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Old 07-17-2019, 10:33 PM   #1
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PDM Failure Part 2

In this thread I covered the failure of my power distribution module (PDM) that was part of Freightliner FL788 formal recall that happened serveral weeks after mine was replaced.

The PDM failure is typically associated with failure of all or some of the rear lighting and failure of the chassis AC compressor clutch, all fed through the rear chassis PDM.

I just experienced a new "PDM like" failure. After 4,000 miles on the recall new PDM I lost all rear lighting (running lights, turn signal and brakes) as well as chassis air conditioning (clutch would not activate).

Thank goodness the failure happened in storage only 2 miles from a FreightLiner service center. Service center ordered a new PDM and installed today. No change. They spend most of the day trying to get PDM on line using all the Freightliner diagnostics. It appears there must be a problem upstream from the chassis mounted PDM. But so far no answers.
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Old 07-18-2019, 03:01 PM   #2
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Quote:
Originally Posted by turbopilot View Post
In this thread I covered the failure of my power distribution module (PDM) that was part of Freightliner FL788 formal recall that happened serveral weeks after mine was replaced.

The PDM failure is typically associated with failure of all or some of the rear lighting and failure of the chassis AC compressor clutch, all fed through the rear chassis PDM.

I just experienced a new "PDM like" failure. After 4,000 miles on the recall new PDM I lost all rear lighting (running lights, turn signal and brakes) as well as chassis air conditioning (clutch would not activate).

Thank goodness the failure happened in storage only 2 miles from a FreightLiner service center. Service center ordered a new PDM and installed today. No change. They spend most of the day trying to get PDM on line using all the Freightliner diagnostics. It appears there must be a problem upstream from the chassis mounted PDM. But so far no answers.


Any updates on your problem?? Inquiring minds....

Today Iím sitting at Freightliner San Diego waiting for the recall work and other items. No word yet on the PDM recall. They first addressed my complaint of a loud exhaust. They just told me a hose to the compressor blew. Odd for a 24 mos old coach I think.

My dash AC is intermittent but cannot catch problems with the taillights.
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Old 07-18-2019, 03:43 PM   #3
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Any updates on your problem?? Inquiring minds....

Today I’m sitting at Freightliner San Diego waiting for the recall work and other items. No word yet on the PDM recall. They first addressed my complaint of a loud exhaust. They just told me a hose to the compressor blew. Odd for a 24 mos old coach I think.

My dash AC is intermittent but cannot catch problems with the taillights.
They fixed my coach with a yet unspecified module replacement that fed the PDM. I asked for the part number which they said they would send me. So it appears the PDM itself did not fail again. I did get another new PDM and the mystery module replaced in the deal under warranty. Right now everything works.

Since there is no way when driving to know if the rear lights are not working, I would be very suspicious that an intermittent dash AC was an indicator of a failing PDM. AC compressor clutch is fed by the PDM.
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Old 07-18-2019, 05:58 PM   #4
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Any updates on your problem?? Inquiring minds....

Today Iím sitting at Freightliner San Diego waiting for the recall work and other items. No word yet on the PDM recall. They first addressed my complaint of a loud exhaust. They just told me a hose to the compressor blew. Odd for a 24 mos old coach I think.

My dash AC is intermittent but cannot catch problems with the taillights.
If that was the 90 degree rubber elbow feeding into the air compressor I experienced the same failure. Caused by improper hardware on the two P clamps holding the line between the CAC and the compressor. I mention this because it was first repaired by a Cummins shop with the same hardware and it failed again a couple thousand miles later. Second time I repaired it myself with upgraded hardware and sturdier clamps.
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Old 07-18-2019, 11:29 PM   #5
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PDM Failure Part 2

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If that was the 90 degree rubber elbow feeding into the air compressor I experienced the same failure. Caused by improper hardware on the two P clamps holding the line between the CAC and the compressor. I mention this because it was first repaired by a Cummins shop with the same hardware and it failed again a couple thousand miles later. Second time I repaired it myself with upgraded hardware and sturdier clamps.


Helpful info-thank you.they covered it under warranty so I suspect you are correct- I wasnít able to get all the details at pickup.

Also, they said my PDM passed all tests, yet dash AC still intermittent, looks like Iíll have to make another visit after the next trip.
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Old 07-19-2019, 08:46 AM   #6
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Also, they said my PDM passed all tests, yet dash AC still intermittent, looks like I’ll have to make another visit after the next trip.
I am very skeptical about FreightLiner's testing procedures for "suspect" PDM's. There is no way to design a test protocol for an intermittent failure condition. If you have an intermittent dash AC that cannot be explained and a PDM from the the affected serial number range you should consider buying a new PDM on your nickel. It is only one hour of labor to install.

Remember, any time your dash AC fails because of an intermittent PDM, you have no rear lighting on the coach. Very dangerous.
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Old 07-19-2019, 04:01 PM   #7
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They fixed my coach with a yet unspecified module replacement that fed the PDM. I asked for the part number which they said they would send me. So it appears the PDM itself did not fail again. I did get another new PDM and the mystery module replaced in the deal under warranty. Right now everything works.
Followup. Got the detail of the "mystery module" that was replaced in addition to getting a new rear chassis PDM.

