PCM issues??? (Following new alternator)

#1
I have a 2010 carolla with just shy of 100k miles.


yesterday afternoon My dashboard greeted me with a flurry of activity. The ABS, Brake, airbag, and power steering lights were all on. Also, the RPM gauge and speedometer we unresponsive and stayed at zero. Finally the left side dash display would not display any information. It would cycle through the various options (time, active mpg, average mpg, time, etc) but only one option (temperature) worked.

I took it to autozone and they hooked up a voltmeter type (large) device to the battery and diagnosed it as the alternator. So I took it to a mechanic who concurred and replaced it.

However, upon picking the car up the same lights were still on. So the mechanic took it back into the shop and ran some more diagnostics then came back saying it wasn’t throwing any codes but the lights won’t go off. Based on this they are saying it’s the PCM (power control module). But after I turned it back on again, now the check engine light is on (in addition to the others). Brought this up with the mechanic but insisted it was still likely the PCM.

I couldn’t find any mentions of PCM issues when searching this site.

Has anyone run into this before? My concern is that we’re not actually sure it’s the PCM, and if it is then I’m also wondering if the alternator was fine and misdiagnosed due to the faulty PCM.

Any advice is welcomed. Thanks!
 
#2
I would think its not the PCM. ABS, Brakes, Airbags, power steering and cluster are their own modules. I would start looking at your battery, battery cables and grounds, look for corrosion. Youtube search for videos on voltage drop testing. Find a better mechanic with better diagnostic tools that can talk to individual modules to see what codes they have. It also could be one module went bad and is putting out a crappy signal on the canbus and is taking down the bus.
 
#3
I would think its not the PCM. ABS, Brakes, Airbags, power steering and cluster are their own modules. I would start looking at your battery, battery cables and grounds, look for corrosion. Youtube search for videos on voltage drop testing. Find a better mechanic with better diagnostic tools that can talk to individual modules to see what codes they have. It also could be one module went bad and is putting out a crappy signal on the canbus and is taking down the bus.
This is very helpful and thanks for the quick reply.

there is a good bit of corrosion on the battery. The mechanic suggested pouring some coke on it to clean it up. I asked about replacing the battery and he said the battery was still good and holding a charge fine (especially considering the faulty alternator).

however, the battery is at least several years old. It’s still within warranty since it’s an 84 mo Toyota battery, but it’s getting up there and my 4Runner didn’t make it the full 84 months before needing to be replaced so I was worried about the battery as well.

pwrhaps I should clean the batter and see if that works. If not perhaps change the battery before trying the PCM?
 
#4
Don't replace the PCM until you have actual proof its at fault. Have your battery load tested and really inspect the battery cables and grounds for corrosion. Not uncommon for a bad alternator to kill a battery. Please, before you start replacing modules or the PCM, spend $100ish for an hour diagnostic time with someone that actually does understand the networks in cars and can really troubleshoot modules and PCMs. The crap gets expensive quick and if you can spend a $100 and that person can prove to you that X is the problem absolutely, its well spent.
 
#5
thanks. I will definitely clean up the battery tomorrow or this weekend when I get a chance. If that doesn’t clear it up I’ll inspect the cables then look into a new battery. Auto zone should be able to test it for me to make sure. I’ll report back if I’m still having issues, before I move ahead with a new pcm.

really appreciate your input.
 
#6
Don't replace the PCM until you have actual proof its at fault. Have your battery load tested and really inspect the battery cables and grounds for corrosion. Not uncommon for a bad alternator to kill a battery. Please, before you start replacing modules or the PCM, spend $100ish for an hour diagnostic time with someone that actually does understand the networks in cars and can really troubleshoot modules and PCMs. The crap gets expensive quick and if you can spend a $100 and that person can prove to you that X is the problem absolutely, its well spent.
thanks again. I’ve read more and talked to some other people. It’s so uncommon for the pcm to go up that I’m thinking if it is actually bad it has to be the result of something else, and thinking just putting in a new one would fail soon anyway.

I cleaned up the terminals this weekend and got rid of any corrosion. Auto zone said the battery was only at 60%, which makes sense since I haven’t driven it since I got the new alternator in (but I would have expected the shop to charge the battery). So I took it for a drive to charge the battery and came back and was told there is no charge in the battery... but no issues starting multiple times, so I’m really confused (let’s see if it starts tomorrow morning... haha)

I have a diagnostic appointment tomorrow at a different shop so hopefully they can figure it out. If not I’m taking it to the dealership.
They say things come in threes, so hopefully this is it since I have a plumbing and deck issue to deal with as well haha.

really appreciate your input gedster.
 
#7
Update: Solved!

gedster, thanks again for your input, and steering me away from the (incorrect) PCM conclusion.

i took it to Toyota and after a full day of investigating, the determined the steering sensor went bad. They also said it looked like the spiral cable assembly was chewed on and will need to be replaced soon, so they recommended doing it while everything was apart. All in all it’s going to cost $1380 but this will solve the problem. It’s still not clear if the car needed a new alternator, but that’s done now so it should last. Another mechanic I spoke with doubted the alternator was even bad since this issue was unrelated, the battery was charged, and the battery light never came on.

hope this info helps if anyone finds themselves in a similar situation.
 
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