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Old 08-24-2007, 11:46 AM   #1
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I've noticed on my coach and several other 05-06 chassis with the MBE926 engine that the boost gauge reads almost 15# when the engine is at idle or key-on-engine-off. The gauges are those supplied by FL manufactured by Medallion.
They are feed through a MMDC connected to the J1939 data bus.
Anyone know of a fix for this issue?
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Old 08-24-2007, 11:46 AM   #2
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I've noticed on my coach and several other 05-06 chassis with the MBE926 engine that the boost gauge reads almost 15# when the engine is at idle or key-on-engine-off. The gauges are those supplied by FL manufactured by Medallion.
They are feed through a MMDC connected to the J1939 data bus.
Anyone know of a fix for this issue?
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Old 08-28-2007, 08:08 AM   #3
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Anyone out there with the MBE926 engine, do you have a boost gauge, and does it function properly?
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Old 08-28-2007, 09:05 AM   #4
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A question I would ask first is "Assuming the qauge is electronic, How is it programmed?" 15 PSI is normal atmospheric pressure at sea level. The 15# reading may be perfectly accurate under those circumstances. Other boost gauges measure the difference above ambient.
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Old 08-28-2007, 09:10 AM   #5
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Since you didn't get a reply to your first query, let me offer a couple of suggestions:
1. Your FL Service location should be able to hookup their computer to your data port and see if they are getting a reading similar to your display. Alternatively, you could track down someone near you like me who has VMSpc. I don't have a boost gauge on my console but VMSpc can display it through the diagnostic port.
2. Freightliner Custom Chassis has been very helpful to me in cases like yours. I called them about my wacko speedometer and they sent me complete hardcopy wiring diagram for my chassis. That has helped me alot. I ended up finding that myself after my local Freightlienr service center worked on several times and didn't fix it, for me or for the dealer that I bought it from. If there are any Service Bulletins relative to your problem, they will know about them. Often, Service Bulletins are used to alert dealers to problems without Freightliner actually doing a recall.
3. If I were in your shoes, I'd consider contacting Medallion directly. While some companies have lousy customer relations, I've been lucky in contacting secondary suppliers for our MH. Ramco (mirrors) Dometic (slide topper), Koolomatic (fan), Evans (dash A/C &controls) have all been kind enough to give me or direct me to the correct answers. If you have a specific query that you want to pose, you might give it a try.
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Old 08-28-2007, 10:30 AM   #6
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It's very common for European manufacturers to read boost in terms of absolute pressure, whereas American engine manufacturers normally express boost in terms of gauge pressure. The difference between these two, expressed in PSI, is about 14.7 at sea level, so it may well be that the boost gauge for the Mercedes engine is calibrated to read absolute pressure.

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Old 08-28-2007, 05:20 PM   #7
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Thanks for the replies. I did ask a FL service center about this while they were doing the tie rod recall, but they didn't find any service bulletins pertaining to it. When engine is under load the gauge will max out at 50#.
I e-mailed Medallion but they did not respond. I'll try calling them next. Yes the Medallion gauges are electronic.
I've seen the boost gauge used with a Cummins engine on this chassis and it displays what I consider normal, 0# with key-on-engine-off. Even if they're using the European method, they shouldn't allow the gauge to max out.
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Old 08-28-2007, 06:15 PM   #8
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Have an MBE but no boost gauge.
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Old 08-29-2007, 05:25 AM   #9
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I agree - the gauge should not be maxing out. Since it's electronic, see if Freightliner can change the boost gauge units of measure from PSIA to PSIG in the ECU or PCU. This would make it read like the Cummins (i.e., 0 PSIG with the key on and engine off).

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Old 05-16-2009, 03:56 PM   #10
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Boost Pressure on the MBE 926

RustyJC gave you the correct answer. Freightliner imported the MBE926 from Europe, their engineers had the instrumentation report the Absolute pressure and not the gauge pressure. ASE requirements are to display gauge pressure while the european displays the Absolute pressure which is atmospheric pressure plus boost pressure which will result in the display of absolute.
Not sure how your displayed pressure is displaying 50 #, The best mine has displayed was 45 #, which would be 14 # (atmosphiric) + 31 # (engine boost). Varified by my VMSpc, my engine boost is at best 31 #.
Freightliner engineering recognizes this problem and has decided to ignore it. To correct the issue would mean a special gauge set for the MBE926 MMDC and LBCU users.
Freightliner told me they ONLY built 916 RV chassis with the MBE926, so it is just not worth their effort to change it.
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Old 05-16-2009, 11:41 PM   #11
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I have what seems like a similar problem on my 2005 Cummins ISL 400. It has been a problem from day one where the boost gauge pegs to 50# when the engine reaches maximum boost of about 32#. Freightliner has put in a new MMDC & boost gauge without change in the problem. They have given up as to how to fix the boost gauge problem. The MMDC gets the boost reading from the ECM and according to the laptop hooked to the diagnostic plug, the ECM is putting out the correct reading. All US made components involved.
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