Check your AFM boys and girls (re. hi rpm miss)

#49

That sounds like a great price for a reman AFM. I might get one before my next trip to the dyno.

#50

Tell ya what, I’ll come run with you in SE30, you just can’t tech or dyno my car :whistle:[/quote]

Tell ya what, I’ll give you the winter to sort out your tech problems, but in the meantime, come on out and see the fun we’re having!!

#51

The invitation of course goes both ways… but you guys can come run ITS legally any day :stuck_out_tongue:

Ask Tom Tiede about our group, I’m sure its equally as fun as the NASA Midwest folks… I really do hope to get out and enjoy a NASA event sometime this summer.

Great shot attached of the start of what seems like an E30 that got in the wrong run group… thats my lone E30 trying to get thru the pack of Mia’s and Tia’s

Photo courtesy of Larry Best of Chicagoland Sports Car Club.

#52

I’m very familiear with you guys, just can’t fit you in the schedule. How competitive would a SE30 be in ITS anyways?

I’m sure our paths will cross soon.

#53

In the midwest I would think the car would be fairly competitive… in the Southeast and Northeast where IT is crazy fast and you have ARRC cars to contend with, I’m doubtful of a SE30’s competitiveness.

Unfortunately ITS only sees 5-7 cars on average here in the midwest.

ITA usually has a large car count as the spec miata guys run dual classes and thats where their car fits.

That holds true for MCSCC or SCCA.

#54

Update: I installed the new Borg Warner reman piece for this weekends sprint races at Blackhawk Farms. Car ran great - no high RPM misfire and pulled excellant all the way to the rev limiter (7000RPM on my MarkD chip).

The new piece bench tested correctly as per the 944guy’s proceedure, and it also showed a loss in the resistance as others have discovered.

Attached is a photo of the old piece showing the strange spot on the end of the AFM circuit board. As the pickup swept into this range it started to do odd things with both voltage and resistance (see earlier post for details)

#55

When we experience issues like these the first thing we do is swap a known good AFM off one of our other cars onto the trouble maker.

If the issue is resolved we’ll normally try a re-man unit; but lately I’ve taken to buying new in box Bosch Units (via the Bosch Pn: 0 280 202 082) from BMA; they were $350-400 from memory, supplied in genuine Bosch box with genuine Bosch plastic bag. Clean, never opened.

Bolted them on and the cars run perfectly. Problem solved.

#56

[quote=“djs325” post=65992]When we experience issues like these the first thing we do is swap a known good AFM off one of our other cars onto the trouble maker.

If the issue is resolved we’ll normally try a re-man unit; but lately I’ve taken to buying new in box Bosch Units (via the Bosch Pn: 0 280 202 082) from BMA; they were $350-400 from memory, supplied in genuine Bosch box with genuine Bosch plastic bag. Clean, never opened.

Bolted them on and the cars run perfectly. Problem solved.[/quote]
Do you perceive the re-man units to be properly calibrated? It would be easy to imagine some outfit disassembling old AFMs, cleaning the housings, slapping in new parts but not doing the careful laser-etch calibrating that Bosch did.

#57

That is the kicker isn’t it - with a reman unit you can never have quite the same faith. The units sourced direct from Bosch have been bolt-in & leave. If you need to fine tune for A/F ratio then so be it; but no other adjustment or fiddling required!

[quote=“Ranger” post=66005][quote=“djs325” post=65992]When we experience issues like these the first thing we do is swap a known good AFM off one of our other cars onto the trouble maker.

If the issue is resolved we’ll normally try a re-man unit; but lately I’ve taken to buying new in box Bosch Units (via the Bosch Pn: 0 280 202 082) from BMA; they were $350-400 from memory, supplied in genuine Bosch box with genuine Bosch plastic bag. Clean, never opened.

Bolted them on and the cars run perfectly. Problem solved.[/quote]
Do you perceive the re-man units to be properly calibrated? It would be easy to imagine some outfit disassembling old AFMs, cleaning the housings, slapping in new parts but not doing the careful laser-etch calibrating that Bosch did.[/quote]

#58

[quote=“djs325” post=66324]That is the kicker isn’t it - with a reman unit you can never have quite the same faith. The units sourced direct from Bosch have been bolt-in & leave. If you need to fine tune for A/F ratio then so be it; but no other adjustment or fiddling required!
[/quote]
The problem is that it’s the well-intended attempts to tweak the AFM in order to fine tune the A/F that screws them up. Once the AFM’s spring tension has been screwed with, there’s no way to be sure of getting it back to the factory calibration.

#59

Reviving this thread because it’s the one that always comes up when I’m looking up AFM info. :slight_smile: Figured I’d add my 2 cents while I’m in here.

