Car in shop, trouble diagnosing, battery light

[mark2000]

Hi all, the mechanics took a look at my P5 03 today and couldn't diagnose the problem.

Problem started last night, battery light came on. Light went off as I drove. Back on at all stop lights, eventually, it was almost always on by the end of my 15 minute city drive home. No other issues appeared to be wrong.

Today, in hopes it was an intermittent problem that would resolve, I pulled the battery terminals and cleaned them (lots of corrosion). Inspected IAT Sensor wires. No dice, the battery light came back on. I barely made it the 10 minute drive to the shop - rough idling, trouble shifting into second, a stall, battery light on and off, etc. Shop did the usual tests:
- Alternator/Battery/Starter - all showing good.
- "Ignition system" (don't know what that is) proved to be OK.
- Cleaned and inspected spark plugs and wiring (last changed 2-3 years ago)

Tomorrow, they will check fuel-related systems and more. The mechanic suggested it could be the transmission. In my experience, transmission problems are gradual, not sudden - I have had no issues with the transmission to date. Mechanic says "there's not enough power being delivered when I press the gas".

I would like to go back to the shop with a couple ideas in the morning in hopes of staving off huge labour costs for diagnostics which don't find anything. Any ideas of what else this could be?

 

[Maroon]

It's time for you to find a mechanic that doesn't try blaming your ignition system, fuel system, and (for the love of God) the transmission for a battery light. Find a new mechanic ASAP.

I think you simply have a bad alternator. When your alternator dies, your battery drains. And when your battery is drained, nothing works as it should.

Maybe I'm wrong, but from the description you've given, I think you're being taken for a ride.

 

[mark2000]

Hey there, I sincerely appreciate the response so quickly!

Well, they showed me on paper, the voltages and test results of the battery and alternator and they are looking fine. Which makes sense, as the battery is not that old, I changed it myself not long ago. Voltages are looking to be right on spec. The mechanic who worked on the spark plugs and wires (right in front of me) fully admitted that he didn't know what was wrong and will have to ask a more senior technician tomorrow - so I know that he wasn't as experienced as he could have been and he was actually pretty good about it, talking me thru his work, and having me hang out in the garage - so I don't think there's anything sinister there in terms of him coming up with bogus solutions.

The car has 150,000 KMs (not miles) on it and is only used about 2 times a week for city driving.

I need it repaired by Thursday and my fear is that they will come back and say its a transmission problem. This shop does not do work on transmissions, and therefore I won't get it fixed in time. In reality, do these symptoms indicate anything that could be a transmission (automatic) problem?

 

[MrGiggles]

I doubt that it's a transmission problem. The shifting issues are likely a side affect of whatever is going on.

Unfortunately intermittent issues like these are hard to troubleshoot, but by the symptoms it may be the PCM going out.

Have you spilled anything down the cupholders lately? The PCM is right below that, many have died from having beverages spilled in them.

 

[mark2000]

I haven't spilled anything down the cupholders any time recently, but it's happened in the past.

What is the cost to replace a PCM these days if I get one used? Parts/labour. Is it worth it?

To me, I agree - the erratic shifting sounds like a RESULT of whatever is going on - not the problem itself.

 

[Maroon]

Well, good. It sounds like you're more on top of it than I thought. Some people drop off the car and go with whatever the mechanic says.

I still think the guy is way off base by checking ignition, fuel, and the transmission. He sounds inexperienced and because of that, I think he could have checked the alternator incorrectly. What was the voltage output at idle?

Nothing indicates that the transmission is bad. Everything you've described is more than likely a symptom not a cause.

 

[mark2000]

I don't have the sheet with me but from what I remember, everything was at the 12/13v range, andwithin a tenth of a volt off what their spec said. I'm sorry I can't be more specific right now because I don't have the sheet yet (I still have to pay).

I DO think there may be merit in his assertion that it could be related to something, re: fuel injection system. Perhaps the fuel injectors or the EGR valve(s)(?).

