'76 280Z Question on dist/manifold vacuum, dist timing, fuel pressure, brake booster

[FastWoman]

It might also be that there's some extra resistance between the potentiometer wiper and the surface of the carbon trace. In other words, that contact might not be completely clean.

In fact it might just be the nature of the beast that there will often be a bit of random resistance between the carbon trace and the wiper, so that measurement might be somewhat meaningless (the reason they don't tell you to take resistance measurements off of the #7 contact in the FSM).

Again, and I can't stress this enough, it's the voltage that matters, not the resistances. The AFM inputs to the ECU are probably very high impedance, so that the wiper-to-carbon-trace resistance wouldn't matter very much at all. They probably have the #7 pin of the AFM wired directly to the input of a voltage follower circuit on one of the ECU's boards.

So if you follow the FSM procedures, you would confirm the 6-8 and 8-9 resistances (and check your work by ensuring that your 6-9 measurement is the sum of the two) and then hook the AFM up to power to see if you have smooth voltage fluctuation from bottom to top as you move the vane. To do this, I'm pretty sure you will need a new multimeter, as I think you would have a digital one. You in fact need a crappy needle-type meter (e.g. Radio Shack for $15) like they had back in the 70's. The crappier the better (as crappier meters don't have damping in the movement). Then move the vane slowly and see if the needle moves slowly (as it should) or whether it has points where the needle jumps (indicating a faulty wiper-to-carbon-trace contact or (theoretically possible, but never seen it) breaks/gaps in the carbon trace.

The crappy meter won't be a waste of money. Throw it in the glove compartment for roadside emergencies.

Trust me. Please.

Edited February 6, 2011 by FastWoman

[FastWoman]

Oh, and I forgot, to do the test, you will need some test leads from Radio shack. You will probably need some test leads with alligator clips. You might need to make your own. Just get some wire and a bag of small clips. You'll also need a soldering iron and some solder. This is a great opportunity to learn to solder. (Very easy.) I'd make myself 4 leads with clips on both ends, just for general purpose (to have in the future). You'd have one from the #6 to your battery's (+) (might need a bigger clip), one from #9 to ground (I might have #6 and #9 backwards from what the FSM says, but it really doesn't matter for this test), one for the #7 to one of your test probes, and one for the #8 to the other test probe. