Engineering Investigations
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Resistor_Smoker
The car was a Daihatsu Charade, a 1990 model. The lights did work by switching ...
JobT
Talking of strange car wiring faults and quality control, on one occasion the ...
Lights aren't so bright (literally and figuratively)
Taro Deneve
10/10/2011 11:52 AM EDT
The problem was simple: One headlight in the car was brighter than the other. My father, who is a mechanic, immediately diagnosed the problem as a poor ground. He connected a new wire from the terminal marked ground on the light, but this only made a marginal improvement. My brother then knew that it must be a faulty wire and ran a second wire from the switch to the high beam positive. However, this had no improvement. Connecting it to the low beam didn't result in any improvement. The light itself operated fine when a positive wire was run directly from the battery, eliminating it from the problem.
It was then my turn to have a go at it. I decided to measure the voltages of the good light, and compare them to that of the bad light. However, when I disconnected the good light, the bad light went out. When I disconnected the bad light, it had no influence on the brightness of the good light. I knew that the lights were wire up in parallel so the good light should have no influence on the bad light. It was as if the good light was behaving as a load in parallel, but the bad light was acting like a load in series.
I then decided to ground the ground terminal on the good light to make sure it was operating normally. However, when I did this, I blew the fuse. This should have been enough for me to work out what the fault was but, at the time, I was still at a loss.
I then decided to check the operation of the switch. I got my father to disassemble the steering wheel cover so we could access the switch. Upon inspection, it appeared that the switch was switching the negatives to the ground, rather than the positives to the light.
I now had the feeling I knew what the problem was. I spent a quarter of an hour drawing up the circuit diagram of the lights. The only way I could make it work, and replicate the fault behavior, was if the ground was actually +12V and the high and low beams were both ground.
This explained why the lights were behaving like they were. With the positive terminal (marked ground) grounded on the bad light, a small amount of electricity was flowing from the good light to the bad. This was because the path to ground was of lower resistance through the filament than through the switch. Of course, as the light brightened, the resistance would increase, sending more through the switch. This is why the lights were also dim.
The solution was simple, I just ran another wire off the +12V from the head light circuit to the “ground” terminal of the bad light.
I had mixed feelings about finding the solution to this problem. On the one hand, I did solve it, on the other, I could have solved it a lot quicker, had I realized what the symptoms meant.
Taro Deneve is a Systems Engineer with experience in Grid Connect Solar Electricity Systems, ATM repair, laser printer repair, power tool repair and an Amateur Radio Operator.
It was then my turn to have a go at it. I decided to measure the voltages of the good light, and compare them to that of the bad light. However, when I disconnected the good light, the bad light went out. When I disconnected the bad light, it had no influence on the brightness of the good light. I knew that the lights were wire up in parallel so the good light should have no influence on the bad light. It was as if the good light was behaving as a load in parallel, but the bad light was acting like a load in series.
I then decided to ground the ground terminal on the good light to make sure it was operating normally. However, when I did this, I blew the fuse. This should have been enough for me to work out what the fault was but, at the time, I was still at a loss.
I then decided to check the operation of the switch. I got my father to disassemble the steering wheel cover so we could access the switch. Upon inspection, it appeared that the switch was switching the negatives to the ground, rather than the positives to the light.
I now had the feeling I knew what the problem was. I spent a quarter of an hour drawing up the circuit diagram of the lights. The only way I could make it work, and replicate the fault behavior, was if the ground was actually +12V and the high and low beams were both ground.
This explained why the lights were behaving like they were. With the positive terminal (marked ground) grounded on the bad light, a small amount of electricity was flowing from the good light to the bad. This was because the path to ground was of lower resistance through the filament than through the switch. Of course, as the light brightened, the resistance would increase, sending more through the switch. This is why the lights were also dim.
The solution was simple, I just ran another wire off the +12V from the head light circuit to the “ground” terminal of the bad light.
I had mixed feelings about finding the solution to this problem. On the one hand, I did solve it, on the other, I could have solved it a lot quicker, had I realized what the symptoms meant.
Taro Deneve is a Systems Engineer with experience in Grid Connect Solar Electricity Systems, ATM repair, laser printer repair, power tool repair and an Amateur Radio Operator.
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zeeglen
10/10/2011 3:42 PM EDT
There was a time and place where one dim headlight was a common occurrence.
In winter the police would go through a pub parking lot and on each vehicle wipe one headlight clean. The other headlight was left mud-spattered.
The idea was that at night any other police patrol could easily see that the oncoming vehicle had been parked at a pub since one headlight had been wiped clean, and thus the driver of said vehicle was pulled over for a sobriety test.
This worked for a while, then pub patrons began wiping both their headlights clean before driving home at closing time.
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David Ashton
10/11/2011 6:57 AM EDT
Reminds me of a nice story. Police waiting outside a pub one night. Guy comes out, staggers/weaves his way to his car. Gets in, sits for a bit. The switches the wipers on. Then the lights. then starts it. Goes forward into the hedge a bit, then backs out of the parking. Eventually exits the pub, narrowly missing the gate. Police stop him 20 yards down the road and breath test him while the other patrons are leaving. Guy slurs his words, stuffs the cops around a bit, then blows into the machine. Reading is zero. Cops test again. Zero. "What's going on?" mutters the cop. "You're drunk as a skunk!" "Not really," guy replies, suddenly sober. "I'm the designated decoy!"