The reason think he might find something down that road, is because if the gas isn't being properly
Injected into the engine, the engine won't run properly, causing erratic RPM, which then leads to problems keeping the alternator charged consistently, which then could lead to a host of other electrical problems that cars are sensitive to and cause the battery light to go on. And because the problem (stalling, jumpiness from a stop) seems to happen the worst when you first try to give it gas, he might have something there.

any chance this could be the right path To start looking into next?

 

[MrGiggles]

His theory has merit. Alternators don't charge well or at all at low RPM. Sticking EGR valves frequently cause rough running and low RPM on these engines. If this had been the case, the battery light would go out as soon as you gave it a little throttle and got the RPM to at least 700.

However, the battery would have to have been weak in order to cause all of the electrical problems. This should have failed the test, unless they charged it beforehand, which should have been noted. I don't think the shop would be dumb enough to overlook a dead battery and alternator light and continue to pursue other potential problems.

Fuel injectors seldom cause problems, they are very robust devices.

 

[Maroon]

Your rpm's would have to be really low to set off the battery light. I've always had idle issue with my car, and no matter how bad it's been the engine has to be practically stalled to set off the light. 2-300 rpm and the light isn't on.

You said the light was almost always on at the end of your drive. This indicates to me that the light is on even off idle. Does the light go off when you rev the engine while parked? If it stays on then the issue isn't associated with the idle.

Your alternator should be putting out about 13.5 volts at idle. <13.0V is bad.

 

[pcb]

... Have you spilled anything down the cupholders lately? The PCM is right below that, many have died from having beverages spilled in them.

The PCM is located under a kick plate under the passenger side floor.

Our car has a control wire running from the PCM to the alternator.

If that wire has a bad connection or is broken, the alternator effectively shuts off.

Make sure you check that wire for proper connection.

 

[mark2000]

Wow, that looks like a really small part and by the looks of it, a DIY repair. How much do these typically cost if pulled from a junkyard and are they easy to install myself?

 

[mark2000]

Hey all
The shop tried new spark plugs and checked the "fuel lines" for blockages (sorry if I'm not saying that right but I'm not 100% sure what I'm talking about when it comes to cars sometimes). They said none of those had any impact.

They said that there seemed to be an air pressure issue in the intake (or something like that) and are going to test if changing the hose (I think they said the air intake hose) makes a difference. I told them about how the IAT sensor is on the same circuit as the battery light in this car and maybe that's why the light was the first indication of problems.

Is it Possible there are issues with the intake hose or the sensor, which would then lead to sensor issues and fuel mixture issues, causing these wonky stalling, accelerating and misfiring issues and making the battery light go on? Is this a plausible route to be heading down based on the symptoms?

 

[njaremka]

if thats the original alternator in the car, and you have over 150k kilometers, your alternator is going out. as was mentioned, time to find a new mechanic. at idle, the alternator should be putting out a constant 13.5 volts to the battery, measured at the battery terminals.

 

[mark2000]

The alternator was showing over 13 I am pretty sure. It was around 13.5-13.7 from what I recall (I'll check for sure later). However, here's a supposition which may rule it out: if the alternator started failing two days ago, and the car has been driven (15-20 minutes home and another 15 to the shop) and if the car has been running at the shop while they've been trying to diagnose it, wouldn't there be more symptoms by now? Ie, the shop should be seeing dimming headlights, etc, causing them to re-consider it being the alternator.

I want to take your advice, but if The diagnostics don't show an issue with the alternator, how could it still be the issue? They did say if I wanted to replace it, it would only be about $250, but they didn't recommend I go ahead with that repair, as they didn't diagnose any problems with the alternator and were not confident it would fix the car.

 

[Maroon]

Hey all
The shop tried new spark plugs and checked the "fuel lines" for blockages (sorry if I'm not saying that right but I'm not 100% sure what I'm talking about when it comes to cars sometimes). They said none of those had any impact.

They said that there seemed to be an air pressure issue in the intake (or something like that) and are going to test if changing the hose (I think they said the air intake hose) makes a difference. I told them about how the IAT sensor is on the same circuit as the battery light in this car and maybe that's why the light was the first indication of problems.

