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Old 10-23-2021, 11:02 PM   #1
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Air Pressure Problem

The Low Air Pressure warning light came on and off a couple times on my 2009 Tiffin Phaeton 36QHS. Then the alarm came on and off along with the light. The secondary air pressure gauge started fluctuating between 45-120. Then the light and alarm came on and stayed on. The brakes worked normally the whole time. I turned the engine off and when I started it back up everything was back to normal. This has happened twice and each time it occurred after driving for about an hour. Anyone had a similar problem and knows how to fix?
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Old 10-24-2021, 05:53 AM   #2
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I believe that is an issue with the VDU unit or something similar to that name. It has been discussed many times that when that unit goes bad it creates that symptom.
Do a search here for VDU . . . Tons of info comes up on it
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Old 10-24-2021, 07:15 AM   #3
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The other source for that type of problem can be a loose ground connection for the dash gauges. That can cause the intermittent drop out as you described.
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Old 10-24-2021, 12:37 PM   #4
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There is page after page regarding similar issues with the Light Bar Control Unit (LBCU) and the twisted pair wiring that connects it and the gauges.

FL says the initial fix is to replace the white wires with the newer black ones. That works sometimes. Others were able to use a good contact cleaner (safe for plastic) and clean the connections.

Search for gauges and LBCU and prepare for lots of reading. I personally fought this and other gauge issues for years, and have now gone to a Fire Tablet velcroed to the dash with a BlueFire dongle plugged into the diag port under the dash. You still need the air pressure gauges though hooked to the LBCU as the data port doesn't send that data out from the ECM.

Best of luck.
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Old 10-24-2021, 12:38 PM   #5
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Quote:
Originally Posted by Ranger Smith View Post
I believe that is an issue with the VDU unit or something similar to that name. It has been discussed many times that when that unit goes bad it creates that symptom.
Do a search here for VDU . . . Tons of info comes up on it
I don't have info on when the VDU was no longer used...but I know that my 2008 and FredT's 2009 Freightliner chassis uses the LBCU and not a VDU.
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Old 10-24-2021, 10:07 PM   #6
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Quote:
Originally Posted by RichKimball View Post
I don't have info on when the VDU was no longer used...but I know that my 2008 and FredT's 2009 Freightliner chassis uses the LBCU and not a VDU.
VDC/VDU ( Vehicle Data Computer or Unit )was the first Freightliner electronic dash gauge control from 1999 to 2003 .

Then updated to the MMDC ( Multi Module Data Center ) to 2007.

Then the LBCU ( Light Bad Control Unit ) for which other members report installing a change up wiring harness.
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Old 11-17-2021, 08:04 AM   #7
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Air Pressure Problem Fix

The Freightliner Service Center found “unit was equipped with old style daisy chain jumper harnesses, (white in color). Checked for proper terminal tension at jumper harness, found unit needs updated jumper harness, found weak connections at several jumpers. Found air gauges have integrated PSI Transducer for low air sensor warning, recommend replacing both air gages due to known internal circuit issues, air leaks.” Parts were $931.31 and labor to install was $928.20. Thanks to everyone who responded.
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