The chip standing up next to the capacitors seems to check good with an ohm meter, but not knowing exactly how it operates can't tell for sure. I figure it is used to control the voltage to the rest of the board since 12V ends up at one of the pins. I need to check resistance from the connector in the truck back to the sending unit to see what reading it has.

Panhandleslim65

Checked the resistance from the connector back to the sending unit to ground and had an open. Apparently there is an open in the sending unit circuit that operates the low light or the wire that connects to the sender is loose or broken. Apparently that when there is an open the relay board flashes the low fuel light at a preset interval (13 seconds). When I get into the tank I will update findings. Will probably be a few weeks since it is cold here and I have to work in the driveway. Also I drive this truck most of the time.