Is it Possible there are issues with the intake hose or the sensor, which would then lead to sensor issues and fuel mixture issues, causing these wonky stalling, accelerating and misfiring issues and making the battery light go on? Is this a plausible route to be heading down based on the symptoms?

Is the battery light on when you rev the engine to 3000 rpm?

You may be on the right track with that IAT sensor. I didn't realize that is used as an input to the ECU for charging the battery. Maybe you can look in the factory service manual posted on this website for a way to test it.

But I really feel that your mechanic doesn't know what he's doing.

 

[mark2000]

Battery light goes out when I rev to 3k, so I was doing it at all stop lights to try and keep the battery light off. What's that indicate?

 

[peeenl]

There's a lot of wisdom ahead of me here, but I've had to replace a lot of alternators; so I'll chime in. Sometimes the alternator doesn't just 100% fail. it can be an intermittent failure. In my experience, if your battery light comes on, and your battery tests fine; your alternator is beginning to fail. They also will all fail eventually, so I agree that if thats the original; replace it.

You cannot guarantee a test on an alternator. If its beginning to fail, it could pass one minute and fail the next. The pulley could be getting some friction or be off center, the belt could be slipping, there could even be something wound up behind one of the other pulleys. The alternator isn't in the best spot on these engines, but if you're getting into triple digits on mechanic time anyway; you should replace it.

Check your grounds too. I don't even think we should call them grounds. We should call the ghosts, because that's how spooky they are.

 

[Maroon]

Battery light goes out when I rev to 3k, so I was doing it at all stop lights to try and keep the battery light off. What's that indicate?

Solid indicator that your alternator is bad. A bad alternator may only put out sufficient voltage at higher rpm's.

When was the 13.65-13.7V measured? At 800 rpm or 3000 rpm? Was the battery light on at that time? Was it running rough at that time?

 

[mark2000]

Solid indicator that your alternator is bad. A bad alternator may only put out sufficient voltage at higher rpm's.

When was the 13.65-13.7V measured? At 800 rpm or 3000 rpm? Was the battery light on at that time? Was it running rough at that time?

I don't know whether it was measured at idle or while revving (I will find out) but I assume they did their testing however a shop usually does it. If shops usually do it at idle, they probably did it at idle. If shops do it while revving, then that's probably when they did it.

Your assumption of the battery light going off when revving led me to believe the EXACT same Thing as you - that the alternator needed to be replaced. That is why, when I took it into the shop, I suggested they do that as a first step - and they did, but found nothing wrong in the diagnostic.

The other wonky indicators that show it might NOT be the alternator are: 1) the battery light isn't going on any more (perhaps that's changed after they've been running the car a bit) and 2) despite all the testing they've done and the drives the car has had since the battery light first came on, most electrical components are still working fine - ie they've not mentioned dim lights, air conditioning works, etc.

In my experience when an alternator starts to fail, it's a pretty steep slope from there - usually the car dies pretty fast after you see a battery light - the battery only has so much reserve charge left in it right? THIS is the part I cannot reconcile. I agree with you, it would seem to be the alternator, but then how is the car still running?

 

[Maroon]

It's still running because the light turned off and the battery is charging. It's an intermittent problem which has never been off the table. Given this, it's not at all surprising that the car is still running.

Running any of these tests you've mentioned while the battery light isn't on is useless. It's intermittent so you have to test when the symptoms are showing.

Sorry, it's really not clear when the light is on and when it isn't. Or when the car is running rough/low idle or when it isn't. Up until your last post, it sounded like you barely made it into the shop, and it has consistently had the battery light on - at least at low rpm. But now that doesn't seem to be the case.

At this point, I'm not even sure that the battery light has to do with anything. If I had a really bad idle causing stalling - and consequently a battery light - I wouldn't describe the issue as a "battery light", I would describe it as a "stalling problem". That's not what's happening here, is it?

Have you checked the battery ground, power lead (not at the battery, but to wherever it goes first), and the connections at the alternator?

I agree that alternators don't typically fail intermittently. They usually fail at low rpm or fail altogether. But it's not out of the question that it's intermittently failing.

Not sure where the link for the factory service manual went, but I would find it and follow procedure for testing the IAT sensor.