ROW TDI low fuel light - Defender Source
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  #1  
Old September 8th, 2016, 03:41 PM
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ROW TDI low fuel light

My low fuel light on dash is annoying me. I know I can fix this thing. Last result is pulling the bulb, but that feels like such a cop out. I would like it to work properly. Most of the time the light is on, rarely it turns off.

Over 1/2 tank of fuel.

New aftermarket sender, no change between original.

I have the 3 wire sender, 1 wire for low fuel light, 1 for ground, 1 for gauge. Wires are hooked up to correct pins on sender. Ground is good (at sender) as tested with voltmeter.

We tested the sender with a voltmeter and it does increase resistance on that pin when the float is raised.

Power on the low fuel light wire is over 15 volts. That was concerning to me. I do notice at startup the battery light illuminates, then disappears after a quick rev up. Truck has a Magneti Marelli alternator.

Everything else on truck electrical wise works as it should or so it seems. All dash lights appear to function as they should.

I have not tried a new bulb in the dash light.

Any ideas?
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  #2  
Old September 8th, 2016, 04:09 PM
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How many volts coming out of the alternator?
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  #3  
Old September 8th, 2016, 04:10 PM
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If you have a 1991, you should NOT actually be wired to use the sender contact for the low fuel. There is a little electronics module hidden in the loom that turns the light on based on the reading from the gauge sender. It is hidden and taped up and shows nowhere on any manual, parts catalog or wiring diagram but it is there for the 200TDI era trucks. 300TDI get no light...

If you are using the one in the tank, it is simple. There are two contact on the slide. One has variable resistance and the other is a plain wire and they go to a common ground. It is easy to have the wires mixed up and weird things happen.

The strips on the sender do wear out as well.
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  #4  
Old September 8th, 2016, 04:15 PM
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Quote:
Originally Posted by Red90 View Post
If you have a 1991, you should NOT actually be wired to use the sender contact for the low fuel. There is a little electronics module hidden in the loom that turns the light on based on the reading from the gauge sender. It is hidden and taped up and shows nowhere on any manual, parts catalog or wiring diagram but it is there for the 200TDI era trucks. 300TDI get no light...

If you are using the one in the tank, it is simple. There are two contact on the slide. One has variable resistance and the other is a plain wire and they go to a common ground. It is easy to have the wires mixed up and weird things happen.

The strips on the sender do wear out as well.
The sender is brand new. That was my first thought. The sender prior appeared original, and had 3 wires attached. The loom looks original.

The wires are definitely not mixed up, I have them correct.

The wire for the low fuel light is grey with white stripe? at the sender. It is the same color wire as goes into the pin behind the dash bulb.

What really bugs me is that the problem is random. Sometimes the light is off.

------ Follow up post added September 8th, 2016 05:17 PM ------

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Originally Posted by Matt_110 View Post
How many volts coming out of the alternator?
I will test and update.
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Old September 8th, 2016, 04:21 PM
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Haynes manual shows white/slate wire from light to "Fuel Gauge and Transmitter" where it is grounded.
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  #6  
Old September 8th, 2016, 04:37 PM
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My irrational thought is that my alternator is putting out too much voltage for the resistance from sender to put the light out.
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Old September 8th, 2016, 05:00 PM
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Quote:
Originally Posted by st0k3d View Post
My irrational thought is that my alternator is putting out too much voltage for the resistance from sender to put the light out.
That's where I was going, but I could be way off. Worth replacing regulator of you can....IF it is putting out too much.
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  #8  
Old September 8th, 2016, 05:09 PM
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That's where I was going, but I could be way off. Worth replacing regulator of you can....IF it is putting out too much.
$115 for a new Lucas... Hmm
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Old September 8th, 2016, 05:18 PM
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Quote:
Originally Posted by Matt_110 View Post
Haynes manual shows white/slate wire from light to "Fuel Gauge and Transmitter" where it is grounded.
Yes, but the 200TDI era wiring diagram is wrong and not shown anywhere.... They snuck in this module but left the wire to the sender that does nothing. Trust me, I spent a long time figuring out what the hell was setting off that light.

Easy to check how it is wired.

If it is direct wired to the sender:
Disconnect and ground the white/slate wire at the sender. If it is connected to the light, the light will come on instantly.

If you have the low fuel light module:
Ground the gauge wire (green/black). The gauge will go to empty and the light will come on after 10 seconds or something like that.
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  #10  
Old September 8th, 2016, 05:20 PM
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Quote:
Originally Posted by st0k3d View Post
$115 for a new Lucas... Hmm
No. Where are you looking? A complete new OEM alternator is $200. Which alternator do you have?
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Old September 8th, 2016, 05:24 PM
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Quote:
Originally Posted by Red90 View Post
Yes, but the 200TDI era wiring diagram is wrong and not shown anywhere.... They snuck in this module but left the wire to the sender that does nothing. Trust me, I spent a long time figuring out what the hell was setting off that light.

