Serial Data Link Fault

[MAC]

The person who has my former silver Eldorado is getting the following codes:

I022 Panel Dimming Switch Pot Circuit

I032 Loss of ABS/TCS Data

I033 Loss of Supplemental Inflatable Restraint Data

I034 Loss of Powertrain Control Module Data

I037 Loss of Air Conditioning Programmer Data

I039 Loss of Road Sensing Suspension Data

T072 Serial Data Link Fault

The main problem looks to be the T072 code. Apparently, there is a data link/harness fault somewhere.

Yesterday, I bought some electrical connection/circuit board cleaner at Radio Shack and spray a couple of harnesses. I also inspected the wiring along both the left and right rocker panels going to the rear and found nothing that looks unusual. So, I'm thinking that perhaps there is a connection problem in the engine compartment?

Since there is a data fault, once the ignition is in the "ON' position or the car is started, the blower fan will turn on and can not be shut off since the Climate Control Panel (CCP) will not work. I'm told that eventually after driving for a while the data problem goes away and all is back to normal including the ability to control the blower. I don't know what to make of it accept that there is some wiring or harness problem somewhere. I'm also told that this problem occurs 8 of 10 times.

I would appreciate any ideas as to what is going on. I hope the problem isn't with the PCM.

[Cadillac Jim]

I would have to check the schematics in the FSM, but the problem seems to be is the Dash Integration Module (DIM), which is the host for reading and displaying OBD codes, supports the OBD interface for code readers and the Tech II, and runs the DIC and warning lights. I would find that module in your car and check the connector. The key is that the panel dimming switch pot circuit has a problem, and that circuit pretty much goes to the DIM and nowhere else, thus highlighting the DIM connector.

[Logan]

Is the car a 1994?

A classic failure mode of the IPC. All these codes and "--" displayed in the data areas. IPC will show defrost mode, blower full speed.

All the computers talk to each other on a network. Circuit 800 is the network loop. If one of the computers goes bad it will pull down the whole system.

The loop includes: IPC, Derm, ACP, ABS, RRS, PCM.

You can try unplugging each computer one at a time to see if the failure goes away.

You can also measure the communications voltage at the ALDL. I think it's pin M. The voltage should always be varing. Once the voltage locks you will see the IPC lock up about 30 seconds latter.

The IPC can often fail in this mode. Problem is, Delco speedometer repair shops cannot see this failure. To them, "--" data display is normal while plugged into the IPC tester. It sometimes would take 3-4 'reman' clusters to find a good one.

[MAC]

Thank you for the input Jim and Logan!

Yes! It's a '94 Eldorado.

Logan, you're right that the Climate Control Panel, where the outside temp is displayed, will read "--" when the IPC is not functioning. Obviously to communicate that it isn't receiving data.

According to GM's factory Service Manual ("FSM") (pg. 5E2-116) the T072 will set if the:

"EBTCM/EBCM does not get polled by the IPC for information or it does not see PCM information on the data link."

I have reviewed the FSM for the possible causes for the T072 "Serial Data Link Fault" code.

So, it looks like there is either a loose connection along the UART Data Line, Data Link Connector to the EBTCM, a short to ground at the EBTCM, or the EBTCM* has to be replaced. Interesting thing is that there is no referenced to the Instrument Panel Cluster needing to be replaced. The FSM refers to the UART Data Line (pg. 8A-50).

EBTCM = Electronic Brake and Traction Control Module

EBCM = Electronic Brake Control Module

*The EBTCM is located in the trunk in the center of the electronics bay.

[Logan]

When diagnostic trees for the service manual are created...ever logical possible failure is looked at.

Typically, for GM at least, with mutiple codes, the lower code number is where you start.

So, ignore IO22 (false headlamp switch code.)

Also ignore TO72 and start at IO32.

IO32 will get you into the IPC part of the service manual.

The Delco speedo shops equipment cannot 'see' a circuit 800 failure in the IPC. Example: My IPC tested 'ok' on the Delco equipment. I walked the Delco tech out to the car, installed the IPC, had a DVOM on pin M of the ALDL. I told the tech "watch the voltage"...then pressed on the IPC plug in such a way I could duplicate the failure.

After about 10 seconds, the DVOM voltage locked, about 30 seconds latter the IPC went into defrost mode, blower high, and all the data went '--', and all the same codes set.

Mine was in 500 mile failure mode. Every 500 mile the problem would pop up for awhile..then go away. I was lucky I was able to duplicate the problem.

This is and was a fairly well know problem with the IPC and Cadillac web sites. Not well understood by the dealers or Delco. Many dealers were replacing 4-5 IPC's before finding a good one. In every case I've heard of...it turned out being a bad IPC and not one of the other computers.

[MAC]

When diagnostic trees for the service manual are created...ever logical possible failure is looked at.

Typically, for GM at least, with mutiple codes, the lower code number is where you start.

So, ignore IO22 (false headlamp switch code.)

Also ignore TO72 and start at IO32.

IO32 will get you into the IPC part of the service manual.

The Delco speedo shops equipment cannot 'see' a circuit 800 failure in the IPC. Example: My IPC tested 'ok' on the Delco equipment. I walked the Delco tech out to the car, installed the IPC, had a DVOM on pin M of the ALDL. I told the tech "watch the voltage"...then pressed on the IPC plug in such a way I could duplicate the failure.

After about 10 seconds, the DVOM voltage locked, about 30 seconds latter the IPC went into defrost mode, blower high, and all the data went '--', and all the same codes set.

Mine was in 500 mile failure mode. Every 500 mile the problem would pop up for awhile..then go away. I was lucky I was able to duplicate the problem.

This is and was a fairly well know problem with the IPC and Cadillac web sites. Not well understood by the dealers or Delco. Many dealers were replacing 4-5 IPC's before finding a good one. In every case I've heard of...it turned out being a bad IPC and not one of the other computers.

Thank you for the information. Nothing like first-hand experience!

So, at this point, I take it that chances are the IPC needs to be replaced? If so, I have never replaced one and I suspect that a salvage yard would be the best place to look. A new IPC is over $1,000.