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05 Malibu bad PCM, BCM or other cause?

posted Jun 02, 2016 04:34:59 by Jim Ryan
I am a DIY'er and after completing Danner's instructional videos while following along with the book I bought a (mechanics special) project car to reinforce / embed my new knowledge (and to apparently remind me that I'm not as knowledgeable as I thought I was).

The car will only start and run if the humidity is above 90% and then it will try to die but I can keep the RPM's up and keep it running.

All other times (KOEO) it will not start and the lights in the cluster are blinking, gauges are sweeping up and down, odometer blinks "ERROR", relays under the hood are clicking, throttle actuator going crazy and fuel pump relay in the trunk is going nuts. Most of this still goes on even if the engine is running.

The starter will only engage for a split second (random & erratic) using the key, but will crank normally if I use a start button jumped to the starter terminals. Sometimes I have injector pulse and spark, sometimes I don't.

I have checked all powers & grounds with a DVOM, cleaned most all of them anyway and added Stabilant 22 to them just for added protection (including computer connections). All were way under 100mv.

I have scan tool communication, showing a couple ABS codes and one fuel sending unit code. I have disconnected every sensor I can think of, disconnected modules, tried a different battery and tested the data bus with a Uscope (still learning with the scope).

I really think the problem is a PCM, but the intent here is to learn how to be as sure as possible through testing.

Any suggestions would be very much appreciated.
P.S. I bought the car from a friend. It sat in his yard for one month before I got to it. Prior to that it had never left him stranded but had been giving him these driveability problems for months.
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8 replies
IvanKibuka-Kiguli said Jun 02, 2016 04:54:01
Based on the instrument cluster activity and the unusual cranking activity, I would start with the ignition key barrel just to be sure it isn't pulling my leg ;-)

If that checked out, I would have a look at the wiring diagram to check out the power, ground and communication relationships between the PCM, ICM and BCM.

PS: You didn't say which make, model and year the vehicle is. The guys with access to wiring diagrams will likely need that information to help out.
[Last edited Jun 02, 2016 05:02:07]
IvanKibuka-Kiguli said Jun 02, 2016 07:04:20
Hello Jim. Not sure which engine you have got but I looked at the wiring diagrams for the 2.2L and noted the following:

The instrument cluster is fed by 2 powers. One is hot all the time via the 10A 'Cluster/Theft' fuse on the BCM. The other is hot with the key in the run and start positions.

The starter motor is controlled by the 'Starter' relay in series with a 30A 'Starter' fuse. The coil side of the starter relay is controlled by the PCM. Relay is powered by the PNP in series with a 15A 'IGN 1' fuse and 'Run/Crank' relay. There is also a 'Run/Crank' relay whose coil is itself controlled by the BCM after receiving a voltage signal from the ignition switch. This relay sounds very interesting in your case.

The throttle actuator motor is controlled directly by the PCM using 2 wires.

The fuel pump is also controlled by the PCM via the 'Fuel' relay in series with a 15A 'Fuel Pump' fuse. PCM controls the power side of the relay coil.

The injectors are controlled individually by the PCM on their ground sides and are powered by a single 10A 'INJ/Injector' fuse.

The ignition coil module is signaled by the PCM on when and what coil to fire and is powered the 15A "IGN 1' fuse.

If it were me, I would start by looking at voltage drop across the ignition switch contacts in ACC, Start and RUN positions. I hope this is of use to you and good luck with this weird case.
Jim Ryan said Jun 02, 2016 12:40:24
+IvanKibuka-Kiguli I apologize for leaving out the specifics of the car. That is important information to know!

2005 Malibu LS 3.5L 142,000 miles VIN: 1G1ZT52845F312382
Clean car, no rust, no corrosion issues, excellent mechanical condition.

I will focus on the ignition switch next. I had dismissed it before because the problems also happens KOEO, but I can now see how the switch is a major component in the system.

Thank you for the tips. It may be Saturday before I have another chance to work on the car. I will post an update as soon as I know more.
Tyler said Jun 03, 2016 02:18:52
Hey Jim, welcome to the forums!

I like where Ivan is going with the wiring diagram information. I get the sense that you're chasing a common issue, but that may just be wishful thinking!

