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Sherlock Ohms

Car Radio Fires Up Check Engine Light

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Rob Spiegel
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Weird connection with the radio
Rob Spiegel   4/12/2013 11:05:42 AM
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This is an odd problem. Out Sherlock here was able to figure it out, but I wouldn't bet on garage personnel getting this right.

tekochip
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Re: Weird connection with the radio
tekochip   4/12/2013 11:39:32 AM
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You're probably right. The first step in nearly every diagnostic checklist in the shop manual is to check the wiring, but replacing sensors is what usually happens first.
 
Even still, that was such an odd wiring problem, it would be darn hard to find.  Job well done, Sherlock.


Rob Spiegel
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Re: Weird connection with the radio
Rob Spiegel   4/15/2013 12:45:13 PM
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I agree Tekochip -- this would be a hard one for the repair shop. This simple problem could have cost an arm and a leg to fix at the shop. In this case, the solution may have bewewn to go to a dealer's shop, since the dealer may have info prompted by numerous occurances. Then again, maybe not. 

Tom-R
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Re: Weird connection with the radio
Tom-R   4/17/2013 3:55:32 PM
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My car radio problem starts with the sliding door in a van. The automatic door stopped working, so I checked the system and it wasn't the motor or switches. It was something with the board, so I took it out and thought I'd replace it. But at $600 used, I didn't bother. The kids went back to opening that door manually. (Radio is yet to come) Next thing to go was the rear hatch controller. Okay we're not even touching that since I know the price already. Then the CD player in the radio stops working and won't even give my wife back her favorite ones. I'm definitely hating automotive electronics at this point, and figure I need to get something new soon. Now comes the weird part. Ford has a recall on the torque converter. It's something mechanical that I understand. But when you're working near the starter you disconnect the battery to prevent accidentally shorting the contacts or anything else. So the shop had my vans battery disconnected for almost the entire day. A few days after we got the van back my wife thanks me for fixing the CD player. I said it wasn't me, and went out to check it myself. Sure enough it worked like nothing was ever wrong with it. That got me thinking about the battery, and figured the electronics needed to be completely powered off to let it reset. So on a whim I pushed the rear hatch button. It worked too! Now I only regret not leaving the sliding door's controller in place! I know I'd had the battery disconnected for other work, but never for an extended period. That was what it needs for everything to reboot.

Rob Spiegel
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Re: Weird connection with the radio
Rob Spiegel   4/19/2013 12:19:47 AM
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Hey Tom-R. You comment would make a good Made by Monkeys entry. Can I use it? If so, please let me know the make and year of the vehicle.

Thanks, Rob Spiegel

rob.spiegel@ubm.com

lgrant
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Bronze
Re: Weird connection with the radio
lgrant   5/14/2013 9:52:35 PM
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I once bought a used Plymouth Breeze. This car had a lot of problems: I had to have the timing chain replaced just after buying it, and about a month later, the head gasket blew out, complete with chocolate-milkshake-like stuff coming out of the tailpipe.  I got that fixed, and the car ran OK for several months, and then it would periodically lose power and start running really badly.  

A trip to the mechanic revealed the problem: when the head gasket blew out, it allowed cooling water into the crankcase.  Apparently whoever replaced the head gasket did not do an oil change.  So the water in the crankcase rusted out the main bearings.  When they rusted out enough that there was a lot of slop in the crankshaft, the engine would make a lot of noise under load. This was enough to trigger the knock sensor, which is essentially a microphone, and it would mess with the fuel/air mixture, trying to make the noise go away.  This caused the engine to lose power. 

Rather than pouring more money into the car and replacing the main bearings, I just gave it to charity and bought another car. 

Rob Spiegel
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Re: Weird connection with the radio
Rob Spiegel   5/15/2013 4:00:24 PM
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Hey, Igrant. I'd like t use this story as a Made by Monkeys post. Let me know if I can via my emial below:

 

rob.spiegel@ubm.com

Rob Spiegel
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Re: Weird connection with the radio
Rob Spiegel   5/15/2013 3:58:16 PM
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Hey, Tom-R. I'd like t use this story as a Made by Monkeys post. Let me know if I can via my emial below:

 

rob.spiegel@ubm.com

Tim
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Re: Weird connection with the radio
Tim   4/14/2013 10:01:10 PM
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The garage tech would most likely tell you that there is no way that your radio could have any effect on your engine and then spend your money replacing all of your sensors before bothering to pull the radio.