When I first went through the AFM refreshing exercise, I measured using a 9V battery and the testing worked fine. I’m currently in the process of building out an “M20 simulator” to run a Motronic 1.3 ECU against. Still in the early stages, so I’m going through and getting a more concise understanding of the various sensor inputs I need to simulate. I’m on the AFM now, so found myself reading this thread (again).

Scott brought up the “circuit bridge” differentiation between the AFM and a typical “variable resistor”, but I figured I’d add details. Partially to inform those that may not know, partially to be corrected if my understanding is wrong.

The AFM is actually a potentiometer (or trim pot). This is the simplest type of voltage divider. To me, a good way to understand a potentiometer is like an adjustable resistor ladder. A simple resistor ladder is 2 resistors configured serially. The ratio of one resistor to the overall ratio dictates the voltage difference over that resistor.

Examples:

  • If both resistors are equal – say both are 200 Ohm for a total resistance of 400 Ohm – then the voltage between the two resistors is the half of the input voltage.
  • If the one resistor is 3x the other resistor – say 600 Ohm and 200 Ohm, for a total of 800 Ohm – then the voltage is 25% or 75% of the input voltage (depending on which side of the ladder the input voltage is).

Here’s the diagram of the M20/Motronic 1.3 AFM:

This is why simply measuring the resistance between 2 pins on the AFM isn’t sufficient. There are infinite possible resistor combinations that could result in the same output voltage. Based on that diagram, here are the important piece if you’re just going to look at resistance measurements:

  1. Measure the resistance between pins 3 (Vin) and 4 (GND). This is your overall resistance.
  2. With the flap closed, measure pins 2 (Vout) and 4 (GND). You should find very low resistance. Based on the numbers from the 944 page, with a closed flap being about .05 of Vin, then I’d expect a closed resistance of about 1/20 of your total resistance.
  3. With the flap fully open, measure pins 2 (Vout) and 3 (Vin). This should yield a number close to your overall resistance. Based on the numbers from the 944 page, with a closed flap being about .9 of Vin, then I’d expect an open flap resistance of about 9/10 of the overall resistance.
  4. Test intermediate flap angles to ensure linear change of resistance from closed-to-open.

I will test at my next opportunity, but I believe this test should yield identical results to the apply-voltage test. Of course, not saying applying a voltage is hard, but if all you have is a Fluke, it should work.

With that said, I’m still trying to get my head around the “laser calibration” and its purpose. The descriptions seem to all be around “fine tuning the resistance of the sweeper”, but it doesn’t really add up. Here’s the image from the 944 page:

Notice the irregularity in the gold sectionals around the sweep zone. This certainly seems intentional, but why? Could this be adding any kind of significant capacitance to the circuit? Also, if you look at the layout, we’re looking at a more complex bridge circuit. Sort of another resistance ladder running in parallel with the sweeper.

My totally post-midnight tired hunch here is that the surrounding area isn’t about tuning the resistance to make the sweeper’s resistance more precise. Instead, it’s about providing capacitance to prevent ringing/spikes/dips when the sweeper moves – making the voltage reading more precise. Effectively, capacitors work for electrons how water towers work for water pressure – providing localized stabilization of voltage (or water pressure). When the sweeper slides to a different section of the track, there’s probably a finite time it takes for the electrons to reach the equilibrium state needed for a valid measurement. Without the aid of the surround capacitance, these spikes could negatively impact what the ECU reads from the AFM. The capacitance, therefore, would be there to help fill/remove those electrons more quickly so the voltage snaps to its proper reading faster.

Since you have this long serial voltage ladder running parallel to the sweeper, the precision probably comes from the need to ensure every point of contact with the sweeper effectively matches the effective voltage ratio you’d expect at the touch point to the sweeper. If it’s off, you’d be affecting how linear the voltage response is to the flap angle.

The devil’s advocate in me wonders if the speed at which the sweeper moves could really be fast enough to induce the kinds of spikes that would necessitate a solution like the one I’ve described. If not, then I’m totally wrong and there’s another reason for it all. :slight_smile:

Som

#60

The capacitance idea is hard to buy. The copper traces are too far apart to create much capacitance. Besides, as you pointed out, the rate of V change is low enough that a helova lot of capacitance would be needed to smooth the change of R.

My perception of the laser calibration business was to fine tune the resistance ratios across the sweeper conductive surface on the PC board. You mentioned the gold areas, but I’d note instead the green areas where the laser etching occurred.

#61

My quick “I’m at work” response.

