Witch hunt.

[lonesome george]

I have been chasing a intermittent lose of power problem with our 98 series 60 DDEC III for about 8 weeks now and think it is finally fixed. I will try to recall and explain the events, as most of you know explaining things is not my strong suite but I'll try, this may help someone down the line.

About 4k miles into a trip in August we had a sudden lose of power. The turbo boost fell to about 2 psi with the throttle on the floor, then the engine came back to life and spooled up about 28 psi of boost shifted the transmission down and took off like normal, this went back and forth for about 5 minutes, it felt like it was running in and out of fuel. I pumped the throttle a couple of times and the check engine and the retarder lights came on with the throttle a about 50%. All if the instrument panel gauges had normal readings thru the entire event. It was raining cats and dogs. I did'nt have the presents to check the Pro Driver during this event.

Next day. I did a visual inspection of everything I could think of, found nothing out if the ordinary. Changed the fuel filters out of desperation. After about 4 hours of normal running I got a check engine and a stop engine light at the same time for about 5 seconds, no lose of power all instruments normal. Clear warm day.

Next day normal running all day. By now we had arrived at a town that had a Detroit dealer, Fairbanks AK. took the bus in to have the ECM read for codes and some advise. Recorded codes were low coolant level, high coolant temp, throttle position sensor fault low, no active codes. Advise, we don't know, it's not doing anything now and what ever we do will be a guess, honest enough. Cleared the codes.The parts department had a throttle position sensor that looked like the one in the bus so I purchased it just in case.

Next day installed the new throttle position sensor to make sure the bus would run with it, it did, so I left it in. The part was a Bendix not the OEM Williams Controls unit so it was a crap shoot.

Over the next couple of weeks it acted up a few times, sometimes lose of power, sometimes lights and sometimes both with the Pro Driver display reading throttle position sensor fault low sometimes, always with the drivers instruments normal. At this point weather did not seem related to the events.

By now we had arrived at another town with a Detroit dealer, Spokane WA. took the bus in to have them read the ECM. The recorded codes were high coolant temp 17 times and throttle position 5 or 6 times. I told the Tech that I had replaced the TPS and he ask if I had replaced the harness with it, I said no, he said it should have come with the switch, of course the one I got did'nt have a harness. The Tech suggested replacing the coolant temp sending unit for the ECM because we had so many over temp codes and the drivers gauge had never been over 210 degrees, the one for the instrument panel gauge uses a different sending unit. They had a ECM temp sending unit so we purchased it and installed it later that day when things cooled down.

A couple of days with no problems and then the engine went to idle and would do no more, this is usually an indication of a TPS failure of some sort. Stopped the engine and installed the old TPS, the engine started and ran bad for a few seconds and then back to normal. This is getting old, we were on a hill with no shoulder.

Got home and ordered a new TPS from Prevost, which comes with the harness and installed both. At this point I don't believe the problem has been fixed. A good friend that has a bunch of experience with 60's said "you have a short some place", we hooked up his reader and got over temp and TPS codes like every time before.

I decided to start the search with the ECM, took it off the engine for a good inspection. Mine is the older type with machine screws holding the large connectors in place and bail clips on the smaller connectors (the newer type have a cam lock I'm told). What I found was two dirt stains on one side of the connector housing, these looked like a wire draw (read path) into the connector and two of the pins in the ECM on the engine control side had dirt and a stain connecting them together! I cleaned the mess up and resealed the connectors and the engine has performed normally for 1500 miles now. Still have my fingers crossed but believe the real problem was a poor seal at the connectors on the ECM letting dirt and moisture in.

What a PITA !!!

[Jon Wehrenberg]

George, I can tell you from experience the connectors for both the transmission and the engine have to be tight and clean. They are sending very low voltage signals and the least bit of corrosion or dirt, or a bad ground can give the "brain" all sorts of bad information.

In our case it was the transmission 28 pin connector and it was intermittently telling the transmission the internal temp was -20F. The transmission will not shift at those temperatures so ours was locked in whatever gear it was in when it got that goofy signal.

The problem is we don't have occasion to look at our connectors as often as we should. Your explaination was a good one and it points out how hard it is to isolate some problems.

[chtree]

George,

We chased a short that wouldn't short out only about every other time. It can be very frustrating. We even traded ecm with a friend for the weekend. Man it was a long Spring! I sure hope that you got it !


Chris

[garyde]

I'm surprised the Detroit people didn't check the connections with all of the readings you were seeing.
Good job staying with it and tracking iit down.

[rfoster]

Good Job George.. It is a pain to chase crap like that. Congrats on a job well done.

[Pete]

Well, the folks at the DD Alison shop told me to bring my bus by their shop and they would reprogram my transmission after I had my trans fluid changed a Prevost in Ft. Worth. After the change to Transynd, the shifts were unusually hard. They reprogramed the trans and it shifts normally. The computer when connected reported that there is an error code that implies a wire from the ECM to the trans has a open. This wire controls the trans retarder, and that it is not working. This coach is the first I have had with a retarder. I am troubled in that when the retarder is used, the oil temp rises , and the coach noticeably reduces speed. (I guess it has not worked since the coach was purchased). The DD/Alison folks in OKC said it would be easier to run two new wires from the ECM under the drivers seat to the trans, and estimated it would take about 8 hours. They also said it would not hurt the trans to go ahead and continue my trip to the west coast.
I wonder if after reading the above post from George if I may have a fouled connector.

[hhoppe]

Gary D / Jon et all: Is there a cleaning / electric terminal spray we should be using on our terminals to clean and fight corrosion. ?? The above descriptions warrants some pictures of the effected locations.

[Jon Wehrenberg]

I know nothing specific. On mine the problem was so serious and widespread on the transmission connector Allison had to remove the transmission side terminal connector, including all new wire bundle to every internal sensor in the transmission and connector.

At the time we had 100,000 miles on the coach and Allison and Prevost paid for the entire cost except for the transmission fluid.

I have since periodically unplugged and replugged every terminal plug from the DDEC and the transmission brain located at the engine and in the steer compartment. This is something that has to be done if any welding is done on the chassis anyway so it is not a practice that will cause problems. If you break a drive axle outer stud for example and have to weld a nut on it to remove it you need to do this. I think periodically plugging and unplugging them keeps the contacts wiped clean, but I don't know this for sure.

[jelmore]

I know nothing specific. On mine the problem was so serious and widespread on the transmission connector Allison had to remove the transmission side terminal connector, including all new wire bundle to every internal sensor in the transmission and connector.

Jon, can you point to another thread about your woes or describe your symptoms here?

[Jon Wehrenberg]

On mine it was on the first bus, and the transmission would intermittently give a do not shift warning. Unfortunately it would happen at any time such as when we were driving. No pattern to it, just all of a sudden, and when it did that, until the message went away the transmission was literally in whatever gear it was in. It would not upshift, down shift or disengage.

Once we had to slow down unepectedly, we were in fifth, and while our brakes were able to slow us down the transmission was locked in fifth and in fifth the torque converter is locked up, so when we got as slow as we needed to get, the engine stalled. We eventually had a Prevost Service Rep ride with us and with his reader when it went into a do not shift condition he was able to read the transmission temp was -20F, and on that particular model of transmission it would not shift if the internal temsp were below +20F.

Any freaky condition on our engines or transmissions can be created with current paths to ground, bad grounds, corrosion in the connectors between two contacts, etc. I doubt if there is a book out there capable of diagnosing all of the potential weird situations that could occur.