Rob Spiegel
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Re: Weird connection with the radio
Rob Spiegel   4/16/2013 4:07:56 PM
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You are so right about that, Tim. And when they find the true culprit, they will likely tell you that, yeah, it was worn-out sensors.

savaden
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Iron
Re: Weird connection with the radio
savaden   4/15/2013 11:07:32 AM
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I suspect that the poor ground connection was creating noise on the data bus that was confusing or swamping out the data signals. Since typically ground is common to all car functions, it would not have to be part of that particular data bus to wreak havoc. As far as standard troubleshooting technique, I participate in a BBS dedicated to vintage Porsches. Whenever someone posts that their turn signals are not working correctly, the universal reply is "Check your grounds!" Even on old cars this is not so easily done as there are numerous connections. On newer cars (2003 seems to be a bad year for GM) there are grounds hidden all over the place. Anyway, however unlikely the solution to this problem might have been except by chance, the standard troubleshooting procedure to check the grounds first is not unknown to the experienced mechanic.

Rob Spiegel
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Re: Weird connection with the radio
Rob Spiegel   4/16/2013 4:11:53 PM
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That's terrific that your BBS is suggesting that grounds get checked before ripping into other possibilities. 

bob from maine
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Platinum
Unfortunate but true
bob from maine   4/12/2013 1:44:43 PM
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This article demonstrates the problem with modern-day auto technicians. You can teach theory until you're blue in the face but you just can't teach common sense. Every Sherlock Ohms article appearing here should be put into a book and made required reading for all budding technicians. I have yet to see a simple fix that would have been found using 'standard' troubleshooting. There was one where when the truck was parked in a rutted field, it wouldn't start. No problem anywhere else but every time this operator parked in this particular field, the engine wouldn't crank. It turns out the engine ground strap was designed just a little too short and with the left front wheel 8" higher than the other 3 wheels the ground strap would be pulled taught and lose contact. This required the technician (me) to accompany the owner to his work-site and even then I had to use jumper cables to solve the problem which then inferred the battery ground was being lost. Using a jack in the parking lot proved the theory. I lost 4 hours I couldn't bill that day but made a customer for life.

Elizabeth M
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Re: Unfortunate but true
Elizabeth M   4/16/2013 4:33:55 AM
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I had a similar incident occur one time with a Jeep Cherokee many years ago (probably 1999). I think the sensor systems were new then, but a check engine light ruined a trip i was taking with my then boyfriend, as it came on soon after we were driving at night from Phoenix to Utah. The car was new so we didn't know what it meant, even though the car was running fine. In the end it was something silly that triggered the light--sensor-related--and there was nothing wrong with the engine, but we turned around early in the trip and came home. Unfortunately, my CD player/car radio was stolen out of the jeep the next night! Had the check engine light not come on, we would have been away that weekend and it wouldn't have happened. So there you go! Not so much fun when this sort of glitch occurs.

3drob
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Platinum
Re: Unfortunate but true
3drob   4/16/2013 4:29:37 PM
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Older Jeeps (early to mid 1990's I believe) had a more insidious problem.  The radio was on the engine data bus.  If the radio had problems it could swamp/inhibit the bus and the vehicle would die.  Not a nice "limp-home" mode, but "call the tow truck 'cause you aren't going anywhere" mode.  Just a silly System's Engineering oversight (or cost savings), but with huge potential ramifications for customers.  I guess you should consider yourself lucky.

As cars become more complicated (especially drive by wire systems), Systems Engineering becomes the critical link.

ASmith
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Iron
Could it be Data Bus Corruption?
ASmith   4/12/2013 4:01:00 PM
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Not too long ago I had to replace a passenger door latch assembly in our 2007 GMC Yukon.  While doing so, the ignition was on accessory and I was listening to the radio.  (Yes, the first rule is always to disconnect the battery, but this seemed pretty benign as far as the likelihood of causing a dangerous short.) 