I should have added that I didn’t mean capacitance like how capacitors store charge. More like how any material that can hold an electric charge has capacitance. What I was trying to explain is why the conductive areas are shaped the way they are. I get the green areas and the resistance adjustments made by the laser etching. (edit: to clarify, what I get is that the laser etching is used to tune the resistance. obviously I don’t get the overall “why” part, though.) What I don’t get is why the conductive traces have the odd shapes that they do. I don’t think it was by accident, but I couldn’t really come up with a good explanation other than providing a tiny bit of capacitance to help “fill” the local area of the sweeper as current is being drawn. The pad shapes are a more pronounced in the older AFM:

The other thing I realized this morning was that I was concerned that the sweeper rate might not change fast enough to induce a voltage change due to air flow changing. What I didn’t think about was the effect that vibrations might have on the sensor itself. These kinds of higher frequency vibrations would certainly get closer to potentially causing signal spikes. That said, if signal smoothing was the goal, I’m pretty sure there would have been simpler ways to smooth out the signal without affecting the responsiveness of the sensor. shrug

Som

#62

Another reason those conductive areas are shaped the way they are could be simply that it was in some way the cheapest possible layout for them to construct.
Whomever made this board, their first mission was to make a profit, the second was to make it work. At least that is my jaded view of things.

#63

Totally with you on the jaded view part of it. I would expect the “cheapest/easiest” option there, too.

That said, the shape of the traces seem too deliberate to me to suggest this was the “easiest way” to do it. I could have seen it as a way to increase tolerances in the manufacturing process if, say, the resistive elements were being laid onto the board separately and had a wide margin of placement error. Then you’d give it some wide pads to land on and then correct away the differences with the laser etching.

But… the pads have all these little quirks/extensions beyond that pads that don’t seem to contribute to helping tolerance issues. Particularly the 2 large tracks around the top of the images that end nowhere.

As I’m writing this, another possibility I’m thinking is that the pads could be there to help with the automated testing/calibration processes during manufacturing. Like robot arms placing ohmmeters around the resistive areas to measure before laser etching.

That doesn’t seem to explain it all, though. Like how wide/tall the resistive tracks on the edges are vs. the tracks in the middle.

Maybe I need to do more circuit bridge research. :slight_smile:

Som

#64

As a youngster, I worked for an outfit for a while that made multilayer circuit boards. Som, since you’re in San Diego, you probably drive by the building all the time.

The big conductive strips don’t look unusual to me. This device is a big clumsy analog sensor. There’s probably a fair amount of current flowing so it needs big traces to keep the resistance down and sensor accurate. If you look at the design from the standpoint of “need big traces” you can see how the designer used the real estate pretty logically.

Traces that don’t go anywhere. This isn’t necessarily a single layer circuit board. There could be multiple layers inside that aren’t visible. So a trace that appears to stop could pass inside the board to other stuff, or simply pass thru to the other side of the board.

#65

That’s definitely something I hadn’t thought of – going 3 dimensional. I was so in the mindset that this was gonna be “as cheap as possible” that I thought for sure it was a single sided/single layer board. Another reason I didn’t think of multi-layers was that I didn’t think there was a way (at least using commonly available manufacturing methods) to connect multiple layers on a PCB without vias. Are there? (were they available in the 80s?)

Wide traces for minimizing resistance makes sense, but I wonder… if you’re trying to get to that level of precision and sensitivity, would the capacitance you’re adding by going with the wide traces start causing a problem with ringing/signal oscillations? As I understand it, there’s a limit to the benefits of reducing resistance in exchange for capacitance.

The way I think about it is that there are electrons in the trace. A voltage difference induces those electrons to flow in order to try and reach an equilibrium point. As the electrons start to move, you have an inductive effect. This inductive effect is like momentum. So the electrons build up momentum. When the voltage changes, or if the voltage difference starts approaching zero, the electrons need a finite amount of time to react.

Depending on the impedance vs. capacitance you have, you get all those fun oscillation/dampening situations:

  • critical damping (impedance/capacitance at proper levels to ensure a quick arrival at the signal voltage without overshooting/oscillating)
  • underdamping (capacitance is too high for the impedance and your signal overshoots the target/equilibrium point and oscillates before reaching equilibrium)
  • overdamping (impedance is too for the capacitance so your signal is slow to arrive at the equilibrium point).

(I could be muddling terms like voltage/charge/etc, but that’s basically the gist of how I understand it)

To me, there’s a (somewhat) similar analogy to how intake runners are designed for the engine. The air/fuel charge substituting for the electrons, the runners themselves substituting for the traces. The cross-sectional area of the runner is your resistance and the length of your runner is your capacitance. For a given RPM, you’re attempting to maximize the amount of charge that flows through without getting overshoots (back pressure being introduced on the intake side when the valve closes and there’s still charge momentum going towards the cylinder) or undershoots (pressure in the runner decreasing before the valve closes and getting some of the cylinder’s charge back into the runner). What you’d be going for is the “critical damping” situation where the runner’s charge pressure matches the cylinder’s charge pressure so the charge velocity is exactly zero when the valve closes. Again, very rough analogy, and being able to tune this with valve timing makes this aspect of the runner’s dimensions less critical in terms of concerns about damping, but still.