Easy to check how it is wired.

If it is direct wired to the sender:
Disconnect and ground the white/slate wire at the sender. If it is connected to the light, the light will come on instantly.

If you have the low fuel light module:
Ground the gauge wire (green/black). The gauge will go to empty and the light will come on after 10 seconds or something like that.
Damn John, that's some obscure knowledge there, thanks for sharing
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  #12  
Old September 8th, 2016, 05:30 PM
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Quote:
Originally Posted by Red90 View Post
Yes, but the 200TDI era wiring diagram is wrong and not shown anywhere.... They snuck in this module but left the wire to the sender that does nothing. Trust me, I spent a long time figuring out what the hell was setting off that light.

Easy to check how it is wired.

If it is direct wired to the sender:
Disconnect and ground the white/slate wire at the sender. If it is connected to the light, the light will come on instantly.

If you have the low fuel light module:
Ground the gauge wire (green/black). The gauge will go to empty and the light will come on after 10 seconds or something like that.
The low fuel light is illuminated whether the wire it attached at sender or not. It also has voltage through it. How could it not be connected?

I have a continuity tester, I'll see if I can test that tonight.

------ Follow up post added September 8th, 2016 06:32 PM ------

Quote:
Originally Posted by Red90 View Post
No. Where are you looking? A complete new OEM alternator is $200. Which alternator do you have?
LRDirect.

https://www.lrdirect.com/STC234-Alte...keep_https=yes

------ Follow up post added September 8th, 2016 06:34 PM ------

Quote:
Originally Posted by Red90 View Post
No. Where are you looking? A complete new OEM alternator is $200. Which alternator do you have?
I have a Magnetti Marelli installed. Not sure if original.

https://www.lrdirect.com/STC234-Alte...keep_https=yes
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  #13  
Old September 8th, 2016, 05:34 PM
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The attached is what it looks like. Taped in the loom close to the gauges. No part number. Has power, ground, gauge sender wire and low fuel light wire going into it.

If you ever have it die or want one you can adjust, I found these ones: https://www.spiyda.com/magento/index...ht-module.html
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Old September 8th, 2016, 05:50 PM
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I don't doubt what you are saying but I don't think my truck is setup like that. My truck was built 10/1990 model year 91. I'm going to try my continuity tester on the slate wire tonight when I get home. One end at sender other at the harness.
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Old September 8th, 2016, 06:03 PM
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I explained above how to test both styles. Only takes a few seconds.
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Old September 8th, 2016, 06:07 PM
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I explained above how to test both styles. Only takes a few seconds.
My light is already on. It is on whether it is connected to sender or not. Grounding it won't make it light up.
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  #17  
Old September 8th, 2016, 06:10 PM
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Quote:
Originally Posted by st0k3d View Post
The low fuel light is illuminated whether the wire it attached at sender or not. It also has voltage through it. How could it not be connected?

I have a continuity tester, I'll see if I can test that tonight.
The straight low fuel light when connected to the sender directly grounds through the sender. It is very simple. If the light is on with it disconnected, you do not have that style or you have a fault in the wiring. Disconnect the gauge wire. Does the light go out? If so, you have the module. Disconnected is full. Grounded is empty.

Quote:
Originally Posted by st0k3d View Post
Matt suggested that you replace the regulator. Your links are for the entire alternator. A regulator is $10 to $20. They are held on with two bolts and are easy to replace. The part number is RTC5670.
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  #18  
Old September 8th, 2016, 09:51 PM
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Ok I got out to the truck tonight. Start truck, low fuel light on

1. Disconnect low fuel light wire at sender - Light stays on

2. Disconnect ground wire at sender - Light stays on, fuel gauge goes to zero

3. Disconnect fuel gauge wire at sender - Light stays on, fuel gauge goes to zero

4. Swapped bulb with known good bulb. Light stays on no change.

5. Tried my continuity tester. I don't think I was able to make good enough contact to get a signal. (Or grounding somewhere? Not sure if that is what it would do?)

6. Tried test probe with good ground to low fuel light spade connector. No change, light stays on. Touched test probe to fuel gauge wire, and it looked like the gauge went higher then with the proper sender ground wire.

7. I cleaned the ribbon connector on the cluster with contact cleaner and some paper towels. No change

At this point my best guess is I have the standard 3 wire gauge setup, and there is a short somewhere grounding the low fuel light? Maybe it is in the warning cluster itself? Anyone local willing to let me try their warning light cluster for a couple minutes?
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Old September 8th, 2016, 10:58 PM
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Okay. Sorry I was backwards. Ground the gauge wire. It will go to full. See if the light goes out then.
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Old September 8th, 2016, 11:49 PM
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Okay. Sorry I was backwards. Ground the gauge wire. It will go to full. See if the light goes out then.
Gauge goes to full, low fuel light stays on.
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