Disassembly of the door required unplugging the window/lock switch module in the door from the wiring harness.  I discovered that each time the module was unplugged, the radio would change stations!  Factory service manuals later confirmed that the door switch module and the radio were both connected to a common and very simple low-speed (~30kbps if I remember correctly) data bus.  Unplugging the module must have introduced noise or power-down signalling or something onto the bus in a way that made the radio think it was being addressed and commanded to change stations. It makes one wonder what else could have been commanded?

It seems plausible that the radio in the Saturn may have shared a data bus with the engine and/or transmission controllers.  If the bad ground caused it to make the bus inoperable, it could explain the observed behavior. 

I recently watched a diagnostic video online (look for scannerdanner if you are interested) showing a case where a shorted sensor pulled a 5v reference from the engine computer very low and the vehicle would not start -- fuel pump inoperable, no spark, etc.  Perhaps a radio without a ground could be driving a data bus high or low to the point that it impact other related circuits, including something related to a knock sensor.

Ratsky
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Platinum
Re: Could it be Data Bus Corruption?
Ratsky   4/15/2013 1:05:34 PM
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Some more information:  GM, like all automotive manufacturers I am aware of, uses a multi-bus vehicle architecture.  Typically, there's one for "infotainment" (including radio/head unit, navi, and sometimes HVAC and other "comfort" associated subsystems), another for the drive train, another for body-control stuff (doors, windows, etc.), and sometimes even another for safety systems (airbags, etc.).  GM uses a "single-wire CAN bus" for infotainment which is relatively low speed (maybe 100Kbps); they are alone among major manufacturers using this "flavor" which is completely dependent on solid continuous grounding of all elements on that bus.  There is always a gateway as well that allows communication among nodes on the various buses (example: a NAV head unit can get wheel ticks and steering angle from the drive train bus to perform dead-reckoning during loss of GPS signal).  It is possible that garbage on the infotainment bus due to a bad ground oculd create havoc on other buses via the gateway, although a PROPER system design would prevent that.  Just about every other manufacturer uses the 2-wire physical layer version of CAN instead; since that is MOSTLY differential, it is less dependent on grounding, at least within the common-mode tolerance of the (industry-standard) 2-wire drivers.  The MOSTLY is because there is one aspect of signalling that IS ground-dependent even on the 2-wire type.  This is the "wake-up" signal which uses a common-mode DC shift as its signal mechanism.  BTW, the commonly used physical layer for the "body-control" part is LIN, which is a very low-speed single-wire type.  The primary motivations for using single-wire buses at all is very simply the severely inflated cost of copper, and WEIGHT.

William K.
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Platinum
Poor radio ground and bad engine problems.
William K.   4/15/2013 12:17:43 PM
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This is the sort of problem that will pop up when shortcuts are taken in the wiring between different modules in the car. The radio ties to the control module so that it will only function in that vehicle, so a stolen radio is worthless. I learned about that after purchasing a radio from a car in a scrapyard. The radio needs to handshake with the BCM and ECM in order to work.

Now if the connection gets noisy all sorts of bad data will be sent to all of the modules. The reason that the poor ground caused problems is that the harness does not include a dedicated common for the data bus that has a low enough impedance to allow adequate connectivity without the additionsl external grounding connection. It might function correctly with all of the wiring and modules sitting on a display board and no external connection, but in the autobody environment it needs that extra ground connection.

Good job on finding the problem though. 

timbalionguy
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Gold
Multi-function radios!
timbalionguy   5/17/2013 7:00:29 PM
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I went to replace the radio in my 2004 Ford product, after the CD player failed. I was quite surprised-- and puzzled-- to find a whole bunch of wiring going to the radio that had little to do with the function of the radio. I did eventiually figure out which wiring was the radio, and the installation worked just fine.

I later found out that the radio's display was used as part of the OBD2 system, as a diagnostic display. So now, I have great tunes, but no diagnostics. I thought tis was a poor place to do this, as aftermarket car audio is a very popular thing. It soulds like this kind of thing is becoming more common, which means in the future, I am stuck with whatever the vehicle manufacturer installs for a radio.

One other thing I noticed is that Ford used an antenna connector unlike anything I had ever seen before. I had to make a pigtail adapter to allow the installation to be completed. I probably now have one of the very few cars anywhere with BNC connectors in the radio antenna line!

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