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Oil Pressure Switch - Hour Meter

May 17, 2014, 01:00:02 pm

OK, finally got it figured out. All it took was a look at the schematic! The oil pressure switch indicated with the red arrow is the switch for the engine hour meter. This is a fairly common application of which I was unaware. The sender indicated with the blue arrow is for the dash oil gauge. There is another for the ECM. When I took this one out I noticed that one of the wires was burned even though it was enclosed within a loom. I replaced the burned part and put on a new loom. I guess all those years hanging around next to that fire breathing diesel took their toll. Something to keep an eye on.

The part is a Hobbs pressure switch. The number is 76575-4 NO. The four stands for 4#, I believe. The NO indicates "normally open." I got this one on the interweb for $25 delivered.
jor

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Re: Oil Pressure Switch - Hour Meter

Reply #1 – May 17, 2014, 11:56:48 pm

Good job Jor. 
I had that same switch fail but it stuck in the closed position even when the engine was off.  Hour meter ran 24/7 while parked for about a month.  Only way I discovered it was I noticed the red LED on the Allison touch pad was illuminated with ignition off.  I guess the switch does double duty to let the Allison ECU know if the engine is running (at least on my coach - I don't know if that is true on a U320 with an M11.)

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Re: Oil Pressure Switch - Hour Meter

Reply #2 – May 18, 2014, 03:05:42 am

jor... thanks for the info and detailed pics. It's one of those things we all should be aware of. 

I replaced mine last week when James Triana at FOT told me that a faulty oil pressure switch was causing the chatter of the ATEC relays when idling after the engine warmed up to operating temp.  On my 6V92 like John's U300 it was difficult to remove due to it's location between the starter and the motor mount.  It's screws straight in and I had to get the wrench behind it next to the block.  When I removed it finally, I discovered it was a 15psi instead of a 4psi that Billy Jack in parts told me that this build was specified for. Don't know if it was the original and I replaced it with another 15psi.  Always, batteries disconnected when working on these or you'll blow the 5 amp fuse to the switch under the dash. 

I was also getting a check trans light and code 32 (direction signals).  After changing the switch and when the engine was below operating temp, I was able to clear the check trans but not the code 32 on the ATEC code reader.  When the engine warmed up the check trans light would come on again along with the ATEC relay chatter.

I have an Allison HT746 and checked with Ken Williamson at Al-Trans Inc. in Mobile.  They are an Allison dealer and warranty repair specialist for the  AL-MS-LA area.  http://www.facebook.com/pages/AL-Trans-Service-Inc-Allison-Transmission-Dealer/169518986391803

[url=http://www.facebook.com/pages/AL-Trans-Service-Inc-Allison-Transmission-Dealer/169518986391803?id=169518986391803&sk=info]http://www.facebook.com/pages/AL-Trans-Service-Inc-Allison-Transmission-Dealer/169518986391803?id=169518986391803&sk=info[/url]
 
Ken told me the cause should be a faulty forward pressure switch in the electro-hydraulic valve body in the transmission since it hesitates sometimes when shifting from Neutral to Drive and I have to do it twice to get it to shift. I don't think it's the keypad or the connections or grounds as I have checked those also.  He ordered the part and I will bring it to them for the repair next week.  I'm not getting a Do Not Shift Light so it's okay to drive.  I just have to put up with the beeping.  The transmission oil pan will have to be removed to replace the switch and it should take about two hours. Time to get it serviced.  Wish me luck!

Jerry aka Murph


 

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Re: Oil Pressure Switch - Hour Meter

Reply #3 – July 07, 2014, 01:00:44 am

Last week I brought my U300 to Al-Trans in Mobile for service and the ATEC code 32 (direction signals).  The error code is gone and my 746 trans is shifting much better now.  The pan was removed and both forward and reverse pressure switches were replaced along with the fluid pressure switch.  There was sludge along with some metal filings found at the bottom of the pan.  The old fluid looked good and I'm not sure when the trans was last serviced and it still had the original pan gasket.  At 84000 miles, I wanted to have this done to get a benchmark and a condition of the trans.  The valve body was cleaned along with screen inside the pan.  The  springs and solenoids were also checked to make sure everything was working properly.  The trans pressure was tested at operating temperature and was found to be good.  They also adjusted the TPS and cleared the code 32. 

With fresh fluid and spin on filter I was good to go.  Drove it home (about 75 miles) and could really feel a difference in shifting smoothness. No more hesitation when using the keypad.  I only noticed a slightly hard downshift from 2nd to 1st but it could have something to do with the Jake on or just the nature of these Allisons.  I also noticed the trans temp was about 20 degrees cooler than the engine.  I'm going to Oak Mountain state park in Birmingham this week (about 600 mile round trip) and I will be able to tell if there are any more transmission issues to be addressed. 

After changing the oil pressure switch I'm still getting the ATEC chatter at operating temp.  Could be that the new one is bad or something else.  When I jumper the switch the chatter goes away along with trans check light and beeper.  When I get back I'll check with James Triana for more ideas.  If anybody has any suggestions I would love to hear 'em. 

I can't say enough good things about the guys at Al-Trans in Mobile.  Ken Williamson is about as good as they come and he kept me up to date on everything being done.  Sometimes when you have transmission work done all they tell you is it that metal was found and it has to be rebuilt.  Total cost for this service was $386 as opposed to $6500 for rebuilt.  If you are in this area and need trans work this is the place to go.  There were big trucks along with busses and fire trucks while I was there.  These Alabama boys really know their stuff and will save you money!  http://www.facebook.com/pages/AL-Trans-Service-Inc-Allison-Transmission-Dealer/169518986391803

Jerry aka Murph

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