Drain Filter Light Always On

Fuel Filter Article

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Light stays on all time. No water in fuel. Sensor is good.

AMG Service Bulletin

Here's what I found causing my Drain Filter Light on my 2001 to stay on - even when there was no moisture in the fuel filter.

My problem was inside the instrument cluster OIL/VOLTS module - not water in the fuel nor a faulty sensor as is described in Service Information Bulletin 01-03-01.

The way this circuit works is that if moisture is detected in the fuel filter reservoir, it activates the sensor which then sends a ground to the Amber Light Emitting Diode (LED) in the Oil/Volts module to forward bias that LED and turn it on.

There is also a test signal sent to the Drain Filter LED that illuminates it during first activation of the ignition key turn on. That signal should time out and the light should extinguish after a few seconds.

Making this difficult to isolate was the fact that the electrical schematic diagram of this circuit in the 2001 Maint. manual, page 12-83 is wrong and contains errors and omissions associated with this circuit. There is an isolation diode in this circuit although it is not shown on the schematic diagram. The purpose of this diode is to separate the input from the sensor from that of the test signal so the light doesn't turn on or stay on unless the signal from one or the other is present. To cut to the chase, this diode was shorted causing the light to stay on all the time.

First of all, all of the LEDs of the Oil/Volts Module are shown backwards in this schematic and they won't work the way they are drawn. Missing is the isolation diode that is supposed to allow the test signal to turn on the light and isolate it from the sensor input at pin 7 (from the fuel filter). I have advised AM General of the schematic error but don't know if they will make a revision to the manual.

Obviously, maintenance at this level is generally never done by an owner but it may be of interest to a repair shop or to someone with electronic skills who if they have a similar failure as mine, might want to fix this problem without replacing the whole module.

The light in my truck remained on after the test period because the isolation diode was shorted leaving a ground on the LED cathode all the time.

I fixed my unit by replacing the shorted isolation diode. Just to be safe I also replaced the LED. These parts can be found at any Radio Shack for a cost of about $2.00. Sure beats having to replace the AM General 6011984 Oil/Volts Module at a cost of $365.

But if anyone has this problem, first thing to do is verify if there is moisture in the filter reservoir, then check the moisture sensor by removing the Oil/Volts module from the instr. panel and disconnect connector C54. With the ignition key on, look for continuity to ground at the harness side of connector C54 pin 7. If the LED is on when that pin is not at ground, then you've got a problem in the instrument cluster like I had.

I think the Diode is labeled as D6 on the circuit board. It sits right under the Drain Filter LED bulb. You can use a 1N4003 silicone diode to replace it. Just pay attention to the polarity marking on the diode. It should go in to the circuit board holes with the stripe (cathode) in the same hole as the one that comes out. If you replace the LED, pick a 5 mm yellow or white one rated at 3 Volts DC, 20 ma current. The polarity and way it is inserted to the holes also must be the same as the one coming out. The longer lead is the anode. Also, if you hold it up to the light, you can see a difference inside the dome of the light . The cathode will have a larger appearing mass at the base inside the light. The anode will have a smaller appearing mass attached to it inside. You can orient the LED the same way as the others by inserting it so that they all look the same. I think the circuit board may have a marking on it as to which hole is the anode and which is the cathode.

Again, if you decide to pursue this type of repair, use someone who has good near sighted vision and with the dexterity skills of working with removal of parts from PCBs. You would need a very small wattage soldering iron, some solder sucker or a vacuum pump to remove the parts. Excess heat on these small circuit pads where the parts are mounted can destroy the board making the whole exercise a futile effort.

My Drain Filter Light now works fine. It's a PITA to live with a nuisance light and one that I almost decided to live with. But, after fixing it, felt pretty good to know that it hadn't licked me.