Hey Autopians! It’s me, Emily, whom you might remember from a few things I’ve written here, like about how I 3D printed new rubber gaskets for my truck, or how I built an e- bike out of junk that I scavenged. Today I want to talk to you about a compass that lives in my friend Fredy’s 1992 Chevy G20 diesel — nicknamed “Poopandus” — which just exhibited a remarkable phenomenon for us last weekend.
The compass is one of those ball ones of the sort that I distinctly remember living on the dashboard of the neighbor’s RV when I was growing up. I always thought these things were neat, so when I found one for a couple of bucks at a thrift store last year, I grabbed it.
I thought it would look better in Fredy’s van than in my Honda Fit, so that’s where it lives now.
The compass in question, and subject of this article.
Last weekend, Fredy and I were hanging out after visiting a museum as part of an assignment I had been procrastinating on for my ceramics class. We had switched from my Fit to the Chevy van in an attempt to pick up a cool (and extremely heavy) steel cabinet with sliding shelves from the side of the road. We failed to get the cabinet because someone else picked it up while we were swapping vehicles, so we went on with our day and picked up some really excellent fried chicken to eat in the park.
Poopandus, the van, parked elsewhere besides the park where we ate our chicken.
After devouring the chicken and spending some time sitting around and digesting, we climbed back into the van to leave. Just before Fredy started the van, something strange caught my eye. The dashboard compass had spun completely around. A moment before, it showed us facing due south, it now said we were facing north. Then, it flipped back around and showed us facing South again.
I wasn’t sure what I had just witnessed, and remarked on it. Fredy looked at the compass, thought for a second, and said, “Glow plugs?”
Fredy turned the van off, and pressed the glow plug button on the dashboard. Sure enough, the compass flipped from south to north again. As soon as Fredy let go of the button, it spun back to south.
So what’s going on here? It doesn’t take a rocket science to figure it out, but, for the record, I did consult a friend of mine who is a Caltech graduate to make sure I correctly did the math I’m about to explain.
Clearly the van’s electrical system is creating a magnetic field that’s pulling the compass away from true north, but how?
Sorry. It’s Math Time
Since the effect only appears when the glow plug button is depressed, it’s pretty safe to assume that it’s the glow plugs that are responsible. A glow plug is a thin little metal rod that reaches into an engine’s combustion chamber to pre-heat it to facilitate immediate compression-ignition in a diesel during a cold-start.
I did a little bit of research and found that individual glow plugs can draw between 10 and 20 amps each when first powered on. The diesel engine in a G20 has a part that GM calls a “glow plug controller,” so all the current for the glow plugs must pass through it. If we’re conservative and guess each plug is drawing 12 amps, and there are 8 plugs, then…
12 amps * 8 plugs = 96 amps
… 96 amps are passing through the glow plug controller when the button on the dashboard is pressed. That’s a considerable amount of current.
Now we need to figure out how much of a magnetic field 96 amps would generate. Helpfully, physicists have already figured out a formula for this:
Let’s define a few of those variable real quick.
• I: This is the current flowing through the wire, i.e. the amps we calculated above
• π: This is pi, the ratio of a circle’s diameter to its circumference. It’s roughly 3.14159
• µ0: This is a mathematical constant representing the “permeability of free space.” This just describes how well a magnetic field spreads through empty space. In this case, there’s air and a plastic engine cover in the way, but those aren’t going to make much of a difference in this case. The constant’s value is 4 π x10 -7 Henrys per meter. (Yes, there is a scientific unit called Henry; it’s a unit of inductance)
• r: This is distance from the wire carrying the current in meters
• B: This is magnetic flux density, essentially how strong the magnetic field is. This is measured in teslas, which, I have to point out, since you are car people, are not the same as Teslas.
We already know our current, and we know our permeability constant, so all we need to solve this equation is determining how far away the compass is from the wire carrying the current to the glow plugs. This is one of those vans with a short nose and much of the engine in the passenger compartment underneath a doghouse cover, and the glow plug controller is on top of the engine, somewhat towards the end that protrudes into the passenger compartment, as seen here:
Always read the manual before attempting math.
The compass is sitting in a tray on that doghouse cover, so I’m going to take a wild guess that the compass is roughly 6 inches from where the 96 amps is flowing. Six inches is 15.24 centimeters, or 0.1524 meters. I’m also assuming that the wire the current was flowing through was more or less straight, because if it was coiled up, this would get more complicated.
If we plug everything we know and we’ve assumed into the equation, we get this:
I’ll spare everyone, including myself, having to go through all the steps of solving that right here, but if you do solve it, you get a value for B that equals 0.000126 tesla.
