Allison HT740 trans speed sensor issue

JT4SC · September 04, 2017, 02:06:26 PM

Hey everyone,

In March we took our MCI 102C3 bus on a long road trip when the check engine light come on. After trying to find the issue (which for your reading enjoyment can be found here: http://www.busconversions.com/bbs/index.php?topic=31630.30) a Valley Power Systems tech was able to hunt down an old diagnostic computer and determine the issue was a faulty speed sensor. They then told me it wasn't something that had to be dealt with right away and I would be fine continuing with our road trip, which we did with zero issues.

I would now like to deal with the faulty sensor and would like some advise on if this is something I (a novice mechanic to say the least) can try to fix on my own, or is it something I should leave to a pro. I live in San Diego and as best as I can tell the local Valley Power Systems is my best bet for a very expensive pro.

Finally, I hate that I only ask for help on this forum and can never give any advise, however I just installed a new auto TV lift for my 32 inch TV inside the coach, and I can help anyone do it cheap and easy (approx $200 and a couple hours worth of your time). If anyone wanted to do something like this I would LOVE to share with you what worked and didn't work for me. My cell is (619) three 92-four 074.

luvrbus · September 04, 2017, 04:39:20 PM

I am not much help being I never saw a 740 with a speed sensor but it should be straight forward with 1 bolt,o-ring and a plug end harness remove the bolt and R and R with a new sensor.Some of the old Allison 741's had a screw in sensor those you use a meter to set

JT4SC · September 05, 2017, 06:48:55 AM

Thanks Clifford, really appreciate your insight!!
Jim

Sunchaser Art · September 05, 2017, 11:26:44 PM

Funny thing, we have a speed sensor problem that causes one of our shutdown lights to come on. . .ours is an '88 Eagle 15 with a 6V92 (DDECII) in front of an Allison 740. The speedometer works without a problem and the rest of the engine is working well within normal parameters.

The problem first started after our engine rebuild and when they installed the Jakes. The light comes on (almost religiously) once I hit 25-30 mph, which coincidentally, is the speed the Jakes will cut-out. Once I shut the engine down, the light will be off, until I hit 25-30 again. Damn if I can figure it out, but my thoughts are it has something to with the Jake setting in the DDEC.

Our speed sensor is in the front wheel, and I really don't think that's the problem, but I haven't had the gumption to pursue it further--especially since everything else is running well, at least for the last 30K miles or so.

I'd be interested in finding-out if you come up with a solution. In the meantime, I'll watch the pretty blue light come on and not get my hair on fire about it!

blue_goose · September 06, 2017, 04:32:45 AM

What year is your 102C3 and what type of shifter pad do you have.
Jack

JT4SC · September 06, 2017, 05:29:32 AM

Quote from: blue_goose on September 06, 2017, 04:32:45 AM
What year is your 102C3 and what type of shifter pad do you have.
Jack

Hi Jack,
It's a 1993 model year, the last year they built the C3. I'll hunt down the manufacturer of the shifter, it's 4 forward and 1 reverse automatic.

Sunchaser: it looks like we are in the same predicament! I think I may have found the issue, if I can get away from work long enough today I'm going to do more investigating. I believe the issue is where the tranny meets the engine, very close to where the tranny dipstick is (at least on my bus). There is a wire harness with 2 wires coming out, one of which goes to a connector that is loose. It's really hard (and dirty) to see what the exact issue is, but I'll hopefully work on that today and report back my findings.

blue_goose · September 06, 2017, 05:41:08 AM

That will not be a 740 transmission. You will have an electronic transmission in that bus. If the speed sensor isn't working it will not shift like it should. Lots of times it is the plug that goes to the sensor. I had to replace the one on mine.
Jack

luvrbus · September 06, 2017, 07:20:00 AM

He said in another post the transmission was verified as a 740, with a sensor I am thinking 741 maybe

bevans6 · September 06, 2017, 07:59:03 AM

According to MCI, all automatic C3's had the HT740. The D3 was the first with the B500. http://sales.mcicoach.com/preowned/pcoach.nsf/CoachSpec102C3?openform

JT4SC · September 06, 2017, 08:20:56 AM

Quote from: luvrbus on September 06, 2017, 07:20:00 AM
He said in another post the transmission was verified as a 740, with a sensor I am thinking 741 maybe

It's definitely an HT740, I have a picture of the plate on the side of the tranny with the serial # and part # as well. Valley Power wants $700 to find and fix the issue, I think I'll keep working on it myself for now...

luvrbus · September 06, 2017, 08:25:42 AM

You have the only 740 electronic controlled transmission I ever heard of that is neat to know , if the tech used the old Kent/Moore reader it is a 700 series.
I have seen 741's and 755's in the C's behind the 8v92 DDEC engines from the factory from MCI

JT4SC · September 06, 2017, 08:30:40 AM

Quote from: luvrbus on September 06, 2017, 08:25:42 AM
You have the only 740 electronic controlled transmission I ever heard of

I knew I was lucky!

What I have seen is the 1993 C3 model had alot of changes from every other C3 model year, I believe because MCI was trying to determine whether or not to use those changes on their new D3 model. This may have been one of those??

bevans6 · September 06, 2017, 08:57:59 AM

Everything I have been able to find suggests that a HT 741 would have an ATEC computer, and a 740 is pure mechanical with a cable shifter. I have found examples of a DDEC engine with a HT 740 that used a speed sensor on the front wheel. Could that be what this is? I guess what I am thinking, from this thread and the original thread, is that it would be easy to tell if it's an ATEC issue just by does it have an ATEC or not. If it's not ATEC, and if it doesn't have an extra set of wires coming out of the transmission (tail shaft area probably if it's a road speed sensor) maybe it's a sensor some other place?

Brian

blue_goose · September 06, 2017, 09:16:59 AM

My 1990 102C3 had a HT755 from the factory. It now has a B500 with a series 50 engine. If you have a HT740 the only reason for a speed sensor would be for the cruise control.
Jack

Iceni John · September 06, 2017, 01:42:10 PM

Doesn't DDEC II use a road speed input, presumably for its cruise control?

My DDEC II has cruise control (but it's not connected and won't ever be - I refuse to use cruise!). My Teleflex speedometer has its own pickup at the left front wheel. My HT740 has a mystery sensor on its nose, near the output, with two or three wires running to who-knows-where. They're not DDEC wires, and not part of the original factory wiring harnesses. Is this a DDEC speed sensor, and if so how does it connect to the ECM? Are tire sizes and axle ratios programmed into DDEC for it?

Just wondering.

John