The problem was with the "Chassis Module Bulkhead Control" (PN 012F/A66-03087-000). This module is up front on the bulkhead in front of driver's seat. The interesting thing is that the failure mode of this module was identical to the failure mode of my PDM (no rear lights and no power to AC clutch).
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Old 07-19-2019, 04:19 PM   #8
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I am very skeptical about FreightLiner's testing procedures for "suspect" PDM's. There is no way to design a test protocol for an intermittent failure condition. If you have an intermittent dash AC that cannot be explained and a PDM from the the affected serial number range you should consider buying a new PDM on your nickel. It is only one hour of labor to install.

Remember, any time your dash AC fails because of an intermittent PDM, you have no rear lighting on the coach. Very dangerous.
I believe they are placing the suspect circuitry under a controlled current and measuring voltage drop. Which could identify loose connections or weak solder joints. Note I said could, not would. I wouldn’t fly one of those but it’s probably good enough for ground transportation. Just my opinion.

On our coach there were no symptoms discernible to the driver (and we check lights every day). However they tested the PDM, it failed the test and was replaced.

PS I don’t think it’s a given that ac failure guarantees light failure or vice versa. The PDM has many outputs and I can envision many failure modes that would be partial.
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Old 07-19-2019, 06:39 PM   #9
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I believe they are placing the suspect circuitry under a controlled current and measuring voltage drop. Which could identify loose connections or weak solder joints. Note I said could, not would. I wouldnít fly one of those but itís probably good enough for ground transportation. Just my opinion.

On our coach there were no symptoms discernible to the driver (and we check lights every day). However they tested the PDM, it failed the test and was replaced.

PS I donít think itís a given that ac failure guarantees light failure or vice versa. The PDM has many outputs and I can envision many failure modes that would be partial.
All true, but we are dealing with 12,000 PDM's that can fail in a wide variety of scenarios. To the best of my knowledge only one failure will be known to a driver moving down the road: AC clutch power lost (if AC is engaged). The nominal failure results in a total black out of the rear of the coach and toad lighting with absolutely no feedback for the driver.

So FreightLiner has apparently decided it is in their interests to test versus replace these PDM's. Each of the 12,000 owners of these PDM's needs to decide whether they want to take the risk. I got mine replaced for free but if mine tested ok in the recall, I would pay the $600 necessary to make sure I had a good PDM. My PDM first failed at 18 months and 11,000 miles. I discovered the failure during a routine check of lighting at a time of the year when I was not using the AC compressor.
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Old 07-19-2019, 06:58 PM   #10
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If that was the 90 degree rubber elbow feeding into the air compressor I experienced the same failure. Caused by improper hardware on the two P clamps holding the line between the CAC and the compressor. I mention this because it was first repaired by a Cummins shop with the same hardware and it failed again a couple thousand miles later. Second time I repaired it myself with upgraded hardware and sturdier clamps.
Is this a constant loud roar? Just yesterday mine sounded like a jet engine when the Jake brake was engaged and the Jake did not shut off right away. The noise continued until I hit the brake pedal with the engine brake switch off.
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Old 07-19-2019, 07:09 PM   #11
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Is this a constant loud roar? Just yesterday mine sounded like a jet engine when the Jake brake was engaged and the Jake did not shut off right away. The noise continued until I hit the brake pedal with the engine brake switch off.
No, a leak at the compressor inlet is a boost leak and will be heard at WOT or heavy boost condition. Can sound like an exhaust leak. You have a larger series engine and almost certainly different plumbing to the compressor than our smaller coaches with ISB. (Apologies to the OP for OT drift).
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Old 07-19-2019, 07:22 PM   #12
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As an on topic comment, I’ll add the PDM controls power to all the dash accessories as well. Found this out because when our new PDM was installed a fuse was left out of it. As a result it would start, and the LBCU and transmission panel and OEM gauge cluster worked. But everything else in the dash was dead. No climate control, cameras, entertainment, nav, etc. I believe there’s a larger circuit in the PDM that feeds the whole sub panel in the left front.

Tech who did our warranty work didn’t realize the new PDM ships without that fuse. As soon as I pointed out the problem another tech who had done a number of these knew immediately what the cause was.
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Old 07-19-2019, 07:37 PM   #13
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We had the recall done last week and now are back in the shop with another dash problem. They are saying the light bar that basically gives you all the info and the warning lights on the top side of the instrument cluster has failed. I can't help but think this is all related?
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Old 07-19-2019, 07:46 PM   #14
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As an on topic comment, Iíll add the PDM controls power to all the dash accessories as well. Found this out because when our new PDM was installed a fuse was left out of it. As a result it would start, and the LBCU and transmission panel and OEM gauge cluster worked. But everything else in the dash was dead. No climate control, cameras, entertainment, nav, etc. I believe thereís a larger circuit in the PDM that feeds the whole sub panel in the left front.

Tech who did our warranty work didnít realize the new PDM ships without that fuse. As soon as I pointed out the problem another tech who had done a number of these knew immediately what the cause was.
That's interesting. Based on the schematics on the FreightLiner web site it appeared the PDM in the rear controlled most electrical activity in the rear of the coach (fuses in the PDM correspond with aft mounted components). The "RV multiplexer" (which I think is the large module in the front driver side cargo bay) is responsible for most electrical connections up front. For instance, all headlight wiring comes out of the "RV multiplexer" through the chassis bulkhead module (module that just failed on my coach).

FrightLiner schematics show brake light wiring coming "RV multiplexer" up front while marker lights, turn signals and trailer connector wiring come out of the aft chassis PDM. Even more surprising the schematics show power for the AC clutch is coming from the "RV multiplexer". So it is a mixed bag for sure.
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