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Check Memo for Intermittent Electronic Problems

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a.saji
User Rank
Silver
Re: CHECK MEMO
a.saji   3/27/2014 6:35:06 AM
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Even I faced something very much similar but the base and the end result of it was different.   

bobjengr
User Rank
Platinum
CHECK MEMO
bobjengr   3/26/2014 6:21:46 PM
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Excellent post Jim.  I've been doing this for a while and discovered when diagnosing a system, try to work with someone like the maintenance man you mentioned in your post.  They seem to know more about hardware and equipment than someone not involved with product on a daily basis. Also, (more importantly) they know about how other hardware interfaces with equipment.   With the exception of the letter he wrote to management, I would bet there is no written record or drawing change to support his actions.  It took me awhile to learn this but I had a very similar experience some time ago.  Again, excellent post.

TRCSr
User Rank
Silver
Same problem with a Subaru
TRCSr   3/25/2014 1:31:27 PM
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I had a Subaru station wagon that was a '89 model. After a few years my wife, the principal driver, complained that the engine temperature light kep coming on and the guage had the needle pegged to the "H" side during warn weather. She would turn off the A/C, which was a problem in DC in the summer, and even turned on the heater, which wa a bigger problem, as I had told her to do if the water temp. got too high. Neither of these attempts made a difference in the problem. I took it to my local Subaru repair shop and they dianosed it as a radiator problem and replaced that (of course this was not covered by the warranty by this time) and the problem remained. Next they replaced the thermostat a second time as it was replaced when the radiator was replaced, again with no effect. They also wanted to replace the water pump, but I told them to hold off on that for a while. One day when the wife returned home from work, as usual with all windows open and the sun roof opened, I immediate set to work to find out what the temperature actually was. I measured it several ways and all showed normal temp. values. Ah Ha! It was not that the water was overheating, it was the temp. sensor. I measure the sensor value at several different temps. as the engine cooled down and was able to get a R. vs. T curve. So, the sensor was not the problem, but in the process of doing all of these I noticed that there was no ground wire from the sensor to the vehicle ground; they were depending on the mounting points for the ground return. And, the metal parts at the mounting points were pretty badly corroded. As I recall the housing that the sensor was in was Al. and the point on the vehicle was steel. I attaced a good solid copper wire directly from the sensor to chassis ground and for the next 50K or 60K miles we had no further problems with that.

Vern Klukas
User Rank
Iron
Another Bad Ground
Vern Klukas   3/24/2014 10:05:51 PM
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Many years ago, as a mechanic I learned that Fiat 124 convertibles had a similar grounding problem. The engine block to frame ground would often be bad. This wouldn't stop the car from running, because there was another ground path through the clutch cable. But over time, the current drawn during starting would temper the cable until it was dead soft, eventually snapping. We thought that what happened was the cable would start to fail, strand by starnd, until the remaining strands were heated to red heat and failed. We learned to always check the ground if a Fiat arrived with a failed clutch cable if the cable end showed blueing from heating.

Yours

Vern

Larry M
User Rank
Platinum
People don't understand grounding well
Larry M   3/24/2014 7:48:11 PM
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As noted, people don't understand grounding well.

In one instance a friend had some engine work done on his car and afterwards the air conditioning did not work. He thought the freon pressure was too low (setting a limit switch) and added some to no effect. Then maybe too high so he bled it off, still to no effect. He brought over to ask me to have a look at it.

A quick check of the compressor clutch showed that it had 12 volts across it but was not engaging. A resistance check of the clutch coil showed that it wasn't open. I observed that there was only a single wire going to the clutch coil, and that the compressor was mounted on rubber vibration-isolating bushings. I measured the voltage from the compressor case to engine ground--about 10.5 volts. Then I removed a lead from the voltmeter and held one end to the compressor case and the other to engine ground--and the clutch kicked in. I asked my friend whether the engine mechanic might possibly have discarded a ground wire going from the compressor to the engine block and he agreed that it was possible. I recommended he avoid that mechanic in the future.

In a different scenario, we've all seen in past years a car with a dim incandescent headlight. And, sadly, most of us have seen mechanics replace the headlight before figuring out that the ground lead for the headlight socket has high resistance to the auto body. (There's a reason the star washer is under the wire eyelet and not above it.)

In a much more interesting and non-automotive scenario, in the 1990s I was working for a large computer company designing PCMCIA cards (credit-card-sized feature cards for laptops, usually modem or network cards). I was called in to help another team. Their design worked in all of the company's laptops except that in one laptop it only worked in the upper slot, not the lower one. Careful study of logic analyzer traces and a couple of synchronized oscilloscope traces revealed "ground-bounce." At the start of an innocent read-cycle, a glitchy circuit caused all sixteen data lines on the card to go from high to low. They stabilized shortly afterwords, well before the data was supposed to be valid.

The problem was that as all sixteen lines were pulled low, there was so much current pulled through the ground trace on the system board that there was a significant voltage drop across the ground line. This made ground at the chip appear to rise closer to Vcc (+5 volts). The result was that negative active signals like -ChipSelect and -Write appeared to go active. Even though they were logically high (say, 4 volts), ground at the chip got so close to Vcc that they acted as low signals, not high, and garbage data was written to the PCMCIA card. This only happened on one of the two sockets, because apparently one socket was connected directly to the ground plane and the other was connected via an undersized surface trace.

We notified the department responsible for the failing laptop design and pronounced our product ready for shipment.

William K.
User Rank
Platinum
Re: Poor grounding
William K.   3/24/2014 7:28:19 PM
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SherpaDoug , I find it amazing that anybody would put a ground wire lug in that area over a fuel tank. That is just careless, and rather dumb. But thanks for the clarifying of who did it. In the auto plants they are very careful about fuel tanks.

SherpaDoug
User Rank
Gold
Re: Poor grounding
SherpaDoug   3/24/2014 7:07:08 PM
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It was about 15 years ago.  The dealer had restyled the interior which moved the dome lights and changed the wiring.

William K.
User Rank
Platinum
Re: Poor grounding
William K.   3/24/2014 4:04:47 PM
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SherpaDoug, A grounding connection screwed into the gas tank? (I wonder if that was an OEM thing or if somebody had added the lights later on. On occasion the assembly workers mess up, but not with things like that. Penetrating the gas tank is a big deal, aside from the safety issue it is also an emissions fault. So it would be good to know if it was aftermarket, and if an OEM fault, what model and year?

SherpaDoug
User Rank
Gold
Poor grounding
SherpaDoug   3/24/2014 3:52:36 PM
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My father in law bought a new car and found the garage always smelled of gasoline after the car had been sitting overnight.  The dealer could find nothing, so he brought it to an independant mechanic who discovered some interior lights were grounded to a sheet metal screw driven into the gas tank.

Cadman-LT
User Rank
Platinum
Memo
Cadman-LT   3/15/2014 5:52:43 AM
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That's sounds like the most expensive overlooked memo ever! Ahh screw it, it was an oil company...like they can't afford it!

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