“It checks out to me as long as those units are right,” said my friend, the Caltech graduate, about my math, while also declining to be named for this piece (clearly they’re confident).
According to Wikipedia, Earth’s magnetic field varies between 0.000022 tesla and 0.000067 tesla. Generally speaking, Earth’s magnetic field is strongest at the poles and weakest at the equator.
Fredy and I live in LA, which is closer to the equator than it is to the North Pole, so it’s safe to assume the Earth’s magnetic field isn’t at its strongest here. If we did our magnetic field calculations correctly, then the magnetic field from the glow plugs was probably much stronger than the Earth’s magnetic field was in the vicinity of where we were eating our fried chicken on a pleasant Sunday afternoon. So, it makes a lot of sense why the compass spun around when the glow plugs were powered on.
So, what to make of all this? I dunno. Science and math can explain a lot about what we experience in the world? Fried chicken is good? Even when you procrastinate in school, you might still learn something?
Bonus fried chicken photo.
(All pics Emily Velasco; Top Images also includes images from Wikimedia Commons, Chilton’s)
Scavenging stuff from the side of the road, glow plugs, and fried chicken?
Where were you when I was young and ready to mingle? 😉
You glossed over the “(long straight wire)” part of the equation…explain that piece?
Interesting article. Sorry you didn’t get the filing cabinet.
My ’17 Accord has a compass display as part of its infotainment system (in the upper non-touchscreen area for those unfamiliar with Accords’ displays of that era).
I have noticed that when I put my iPhone in the little cubby in front of the shifter, the compass display gets inconsistently whacky. It doesn’t matter whether the phone is plugged into a charging cable or not. And there is no inductive charging pad anywhere in that car. So, I’m guessing that the compass sensor is somewhere near there and being influenced either whatever ferrous metal is in the phone or a magnetic field generated therein. The display also shows the exterior temperature, but it gets heat-soaked when the car is parked, particularly in direct sunshine. I assume that sensor is up front somewhere and displays reasonable temperatures after the airflow cools off the sensor.
I used to fly a Cessna that checking the calibration of its old-school compass was part of its annual inspection and they used to put tiny magnets nearby to calibrate it at the factory.
My 2001 Jetta TDI had glow plugs and a controller, but no compass, so I have no idea if it would have done the same thing. I had a very early and primitive Garmin hand-held GPS that I used in both the plane and the car, but I believe it calculated what direction we were going by drawing a “line” between Lat/Long coordinates while in motion.
Apropos of nothing, I read your Prius review from a few years ago and my guess, based on the nav screen, is that you were put up at L’Auberge Del Mar. Back in the mid-70s I lived in the little brown condo/apartment complex across the entrance drive towards the ocean for a couple of years while in college at UCSD. But the hotel and spa did not exist while I was there. My late uncle was a seismic engineering professor at Cal Tech and consultant for building projects around the Pacific rim and a cousin was a science writer for JPL.
UFOs, it’s caused by UFO’s.
…a much simpler explanation
Cool! Important thing for everyone to remember: The earth is a very large magnet, but it’s not a particularly strong magnet.
I learned that your space key seems to be sticking. Interesting article though.
It’s the CMS 🙁
*sigh*
It is not the glow plugs themselves doing it.
That van has a pushbutton for the glow plugs, which means it has had a manual control retrofitted.
The most common way of doing that is to use a large, usually Ford-style, starter solenoid. That solenoid makes a pretty significant magnetic field.
On a van, I would imagine it is located near the doghouse, which is plastic, and does not provide shielding for the magnetic lines of force.
Even the factory timer setup used a large relay, but it was, IIRC, on the engine which is why they had a high failure rate.
I mentioned the glow plug controller right there in the article. I even highlighted it on a page of the service manual and included a photo of that in the article.
*sigh* if only reading the article were required in order to comment.
lol
It does seem like a coil could generate more interference than straight wire.
I got a portable electronic compass a few years ago, and it was trying to set, maybe that’s par for one of those?
The glow plug harness itself would also carry on the order of 300-400 amps (split second) falling to 100 amps or so by the end of the preheat. At least that’s what I’ve measured from the 7.3 IDIs.
That would cause significantly a lot of distributed along-the-conductor magnetic field as well.
They likely both contribute, as all of those starter style solenoids on dead glow plug controllers that I’ve taken apart aren’t fully closed loop iron cores, so they’re just blasting that solenoidal field out everywhere!
No mu metal shielding?
That is a funny thought, coating the inside of the doghouse in mu metal to let your old school floaty ball compass work.
But do we really think Ford or GM could spare the money for said shielding back then………… or even today
I was thinking more of shielding coils and relays in the traditional style, though mu metal foil is available.
