Community Forums

desertdd · 09-10-2018, 06:44 AM

I posted awhile back about erratic air pressure readings on 2007 Discovery with a Freightliner chassis even after I changed the filter in the dryer. I took the dash apart and checked all of the wire connections on the gauges. I didn't observe anything amiss. I put everything together and went on a 1000 mile trip and no erratic readings. What is your guess what changed. My guess is that there was moisture in the system and it took some running to eliminate it. I did notice the air lines are hooked directly to the gauges. Not complaining though!

Pigman1 · 09-10-2018, 07:42 AM

I had a 2008 Freightliner chassis with similar problems. All your dash gauges are fed from a multiplex bus which means all data, both air pressures, temp,oil pressure, etc are fed over one wire. The gauges are then daisy chained at the dash and each gauge picks the data bit it needs to display what it needs. When you pulled and re-seated the plugs you got a better connection somewhere in the chain and all now works. I solved my problem by pulling all the dash jumpers and putting them back together differently. I think Freightliner put out a service bulletin on the issue, but you solved your own problem, so.....??

twinboat · 09-10-2018, 07:49 AM

Water in the system probably didn't cause erratic readings.

Before air dryers, we would discover that some drivers weren't draining there air tanks when the truck came in for service.

Everything looked and worked fine except water coming it the relay valves. We would find quarters of water in the tanks.

In freezing weather it was a different story.

FIRE UP · 09-10-2018, 08:58 PM

Quote:

Originally Posted by desertdd

I posted awhile back about erratic air pressure readings on 2007 Discovery with a Freightliner chassis even after I changed the filter in the dryer. I took the dash apart and checked all of the wire connections on the gauges. I didn't observe anything amiss. I put everything together and went on a 1000 mile trip and no erratic readings. What is your guess what changed. My guess is that there was moisture in the system and it took some running to eliminate it. I did notice the air lines are hooked directly to the gauges. Not complaining though!

Your air LINES GO DIRECTLY TO YOUR GAUGES? Wow, that's a new one on me. For years, on Freightliner chassis's, the air system is plumbed into either the VDC or, in the case of coaches '04 or newer, they're plumbed into an MMDC. The MMDC or, Multi-Module-Data-Controller is the main computer that, similar to what Pig-man states deciphers all the info from the engine ECM, the transmission TCM, the ABS system and, the AIR system. Once all the information is processed, it (The MMDC) sends the appropriate information to each set of gauges.

Both the VDC and the MMDC, have what's called "transducers" that the computers use for interpretation, so the correct pressure information can be sent to the gauges.

So, this is why I'm kinda surprised you have air lines going directly to your gauges. Wow.
Scott

RichKimball · 09-11-2018, 07:07 AM

In troubleshooting erratic air gauges I too found on my 08 Freightliner chassis that air pressure seems to be connected directly to the gauges.

Quote:

Originally Posted by FIRE UP

[/COLOR][/U][/I][/B]

Your air LINES GO DIRECTLY TO YOUR GAUGES? Wow, that's a new one on me. For years, on Freightliner chassis's, the air system is plumbed into either the VDC or, in the case of coaches '04 or newer, they're plumbed into an MMDC. The MMDC or, Multi-Module-Data-Controller is the main computer that, similar to what Pig-man states deciphers all the info from the engine ECM, the transmission TCM, the ABS system and, the AIR system. Once all the information is processed, it (The MMDC) sends the appropriate information to each set of gauges.

Both the VDC and the MMDC, have what's called "transducers" that the computers use for interpretation, so the correct pressure information can be sent to the gauges.

So, this is why I'm kinda surprised you have air lines going directly to your gauges. Wow.
Scott

FIRE UP · 09-11-2018, 10:48 AM

Quote:

Originally Posted by mevman

In troubleshooting erratic air gauges I too found on my 08 Freightliner chassis that air pressure seems to be connected directly to the gauges.

Roger that, always learning on this forum. Thanks
Scott

Darwin · 09-18-2018, 05:59 PM

I have u 2007 Freightliner and had a similar problem. Gauges would start bouncing and low air pressure alarm would go off. One shop suggested water in the system. I fought the problem several years, bleeding air daily and changing air drier. I contacted Freightliner a second time, they sent new wiring harness which were installed in the dash which solved my problem. I suggest you call Freightliner with you serial number

RichKimball · 09-19-2018, 06:47 AM

Wow that's great. I was having issues too, and went in to the dash area to check on wiring etc. I pulled the connectors on the gauges and some other areas and put everything back. Since then I haven't had a air gauge issue so I can believe that wiring and not the module can be the culprit.

Quote:

Originally Posted by Darwin

I have u 2007 Freightliner and had a similar problem. Gauges would start bouncing and low air pressure alarm would go off. One shop suggested water in the system. I fought the problem several years, bleeding air daily and changing air drier. I contacted Freightliner a second time, they sent new wiring harness which were installed in the dash which solved my problem. I suggest you call Freightliner with you serial number