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Oh_Roy
10/18/2011 5:59 PM EDT
I like the story
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Bert22306
10/10/2011 5:14 PM EDT
"There was a time and place where one dim headlight was a common occurrence."
So true! This ties back to the recent thread on whether quality of products has been getting worse lately.
The reason for that dim headlight, or other light, was often nothing more than a bad ground. And the bad ground was caused by shoddy design and shoddy workmanship, or most often both.
My 1980 Mustang had a weak blinker light, up front on the right. Eventually, it got so bad that the right blinkers quit blinking altogether. Not enough current through the mechanical blinking gizmo in the fuse box.
A quick peak under the front bumber revealed a bad ground. Easy enough to fix, only because thank goodness I looked at the most likely place first, and the problem was there. It could have been a nightmare to find.
But it should never have happened. It's not so hard to understand that connections, especially in places where water and winter slush are likely to go, have to be made obsessively water tight? Or better yet, protect the connectors from the elements.
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Sheetal.Pandey
10/11/2011 4:54 AM EDT
Its fun to solve these practical problems. I remember when I finished engineering, and our TV was not working, my mother asked me can you repair it. I said NO,dont know how to repair the TV. I remember opening a transistor and microwave, was successful in microwave case.
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sharps_eng
10/11/2011 5:28 PM EDT
But if you fix your gizmo, you end up with an old but still-working gizmo, not the new gizmo you really want. Much better to give it to a promising young'un to dismantle, so that you will definitely never use it again.
I had to grit my teeth when I heaved our trusty old SONY 26in TV, resolving not to fix it because we wanted a nice new flat screen.
I don't want smooth reliable analog TV, I want a TV that judders and can crash, just like all my other digital gizmos.
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LiketoBike
10/14/2011 2:44 PM EDT
I must be entering old-fart-hood :-) Half the time at least, I want the old but still-working gizmo because I am more familiar with it, or it works better, or the new one omits some feature I want, or maybe I just have a nostalgic attachment to it (e.g., it belonged to a family member).
Maybe this ties into the quality thread :-)
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WKetel
10/14/2011 9:13 PM EDT
Liketobike is certainly correct about quality. Each generation of product is less robust and dependable, with new features that are useless and performing the intended task less adequately than the previous generation. So I repair the old one because I don't want the new one because it does not provide the function that I am looking for. Progress is seldom improvement in this era.
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bk1
10/14/2011 6:01 PM EDT
Poor grounds have been wreaking havoc in cars forever. In my first car, a 1970 model, all the lights would occasionally dim when I hit a bump or moved the auto trans shift lever. I searched all over for the culprit, using the vehicle schematic (all one page of it) for help. I finally found the problem working under the hood at night when I saw sparks jump across the well-worn transmission shift linkage! Since the engine, trans, rear suspension, and exhaust all mount with rubber bushings, a failed ground strap from the motor to the body caused the entire drivetrain to be isolated from the chassis. A simple fix after weeks of fruitless troubleshooting.
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WKetel
10/14/2011 9:17 PM EDT
ON the car with the headlights problem, I wonder if the headlight switch was originally intended to be switching the ground return for the lights. I can see that it would take less wire to control them that way, and it could avoid the problem of rusty ground connections at the lights. It would be interesting to know what make of car it was.
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wu0f
10/19/2011 8:33 AM EDT
On several of the newer cars I have had occasion to work on I have found that the lights all work by switching the ground. That way the computer knows what is working or not.
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Duane Benson
10/17/2011 7:28 PM EDT
I had a case a while back where both low beams were intermittent. The high beams always worked but sometimes the low-beams didn't. It was all the more difficult to troubleshoot because sometimes just turning the car off was enough to cause the lights to start working again. Eventually, I was able to verify that the switch was working, even when the lights were not.
What really confused me was when I both verified that the terminals at the lights had a good 12V and both grounds seemed to be pretty darn close to zero ohms.
Eventually, just for kicks, I replaced one of the headlights. Suddenly, only one headlight was intermittent. I replaced the second one and the problem went away completely.
Looking closely at the bulbs, I could see that both had a broken filament that would lay down and make physical contact but could easily be jostled loose. Somehow, I had broken both filaments at the same time in the same manner. I still find that odd and that both went off and on at the same times.
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David Ashton
10/17/2011 7:45 PM EDT
" I still find that odd and that both went off and on at the same times."
That's called Murphy's law... ;-)
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JobT
10/26/2011 8:10 PM EDT
Talking of strange car wiring faults and quality control, on one occasion the fault showed itself as a dimming of the clock display when the brake was applied. After much head scratching and work I finally found that one of the double filament rear lights had a short between the filaments inside the bulb. I never quite worked out why the clock display was affected, but replacing the bulb fixed it.
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Resistor_Smoker
11/6/2011 2:24 AM EST
The car was a Daihatsu Charade, a 1990 model. The lights did work by switching the ground connection, but I guess that the lights were built for positive supply switching since they were fed with +12 V to the ground connection and the high and low beams went to the ground.
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