For me, given the number of different symptoms you have, I'd suggest picking the most repeatable one and focus on it. The starter engagement issue, or the instrument cluster, for example. The starter itself and it's power/ground are obviously OK ('cause you can bypass the relay), so I'd probably pull the starter relay and see what's missing when the engagement is erratic. If you find a control issue, then that'll be another strike against the PCM.

The TAC going nuts is downright creepy, 'cause that also points to PCM. I can't think of anything the BCM (or any other module) could do to cause that.

You could also isolate another one of the relays that's acting up, and do the same kind of testing on it, see where it takes you. If you find more signs that point to the PCM, then we'll start thinking about calling a computer.
Jim Ryan said Jun 06, 2016 04:20:09
OK I had some time to investigate further...

I tested the ignition switch to the best of my ability. The schematic doesn't show much detail and doesn't show all the wires at the switch (barrel section). The white (crank) wire shows battery voltage KOEO, but that drops to 4V when cranking.
Is that enough (4V) to satisfy the PCM?.

The white/black wire (5V Ref) stays at 5V key in, 5V key out, 5V on accessory but drops to 4V KOEO and cranking.
Some of the voltage drop is due to the excess loads that are cycling on and off with KOEO.

I verified the powers and grounds at the PCM and BCM but I'm an amateur when it comes to verifying the communication between the two and I'm not sure what to do next for this step. There is what I believe to be a good waveform at the DLC and I do have scan tool communication.

The Run/Crank relay #32 has battery power present from the PCM but the PWR/TRN relay #33 chatters due to a poor ground from the PCM (also verified at the PCM pin 24
"main relay control"). If I touch a grounded test light to the control side/ground terminal, the relay goes silent and the voltage drops to 0 on that ground pin.

I measured 19mv at the PCM main ground wire (backprobed at PCM pin#73 measured to battery neg.) so I believe the main PCM ground is good.

There is no ground signal at the Starter relay #31 measured at the relay control wire and at the PCM #23 pin. Today, this car wouldn't even crank. It seems the symptoms are worsening week by week.

No 5V present on reference A (MAF:gray) and reference B (AC Comp.:gray)
No 5V ref signal measured at the TAC, pin 13 (wht/blk) and also pin 1 (tan)

I also measured constant 12V coming from the PCM pin 37 (fuel pump rly ctrl) and causing the fuel pump to run constantly

This is the first time I have been involved in troubleshooting a potential PCM failure, but it does seem that the PCM is the common thread with all these test results and therefore my educated guess is to replace the PCM.
Would you agree?

Thank you,
[Last edited Jun 06, 2016 04:24:35]
IvanKibuka-Kiguli said Jun 06, 2016 04:59:04
Hi Jim,

Looks like you are strolling down PCM Avenue with this one. It is possible one sensor is shorting out the 5V ref elsewhere (you will need to unplug all sensors that use the 5V ref to prove this) but 12V feeds falling to 4V in KOEC? Weak battery? The fuel pump running all the time is another weirdo. So many things are pointing to a bad PCM but like Paul always does, I am trying to think if there is anything missed (besides unplugging all the sensors mentioned above). Hmmm!
Jim Ryan said Jun 06, 2016 12:41:49
One of the first things I tried when I bought the car was to unplug each of the sensors in hopes of finding a simple fix. No such luck!

I didn't have every one unplugged at the same time but over the past few weeks I have unplugged each, one by one and then tested the car for any improvement. I don't think I missed any sensor / module.

Yes, the voltage would drop to 4V even though I had a battery charger on the battery and I removed the charger only during testing with a scope.

I've considered opening up the PCM case for a visual inspection. I don't know if it is common to see a burnt trace / component or if the failed part won't be easy to find.

My local Pronto auto parts store can get a programmed PCM for me within a couple days so I think I'll order one. If I still have these issues afterwards, I'll just have to consider it money well spent on my education!

Thank you for your guidance, it helped get me through the point where I was stumped and didn't know what to do next.
[Last edited Jun 06, 2016 12:44:58]
IvanKibuka-Kiguli said Jun 06, 2016 17:58:26
Wishing you success, Jim. We don't have many Malibus in Uganda (infact I have never seen one on our roads), so I have never got the opportunity to troubleshoot one. Armchair diagnostics is what I indulged in on this one :-)
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