Will it run without BCM?

[2seater]

I know this is a strange question, but does anyone know if the engine will operate and drive with the BCM disconnected? In addition, is the ALDL port still active with the BCM disconnected? My car is still in storage some miles away so I cannot find my own answer at this time.

[padgett]

It will run but it will not be happy.

[Roadster90]

Padgett,

How come, why for ???

Howdy Two Seater

[padgett]

Well for one, the speed (MPH) input is routed through the BCM and none of the CRT or dash functions will work. I do not recall if the transmission signals go through the BCM.

Onother word the SES light will probably be flashing but I would think it should start. Next time I pull a glove box, I'll check.

[Guest gotitright]

My 91 ran ok with the one plug pulled on the BCM for the climate control. Drove it for a week until I got a new one.

[2seater]

Thanks guys. My car has been dormant for going on two years now and I finally intend to get it back on the road. The reason for the question has to do with getting a clean recording of sensors for tuning of the turbo engine. I have had some success in the past with my Auto X-ray but it has limited capability and sometimes difficulty connecting for recording a run. It was suggested I try disconnecting the BCM to cleanup the interference. I am going to try TunerPro RT for recording and it may work better than what I have tried in the past? Reading any individual parameter available isn't a problem, and the stock diagnostic works fine for that, or a scanner plugged into the ALDL. The problem I have seen in the past has been a recording of several sensors at the same time has sometimes shown an offset or delay from one sensor to the next, like something slows down the data access? I do appreciate the input.

[Roadster90]

HUH ?

Good Luck Hal .... keep us posted with your progress

Nic

[padgett]

Keep in mind that the ALDL is really a LAN with eight or ten different components tied to it. Also it is supposed to be a loop, something that is broken when you remove the ALDL cover.

[2seater]

I understand about the communication link and that is why I asked. I know bits and pieces but am not savvy enough to know the total picture. I know the cover contains a jumper to provide a redundant link but it must be removed to connect the scanner? Frankly, I don't want to change, or disconnect anything to record data, and it would be a method of last resort, it it would work at all. Perhaps I haven't happened upon the right scan tool to record data (this will be the 4th different tool)? The Auto X-Ray does work, but has limited recording and it causes the instrument panel to sort of pulse when connected to record, which gives me reason to question if all of the recorded cells actually line up with each other? Thanks for the input.

P.S. I believe there is hardware/software available to remove the EPROM, plug in there, and both record and operate the car from a laptop? This is really the method of last, last resort, plus more expensive.

Edited April 11, 2010 by 2seater
add postscript (see edit history)

[KDirk]

2seater-

You are correct that the ALDL plug cover completes the loop. If removed for a scan tool to be plugged in, the tool completes the loop in place of the shunt in the cover. Further, the IPC will become less responsive [choppy] due to dropped frames while a scan tool is in the loop. This is normal (even states that this is to be expected in the FSM) as the scanner polls the data bus continuously to gather the data the tool is displaying.

In all GM ALDL (8192 baud) data link equipped vehicles, there is one device that is designated as the bus-master (primary device on the network). In the Reatta and other E-bodies of the late 80's/early 90's vintage, the BCM is the master device in normal operating mode. On other cars (particularly Cadillac) the IPC is often the master device. In any case, I suspect that when a scan tool is attached, it assumes the role of bus-master, knocking the BCM down in the chain of command one notch. If my thinking is correct, the scanner then controls the bus, and the flow of data so it can both monitor and control all the other devices for troubleshooting. This in turn hogs more of the available network resources.

This extra demand on the bus eats up bandwith that would normally be allocated to other devices on the network, thus making them [appear to] respond in not-quite real-time. What I don't know is if the scan tool is given priority to ensure that it's readings are real time, while the rest of the devices (IPC, ECC, ACP, BCM, etc.) are left to operate in this "reduced responsiveness" mode. This would make sense, as the scan tool needs the fastest response times and most accurate data for troubleshooting purposes, but I am not certain if that is what is happening.

There is not a lot of solid information on the low-level workings of this bus (CAN is much better documented since it is a largely standardized system among most car makers) and what I have found is not that detailed in regards to what I am discussing above. I really wish there was a definitive reference available (would have to come from internal GM documents, so pretty unlikely - trade secrets and all that) to make it easier to monitor and interface with this bus from outboard devices.

Obviously, the scan tool makers have this data, but they paid royally for it I'm sure. GM would not simply give it away, so unless they bought it, or reverse engineered it for their own use, there is not much available that we don't already know from the various tuner programs available.

KDirk

[2seater]

Thank you Kevin. Your explanation makes this clearer and I had not seen the reference to the IPC in the manual. Input is much appreciated.

[2seater]

Sorry about bringing up an old thread but I finally answered my original question, sort of. I have been working with TunerPro and a terrific guy there that helped me finally get a good recording interface for charting engine performance. Part of the process is to send a command to stop BCM communication after connecting the laptop to the ALDL. This does have the negative effect of turning off most of the IPC and the climate control goes into a default mode, but the information recorded appears to have very good synchronization. The TunerPro RT program is free and I made my own bare pin connector cable for laptop connection to the ALDL for <$15. Recording capacity limited only by the capacity of the computer, and I did so for over one hour on a drive home. The laptop I use is about (13) years old and is now the "garage computer", so nothing very powerful is needed. I keep one of the selected readouts on mph so I do have a speedometer of sorts. A datastream file is required from the website and I am not certain if the one they developed for me is available yet. PM if additional info. is desired. This program can also be used for modifying programming if you have the chip burning facilities, which I do not.