There might be stronger reasons for blocking interference than a compass.
Some diesel doghouses are better built than gas versions. I got as far as making an offer on a 7.3 non turbo ambulance.
Also, some better acoustic sheeting has annealed lead foil in a layer. Good for general noise, though not emf.
It’s a good idea to read the whole article.
Did you…read the article? I’ve heard reading is fundamental.
Something is eating the spaces between some of the words in this post.
Your friend better watch out you goys or only about one meter2 of handwaveium folded into a tesseract from a functional flux capacitor. Reflect enough tachyons and there goes the neighborhood.
My autocorrect seems corrupted by ai and has done very bizarre things lately.
Maybe others have the same affliction?
Been saying it for ages: autocorrect = autocorrupt
Very accurate
This article just caused a core memory of mine to surface.
My great uncle had an ’85 F250 diesel. It had a bubble dome compass as well, mounted on the dashboard somewhere around the instrument cluster. It would move about 15° when the ignition was turned on.
It’s interesting to see that phenomenon discussed and explained so many years later, even if the source of the magnetic field may not be exactly the same.
The same thing happens with the standby compass in airplanes. The compass needs to be calibrated every so often. It requires performing a procedure called a compass swing where you taxi the aircraft in circles, stopping at precise headings to record what the compass reads in relation to the other navigation instruments. Things like the radios, windshield deice and even the instrument lighting can affect the magnetic compass heading.
“Good morning folks, captain speaking from the cockpit this morning. We’ll be on our way to Seattle in just a few minutes. We’re going to perform a compass swing before we get out to the runway. You may get a little dizzy, so please use the barf bag in the seatback pocket in front of you.”
Hate to break it to you but car compasses don’t work. You would get just as reliable or unreliable heading if you took a Magic 8 ball and just labeled the sides for different headings
I keep an old compass in my car, it once alerted me to the fact I was driving the wrong way around the M25.
All those cars coming at you didn’t give you a clue?
LOL Planes, Trains, and Automobiles reference
Hey, it’s just for fun. It does point north though.
Indeed, a normal compass will mostly work in a car, but you should not attempt to take a bearing or similar.
They work better in diesels than gas vehicles (no HV electrics).
Had a BMW with a (factory) electronic flux gate compass once, which in true German style had a whole section in the manual about setting it to offset true to magnetic north and how to ‘swing’ the car to calibrate it.
Didn’t need emf shielding back then. If modern EVs didn’t have proper emf shielding you would have similar issues. The DC from battery to inverter at full throttle is 1000A – 2000A on many models. The 800V architectures have the lower Amperage.
That’s why manufacturers don’t want AM radio.
They don’t want to shield anything properly.
There are limits to what shielding can do, and AM definitely isn’t within those limits.
Am I misunderstanding that?
Are you saying AM radio can’t work?
If so, I have a few questions.
I just overpowered a force created by the earth itself! Eh, not worth it, sitting back down again.
You won the battle but lost the war
Istheauthor’sspacebarbroken?
My personalfaith requiresthat I not attemptperfection inanything I dobecause only theAlmighty isperfect. Ichoose to expressthat by leaving out somespaces inmy writing.
Gobblesss
I thought maybe you were just an e e cummings fan.
I blame the ultra low profile stuff going on with some of the “fancy” keyboards they are putting in things now, I get keys randomly either not registering or sticking when I don’t want them too.
It’s actually just a CMS issue. It shows up on some browsers on some devices, but not others. It’s probably something to do with the way Word encodes formatting and the CMS misinterpreting it.
I still don’t particularly like these new fangled keys. lol
PS that chicken photo is making me hungry.
Aha, the not-space is the HTML entity for “LINE SEPARATOR”. For example, the HTML for “from my” is “from my” – the browser will break between the “from” and the “my” if there isn’t room on the line for both, but if there is room for both then there won’t be any space between them because a “LINE SEPARATOR” isn’t a space.
yea, exactly
When I was at Cisco there was an idiot that insisted on pasting code into into and out of Word
Hyphens, em dash’s, n dashes, negative signs, would get scrambled, and the many types of white space would switched for non apparent reasons.
Fоr іnstаnсе thіs іs nоt whаt іt lооks lіkе аt аll, just trу аnd sеаrсh fоr thіs.
Ah yes, the dreaded em dash. Our doc writers would introduce them into example command lines in customer doc. We’d get bug reports that copy/pasting the example into the command line didn’t work. We added doc test cases to catch this nonsense. Attempting to educate the doc folks had limited success.
My bank, which rhymes with chaste manhattan, sends me statements as PDFs.
For some stupid reason instead of minus signs the PDF uses em dashs.