Yet another possibility is that the board was designed to for multiple possible configurations. Maybe the resistive material is laid on in a subsequent step to the traces and in some configurations some of those loose end traces get bridged by a resistor. Though I can already see one pair of pads that don’t seem to line up enough to support this theory.

By the way, I enjoy guessing/testing/guessing/blabbing about it, as you can probably tell, even if I never find “the answer”… so feel free to ignore. I won’t be offended. :slight_smile: That said, I haven’t looked at some of this stuff for years… so anyone who feels so inclined to correct me, I’m all ears. :slight_smile:

Som

#66

Re. multi-layer boards in the 80’s. Yes, they were around. I was working for Dibble Electronics there in San Diego ~1985. We could do up to 14 layer boards.

Sorry, but I think the “capacitance that you are adding” exists only in your imagination. I don’t know your background so I apologize if I’m patronizing. A capacitor adds resistance (technically “impedance”) to the circuit as a function of changing V. If V is changing pretty slowly, you’d need a helova lot of capacitance in order to have any effect. The visible traces are so far apart that they would have capacitance for squat unless the change of V was in the Mhz range.

I don’t think that the sweeper arm moves fast enough that there’d be much need to smooth the signal by adding capacitance or inductance. I don’t think the design of the circuit board adds any capacitance at the rates of V change it’s going to see.

Likewise, inductors react to changing I (current). If I changes fast, the inductor adds resistance It’s really impedance, but in DC applications it’s easier and reasonably accurate to think of it as resistance.

Re. your examples. I’d go for more simple ones. Capacitors don’t like changing voltage. If voltage changes fast, the capacitor acts to resist that change in voltage. If the change of voltage is not so much, it takes a lot of capacitance to get the job done.

Inductors don’t like changing current. If I changes fast, the inductor acts to resist that change. If the change is not so much…

In each case the “acts to resist that change” takes the form of adding “impedance” to the circuit. Normal folks don’t have much of a feel for impedance so it’s easier to think of it as resistance, except that the resistance is a function of rate of V or I change. Fast V change means more resistance, for example.

#67

AT WOT, the AFM becomes a switch to tell the ECU to tune the motor with the factory installed map. Unless said switch is unstable, there should be fluctuation in signal. So, theorize that the switch is fluctuating and calling for a different fuel/air mixture intermittently (trying to go to the O2 sensor signal) and you have a really unhappy motor. I wonder what would happen if we just jumpered the WOT switch and tried that?

#68

My background, as far as education, is computer engineering – which is a hybrid of computer science and electrical engineering. That said, most of my professional work has been on the programming side than the hardware side.

The capacitance I’m talking about definitely isn’t made up. I think we might be talking different terms, though – capacitance just means the ability to hold an electric charge. Anything that conducts electricity has a capacitance. Capacitors are components designed to store more charge (have higher capacitance) than the material itself could hold on its own, so to speak. I’m not referring to capacitors, and I’m not suggesting the board layout is meant to create some kind of capacitor component.

You’re absolutely right that the type of circuit we’re talking about might not move fast enough to be affected by the tiny amounts of capacitance we’re talking about. The discussion of trace capacitance and impedance was mostly relevant when discussing high speed (100MHz+) bus design. When you have switching at that rate in a digital circuit, even the tiniest amount of overshoot/oscillation could register as an incorrect 1 or 0 on the other side. So in that case you had to match impedance to the capacitance in your traces. Often this required adding resistors serially to your data lines. But the resistance had to be perfect. Too little and you still get ringing. Too much and the signal can’t switch fast enough.

I get that that was a discussion about high speed digital circuits – and this clearly is not a high speed digital circuit. In slower digital circuits, it doesn’t matter if there’s an oscillation when a signal switches because there’s plenty of time for the signal to stabilize. What I wasn’t (and am still not) sure of is if the concept still potentially applies in what’s supposed to be a fairly precise mechanical/analog sensor application like this. It’s possible it doesn’t… but, again, I’m just spitballing theories to try and explain the trace layout.

Regarding the multi-layer boards, totally on the same page about multi-layer boards being around for decades. What I was curious about was if it was done without vias. Connections between layers in a board are typically done with a drilled hole + plating (vias). Kind of like a rivet. (that explanation is just for anyone else happening to read the thread, btw) The circuit in the AFM doesn’t have vias in the areas where the traces go off into the middle of nowhere. I wasn’t sure if there were ways to connect layers without the drilled holes that were common in the 80s. I’ve since done some more searching and a concept of “solid” or “filled” vias appears to be a thing, which I didn’t know about. Not sure how common it is or why they’re used, but that would be a good explanation for the random traces.

Som