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Old 02-05-2016, 02:22 PM   #15
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Along with the solid advise above you might want to check the connections on the back of the air gauge. A bad ground can drive a person crazy.

I had an 02 with bad VDU. Mine would swing back and forth and sometimes go to zero but if I remember the buzzer would sound. The VDU converts the air pressure to an electrical signal and sends it to the gauge. It's my thinking that the VDU knows there is air so no buzzer but the gauge is stupid and doesn't recognize it. So it could be a faulty gauge. You can swap the airlines at the VDU or you can switch the lines under the dash at the gauges. That would be a good place to start.

As noted previously, make sure your buzzer is working by doing the brake test.




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Old 02-05-2016, 03:18 PM   #16
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My 02 behaved as you describe. The best part was the max gauge indication with a low pressure alarm. The VDU does a couple of things: analog to digital conversion and signal multiplexing. The dash gauges are daisy chained together with each gauge having a unique digital ID. The VDU then sends an ID with a signal to the gauges. This is a pretty common way to reduce the number wires, though it does require some degree of digital logic to be added. There are no air connections behind the dash. If a gauge is suspect it can sometimes be "fixed" by swapping a couple of gauge connectors with each other. The VDU issue mentioned by Skip426 was a situation where there were cold solder joints on the VDU air pressure transducers - cold solder joints = intermittent operation. I fixed my problem by reflowing the solder joints on the transducers. The documents attached in Skip426's response describe how to do that.
I get a low pressure alarm right after start-up for a couple minutes until the air pressure reaches the proper pressure. I would think that would be a good check of the pressure gauge and alarm/buzzer.
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Old 02-05-2016, 09:00 PM   #17
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If you have time, pull the dash panel, and if your gauges have air lines connected, swao them.

Best to release all of the air by pumping the brakes first.
This will confirm it is a gauge.

Regards,

Dan
On his Freightliner chassis, there will not be air lines to gauges behind the dash panel. The VDC (Vehicle Data Computer) has two air lines going into it. The VDC is located on the coach by the body builder.

The gauges are all run by a wiring spider harness. You can actually unplug the wires from the gauges, scramble the order, and reconnect and each gauge will work correctly as each gauge reads all the data on the wires.

I believe his gauge problem is in the VDC, and the repair procedure has been referenced above.

Edit: I see that NMBluept and I were trying to say the same thing at the same time. :-)

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Old 02-06-2016, 02:58 AM   #18
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If your air gauges are electric check out the sending units on the trundle (brake peddle below the floor). You can swap the front to back and check your gauges. It only takes a few seconds and no tools. If the gauges swap then you'll know one of them is bad.
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Old 02-06-2016, 06:09 AM   #19
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In most vehicles if worked on. ( hundreds ) the sendor units on the treddle valve are brake light switch's.

Not saying they are not there also but it could get crowded and confusing on which ones to switch.
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Old 02-06-2016, 06:21 AM   #20
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Thanks to all for the advice. I will update when I get the fix.
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Old 02-08-2016, 12:39 AM   #21
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Here is a link to show you what they look like and where to order one if you need it. Summit always has them in stock and they are the cheapest place you'll find them. Both of mine are on the Trundle along with the brake light switches.

http://www.summitracing.com/parts/SWW-82894/
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Old 02-08-2016, 09:11 AM   #22
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Quote:
Originally Posted by DD788Snipe View Post
Here is a link to show you what they look like and where to order one if you need it. Summit always has them in stock and they are the cheapest place you'll find them. Both of mine are on the Trundle along with the brake light switches.

http://www.summitracing.com/parts/SWW-82894/

DD788Snipe; Please remember you have an 06 Spartan chassis and the OP has an older Freightliner chassis.
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Old 02-09-2016, 01:01 AM   #23
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Yes I understand that but both manufactures used Stewart Warner gauges and sending units. They are pretty common Skip.
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Old 02-09-2016, 10:58 AM   #24
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Yes I understand that but both manufactures used Stewart Warner gauges and sending units. They are pretty common Skip.
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Craig , after June ,9, 2002. ( as per the gauge testing bulletin, in my first post to this tread ) it may well be that Freightliner switched to S/W gauges and sensors on 2003 and newer chassis, I haven't collected any info on newer years, but for the chassis year in question, the problematic VDC was in use.

This air system diagram , shows the brake light switches as the only electrical components attached to the treadle valve, with 1/4" air lines routed to the remote air pressure sensors in the VDC. The system is , in my opinion , overly complex as it relies on 30+ feet of air lines to supply the sensors, and that along with the internal issues with the VDC system may be the reason Freightliner switched to another system. Skip
Attached Files
File Type: pdf Freightliner air brake diagram.pdf (312.3 KB, 21 views)
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Old 02-10-2016, 05:42 PM   #25
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I understand.
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Old 02-18-2016, 06:00 PM   #26
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Problem identified

Sorry for the delay, but thanks to all for the advice. I ended up contacting a local mobile RV repair service who had a technician who was very familiar with hydraulics, and related to Winnebago products. He told me that he suspected the leak to be in a return hydraulic line, and that will give a false reading on my rear air gauge.
When he arrived, he confirmed the leak and told me it probably was related to a recent accident, where a driver fell asleep at the wheel and ran into the rear side of my MH. Anyway, he patched it up to allow me to drive home with no problem. After I got home I took it to a nearby Freightliner service center. They said the line is 15' and it will need to be replaced. My insurance company said the cost of the temporary and permanent repairs should be covered because of the proximity to the previous damage. So, all is well.
Thanks again for your advice. And, good to know that the air gauge reading is dependent on the integrity of hydraulic lines.
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Old 02-18-2016, 06:44 PM   #27
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I'm curious exactly which air gauge you are talking about that is dependant on the hydraulic lines.

The air brake system gauge should read tank pressure regardless of the other systems.

Is it a air suspension, ride height gauge ?
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Old 02-19-2016, 08:34 AM   #28
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twinboat

Actually, I can't answer your question. My dash has two air gauges... Front Air and Rear Air. I assumed that those gauges only reflected air pressure related to air suspension and brakes. I don't know that I'm wrong. But the technician told me that the leak in the hydraulic line could affect the air gauge. As soon as he made the temporary repair in the hydraulic line, the air gauge went to normal.
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