Took a while to figure out what the heck was wrong when I put it into a spreadsheet.
Something is up with how the Autopium CMS handles importing/pasting from Word or GDocs. My vanmongling articles were riddled with random formatting and line break issues at first.
I’m glad the investigation wasn’t done by Torch. No doubt his conclusion would involve chainsaws and poop.
And instead of fried chicken we would have gotten a picture of smashed potted meat.
Somebody show this article to the Insane Clown Posse, pronto.
F’ing magnets!
It’s ALL done with magnets!
Yet nobody knows how they work!
They rely on Wishful Thinking, like most magic.
Glow plugs? In LA?
I’ll bet that van’s noisy AF to ride around in, if my experiences with an ’80s GM van and a ’00s diesel Ford Econoline are any indications.
clackclackclackclackclackclackRAAAAAAAAAAAAAAAAWWWWWWWWWWRRRR hits 25mph clackclackclackclackclackclackclackclackclackclackclackclackclackclackclackclackclack
It’s not especially noisy inside. I wouldn’t say it’s noisier than being in my 82 Toyota pickup
So it’s pretty damn noisy. 😀
I have yet to have a satisfying mac salad.
You should take a cross-country road trip, trying macaroni salad in every city and town you stop in, as a quest to finally find a macaroni salad that satisfies you. You can write about the experience. Jason and crew would definitely publish that.
Especially if many of them were purchased at gas stations. 🙂
Autopia loves poop content!
My Aunt Frankie makes a really good one, IMHO. But no mayo, hers is “Italian” and uses Italian salad dressing. Tri-color rotini, salami, onions and peppers chopped up, when I am not eating it, olives, and parmesan cheese. Mayo-based macaroni salads never taste good to me either.
Now I want to make some. Would go well with the cheddar brats I have in the fridge. Hmmm.
That sounds really good. I want to make that for dinner now
I am familiar with both version you describe, and I can concur that your aunt’s is the superior version. I also don’t mind it with some pancetta in it.
Oooooh – I have a package of that in the fridge I need to use! This is soooo happening tomorrow. Need to go to the store tonight.
Same. The only thing more bland is egg salad.
Practical and direct application of math is the best! Yes I am a nerd, why do you ask?
Is that chicken from Donahoo’s?
You bet it is. How did you know!
Because I went there last month to get chicken to take my my daughter’s graduation from Cal Poly! Love those rolls!
Congrats to her! I’m a CPP graduate myself
I told her you said congratulations!
Go Broncos!
Thanks for running this funny little post of mine! Someone over on Mastodon has just informed me that semi trucks don’t use magnetic compasses because there’s too much iron in the truck and it throws the compass off.
Does anyone know if that’s true?
It sounds plausible for an off-the-shelf compass being lobbed into a truck. A compass that comes with the car ought to have been calibrated by the oem to the magnetic model of the vehicle.
In UAV land, I have to calibrate magnetometers/compasses well away from the steel building. A 1000+ pound lump of iron and thousands of pounds more of steel rails and sheets will definitely mess with a compass that wasn’t compensated for it.
A friend of mine built an electronic dashboard compass for his jeep. He wrote something into the code to let him calibrate it, but he had to drive in circles something like 20 times in an empty parking lot to do it
That sounds very familiar….
You definitely don’t want to flip and roll a semi tractor around all 3 axes to calibrate that compass. Assuming you walk away from the wreck, the compass ought to read very true then!
That’s been the instructions for every car I have owned with a compass built into the mirror. Though not necessarily 20, more like 5-6. And you are supposed to do it if you drive more than X miles from where you last did it – 500? 1000? Can’t remember. I have never bothered.
I don’t think there are any instructions for calibrating the compass in my Mercedes. Probably driven by the GPS rather than an actual compass.
Or it calibrates itself based on the GPS when you are moving.
Either way. But simpler to not have it be an actual magnetic compass in the first place. Not much point when you have GPS baked in.
If you’ve been surrounded by thousands of pounds of ferrous metals, you may be entitled to compensation. Call 1-888-COM-PASS. Operators are standing by.
Given that it’s Masto, we at least know they’re not joking.
Can you explain? I don’t get the reference.
4 glow plugs or 8? The equation picture in the article calculates for 8 glow plugs and 96A, but the article text uses 4 glow plugs and 48A.
We’ve fixed it!
It hasn’t hit the CMS update yet, I don’t think, but yay!
Dammit I got suckered into doing math on this site again?? For a site that celebrates its birthday on March 32nd there sure is an awful lot of math and learning going on around here.
Math yes learing probably not
Missing spaces hit me hard. Everything else ist fine
It’s the glow plugs interacting with the fried chicken. Easyphenomenontoreplicate.