DDEC comes on unreliably

[Iceni John]

Further to BusWarrior's post about keeping the DDEC always connected to the start batteries even when the main battery disconnect is Off, I discovered why this is important some years ago.   I had inadvertently connected my DDEC to the output side of my Cole-Hersee battery disconnect switch, and strange things began to happen.   The CEL and SEL would briefly flash on while driving, even with everything showing OK, but the engine never stopped or hiccuped.   When starting the engine, the CEL and SEL sometimes would not light at all, or sometimes would flash erratically.   This was before I bought a Pro-Link scanner, so I had no way to tell if a real code was being flashed or not.   After I realized that the DDEC must always be connected and I had rewired it correctly, there never were any more of these weirdnesses.

DDEC II draws only about 8mA when the engine's not running, so there's no risk of your start batteries running down if they're healthy or they have a maintainer.

John

[Bill Gerrie]

When you turn the 'ON' switch to run wait for the CEL and SEL lights to go out on their own. About 15 seconds then start engine. This is the DDEC computer self diagnosis.

[Bill Gerrie]

You will have to find the momentary switch on the dash or side dash. It will be marked diagnostic request or overrule. You have to know where it is to flash codes and also if the computer shuts the engine down in the middle of an RR crossing you have the ability to overrule the computer to move the bus off the road. It will give you a few seconds to get to safety. 

[BusNit]

I have a pro link scanner with DDEC II & III cartridge to loan locally. (Mesa/phoenix)

In reading all of this, it is my understanding that the ECM has its own internal battery? Something we should be swapping out?

[Bill Gerrie]

NO. I have never seen one go bad.

[luvrbus]

You will get code 46 warning you if the battery is getting low keep the batteries charged and they don't bad often  www.dieselcontrol.services.

[Bill Gerrie]

You will need to locate the DDL connection to see if it is a 12 rectangle type or a 6 pin round one.

[Sebulba]

When you turn the 'ON' switch to run wait for the CEL and SEL lights to go out on their own. About 15 seconds then start engine. This is the DDEC computer self diagnosis.

Will do.  I just turned on the main switch and waited a few seconds and those lights went out like you all said.

Now I willoficy my start procedure to include this step.

Thanks

Seb

[Sebulba]

You will have to find the momentary switch on the dash or side dash. It will be marked diagnostic request or overrule. You have to know where it is to flash codes and also if the computer shuts the engine down in the middle of an RR crossing you have the ability to overrule the computer to move the bus off the road. It will give you a few seconds to get to safety.

Ok, I will start investigating this.

Thanks

Seb

[Sebulba]

You will need to locate the DDL connection to see if it is a 12 rectangle type or a 6 pin round one.

Oh brother, there is so much to digest in this thread

Where would a guy look for the DDL connection?

Thanks

Seb

[dtcerrato]

Oh brother, there is so much to digest in this thread

Where would a guy look for the DDL connection?

Thanks

Seb

HOW DO YOU EAT AN ELEPHANT?
ONE BITE AT A TIME! 

[Bill Gerrie]

Generally the DDL connector will be under the dash or in the electrical panel at the side.

[Sebulba]

HOW DO YOU EAT AN ELEPHANT?
ONE BITE AT A TIME! 

You got it!

Seb

[Bill Gerrie]

Once you get the hang of how the DDEC system works it is a nice feature to have. I know a lot of guys on here don't like an electronic engine but it is the fear of the unknown. I carry one of each sensor and a spare computer so I could fix almost any electronic issue at the side of the road. Very seldom does anything go wrong.

[chessie4905]

I was introduced to them in 1980 when GM went to Computer Command control. Mileage on all models picked up 3 mpg right off the batt. Some teething issues early on but by 83 they were reliable.
Lucky they went with them on the Detroits. Better fuel economy and less annoying smoke, unless jacked up a lot. You definitely need the scan tool and good manuals. A better one that describes how each sensor works, normal running and stopped voltages, whet episodes can trigger a fault.
GM manuals were great back then in all the help to understand them and properly diagnose problems. Biggest problem was that many mechanics refuse or can't or can't comprehend it. Seems little has changed.
GM needs proprietary scan tool to work on them, or an really expensive aftermarket one now.