87 MCI MC-9 Allison HT 748 4 Speed Auto Trans issue

thepiratebus · May 23, 2011, 04:20:12 PM

OK - latest update. My buddy who has it at his house right now opened up the compartment and stated there were two red led lights one on top of the other as well as two toggle switches (with red covers) to the left of the lights. Which one of the toggles should we try or more importantly trust for the code. He said to the right of the lights were a bunch of relays and such and then two black boxes with Allison transmission on them.

Thanks - at least we feel we are heading in the right direction now.

NJT5047 · May 23, 2011, 07:52:02 PM

One of the lights should be green. PO probably changed out the ATEC light. I'll go check tomorrow and let you know which light is the ATEC.
There's an easy way to sort it out, hold either switch and count the flashes. If you flash out a "25," that indicates no codes on the DDEC ECM (there may be stored codes in the DDEC which would complicate this process). So the other switch is the ATEC.
Both the ATEC and DDEC ECMs are mounted to the right of the switch panel. I'd advise to avoid manipulating the wiring around the ECMs. The wiring might be in excellent condition, and it may not.
If you see signs of water damage, or any corrosion in the compartment, the ECM plugs might have corrosion and become intermittant. Then you'll have to carefully pull each and check. 99% of problems with DDEC electronics are due to corrosion or POs chopping up wiring.
JR

thepiratebus · May 23, 2011, 09:20:16 PM

Thanks - we'll start looking this week at all the connections too.

NJT5047 · May 24, 2011, 04:20:09 PM

Checked the location of the code lights on my NJT MC9 today. The ATEC is the bottom light and bomber switch. And it is green on my coach.
I've seen some coaches that have had all, or some, of the code switches altered or removed. POs do some interesting things with DDEC/ATEC wiring. The electronics are like black magic when things start to fail.
If the code readers don't work, they are simple to get going again.

HTH, JR

thepiratebus · May 25, 2011, 09:45:04 AM

Thanks - going to try and check it all out before weekend. Appreciate everyone's help - frustrating problem.

luvrbus · May 25, 2011, 09:52:12 AM

Comes with the territory of owning a 700 series electronic transmission once you get it straighten out it may last for 10 years or 10 minutes with out problems you never know

good luck

thepiratebus · May 25, 2011, 05:39:38 PM

The codes are in. Hopefully someone can give us a hint as to what this means. Called my buddy and he fired it up this afternoon. He said the bottom led was green once he started it, so that corrects that confusion.

He let it idle for a bit to build pressure and heat up. Once warm he left it in neutral with the brake on. He went and did the bottom switch and the green light flashed once, pause, then 4 times. It repeated this pattern. He then wanted to see what the top one did, so he did that and it flashed quickly 10-12 times. He then did the bottom one again and it did 6, pause, 6. He put it in gear (did not really move) and then he put in neutral and has repeated since (without shutting off) and it continued to do 6,pause, 6.

Personally I think the 1, pause, 4 was the correct code and the other was due to the other switch, but we might try again after it shuts down and starts back up.

Thanks

NJT5047 · May 25, 2011, 07:17:49 PM

The ATEC code "14" is "Forward Pressure Switch" fault. Sorry, but I'm not familiar with the internals in the transmission...so cannot advise what exactly we're talking about, however, almost all of the electronics in an ATEC transmission is serviceable without pulling the trans. This is good.
Another caveat: make sure that the wiring related to the the code producing item isn't damaged. Wiring problems will emulate failed components.
Since the switch is wired thru the main harness, it's probably located on the valve body.
This link will give you an idea of how to check the main harness for short or open circuit...you can move foward and backward and see a little more info. Isn't as good as manual...but. http://www.tpub.com/content/constructionpaletizedsystems/TM-9-2320-364-20-2/css/TM-9-2320-364-20-2_61.htm
I'd bet on a wiring problem...perhaps related to the pan injury.

ATEC Code "66" indicates a comm link issue. There are only 6 leads in the DDEC 1/ATEC link. VSS, TPS, are included. I'd guess that you have no transmision problems, but have a wiring problem.

