Another allison ht755cr question

bigred · February 08, 2015, 04:17:04 PM

Hey Guy's:: I have a question concerning the ht755 cr in my Prevost .Allison mechanic says he can't tell anything is wrong but sure seems like there is to me.When taking off in this thing ,it is as if it is taking off in second or third gear .Man hooked it to the computer and it showed every thing is working as it should .I have tried counting the shifts .As far as I can tell,it shifts the correct number of times. There is no codes stored .The reason I think there is a problem is this.When taking off in reverse ,bus has plenty of power ,but when taking off in drive.it doesn't have enough power to pull a greased string out of a two inch water pipe.If any of you have any idea's ,I sure would be grateful if you would pass them this way!!

RickB · February 08, 2015, 04:25:05 PM

Are you positive it's not a second gear start 754? I found my first gear on the other side of the detente's on the shifter.

RB

bigred · February 08, 2015, 04:38:53 PM

I don't think so .The Allison tech called Allison and they told him it was the HT755CR. It has the shift pad .I have tried manually pushing the "1"button and it did not seem to make any difference.This does have the Atec control system.

luvrbus · February 08, 2015, 05:09:20 PM

They have a forward and reverse pressure switch your reverse is working fine the forward pressure switch is going bad and it won't throw a code till it is dead.

I would start there as they don't cost that much either in the threaded or the bolt down type you will need to remove it anyways to check the pressure

muldoonman · February 08, 2015, 05:58:49 PM

Must be a problem cause my 91 8V92TA with the 755 CR does take off quick , even pulling my pick up tow rig.

TomC · February 08, 2015, 07:09:06 PM

Also remember that 1st gear is 3.692 and reverse is about 5.0-not a good comparison. Good Luck, TomC

bigred · February 09, 2015, 04:54:26 AM

Thanks for the input Guy's.I do realize the difference between reverse and first,but this thing actually struggles to get under way,so I know something ain't what it should be .The pressure switch idea really makes sense since there are no codes showing .The Two Allison techs in our area are a little out when it comes to working on buses.The nearest full service Allison repair facility is Clarke Power in Greensboro,which is about 150 miles from here.They do have a shop and a road service locally(Canton N C ) but these are a lost cause.The Greensboro facility is top notch though!

RickB · February 09, 2015, 05:36:10 AM

I had a slipping 754 second gear start Allison and I put straight 30 weight rotella in it and it's never given me a problem since

Just a thought

RB

luvrbus · February 09, 2015, 05:37:18 AM

The problem could be on the engine side like low turbo boost , fuel and electrical that combo works good (8v92-755CR) till it has a few glitches then the fun begins I am sure Clarke will check the pressure on the transmission and go from there

good luck

muldoonman · February 09, 2015, 06:13:28 AM

Bigred,

Just curious, How many miles on your coach? Has it just started it?

bigred · February 09, 2015, 06:28:30 AM

It has to the best of my knowledge about 150.000 miles on it .It had the speedo changed and the previous owner noted in the literature that he had changed it at 101.000 .The new speedo is showing right at 50,000.

blue_goose · February 09, 2015, 09:01:14 AM

I think Luvrbus hit it on the head with engine problems. If you don't have boost it will not have any power. One of the least costly things you can put on the bus is someway to read engine boost. Before you spend more money on repairs get a boost gauge. All it takes is a one eight air line up to the dash and you are in business. Check around your exhaust for signs of a leak.
Jack

Boomer · February 09, 2015, 09:16:16 AM

Do you have a DDEC II 8V92? The problem you describe acts like a faulty turbo boost sensor, or wiring in that circuit. You are not going to get much fuel until the turbo boost reaches about 4 lbs. And conversely, no boost without fuel, so a fuel restriction somewhere can cause it too. This was a smoke reduction set up on the early DDEC. Starting on hills is dicey.

HB of CJ · February 09, 2015, 09:19:29 AM

If it worked OK before, but is not working now, perhaps it is as simple as the fuel rack not opening quickly enough? Some sort of 3-7 second delay buffer? But you have DDEC.

Could the key be that it seems to romp and roar in reverse, but not in the forward gears? Sounds like it may be something new inside the programing, but I am not so sure.

Or ... like said already before me better, something with the turbo?

HB of CJ (old coot)

bigred · February 09, 2015, 12:43:03 PM

Boomer :It does have the DDEC .This coach has evidently been sitting for a while as the Air box drains really leak at slow idle,so i usually keep it on the fast idle.I always wanted a bus,but I'm beginning to think I'm getting cured of that .It has been a trip so far .Some of you will remember when we took it to a camp ground last July and we had to have it towed because it would not go into gear,then after having it towed to a shop ,the tech went out and started it and it went right into gear and has not missed since.No codes and no problems that the tech could find.Is there such a thing as a haunted bus?? Just wondering!!