Pull the ECM fuse and clear the DDEC codes. See what you get then. You might be reading a whole series of codes. That gets confusing. ALWAYS verify that the ignition master is OFF before pulling any fuses from the DDEC or ATEC. Keep in mind also that the DDEC and ATEC are always hot. They have B+ input that is not switched by the master ig or battery disconnect. Don't short anything out if you pull plugs.
AND keep in mind that both DDEC and ATEC are 12V systems that operate from the center tap. They will be fried if you connect the 12V input lead to the 24V post. Then they'll flast all manner of codes....or non. A good DDEC 1 will read a code "25" when no codes are set. DDEC 1 units are famous for codes such as "low battery voltage," "injector...?" These codes won't set a CEL, and won't have any effect on operation. But the clutter up the code reading process.
A DDEC code "55" indicates a trans comm link (TECL) fault.

Try the DDEC again and see what happens. Read the codes with the engine stopped, not when running.

I've been planning on buying an ATEC manual...worry that something like this might pop up.

JR

luvrbus · May 25, 2011, 07:50:00 PM

The 748 has 2 pressure switches forward and reverse fwiw

good luck

NJT5047 · May 25, 2011, 08:23:10 PM

It looks like the reverse pressure switch is externally mounted? From some pix on the 'net, looks like the FPS is mounted dead center of the valve body.
The FPS can be evaluated by checking for continuity across the trans main harness plug. I'd guess that it should be open when in neutral, or not running, or in reverse. Closed when a forward gear is selected. I'd guess.
Pulling the main harness plug should allow an easy check once the layout of the plug is established while laying under the coach and looking at the plug upside down. I believe the trans plug has tiny alpha stamped into the bakelite part of the plug? That would be nice for someone young enough to focus on them!

I wouldn't replace anything until diagnostics point to a mechanical failure such as a failed pressure switch. Those things rarely fail. Still, could be related to the damaged trans oil pan. The FPS might have been damaged by the displaced filter or whatever holed the oil pan.
I'm still not clear on when this problem presented...did the transmission shift funky prior to the oil pan damage? Or did the problem begin with the oil pan hole?
I'd highly recommend anyone owning an '86 or '87 MC9 with a DDEC 1 and ATEC buy a DDEC 1 Dx manual and proper ATEC transmission manuals and keep them in the coach. Very few shops will work on a DDEC 1. And a good many of those that will have no docs on the system. It's quite different from a DDEC II and up. DDEC 1 is a product of Motorola, DDEC II and up are Detroit designs. DDEC 1 units are dependable, just that they have a ton of extra wiring and connectors and every inch can be problematic on an old coach.
I'm not sure exactly what year th Pirate coach is, but if it's an '87 or later the coach suspension is considerably different from earlier MC9s. '87 MC9 and MC12s share 102A3 style suspension. And, if it's an NJT, everything else is different too.

JR

thepiratebus · May 26, 2011, 05:08:57 AM

Thanks - will look into getting the book this week. We'll try and clear all codes and try again. the fps makes since since that is the main issue we are having. Thanks for all the help

NeoplanAN440 · May 26, 2011, 06:28:53 AM

the forward pressure switch is almost in the center of the valve body. the chances are great that it had been hit when you punctured the pan.

the switch is closed when under pressure ( gear selected)
so in order to measure it , it has to be removed from the body. apply 25-35 psi pressure and check if it closes (0 ohm resistance ).

or if your transmission has the (plastic like) connection plate inside the pan, you have a broken circuit in it.

the pressure switch connection 3 and 4 go to the transmission pan connector pin E and F

but with no pressure there should be no resistance on the switch.
so the only way to test it is to remove the pan and check the switch and internal wiring!

thepiratebus · June 19, 2011, 07:15:30 AM

Thanks for all the help - sorry for the slow reply, but I have been on the road. We finally got around to working on the bus and it seems to be good to go. We did a series of the recommendations and one or all together did the trick. I wish I could pinpoint what fixed it, for other people who may have this issue, but while we had it jacked up we wanted to do it all.

Before ordering the FPS, we decided to go easy and cheap. Step one was to disconnect all battery sources so there was no possibility of power. We assumed this would reset the AT comp. At that time we also located a fuse that seemed to be blown. This looked to be going to a aux source, but not sure what. We had not noticed this fuse being blown before (it had a light on the fuse when blown and we think it blew when we hooked batteries back up). New fuse installed. We then drained all fluid and changed the AT filter.

After it all, it works perfectly. Thanks for all the advice. Happy trails!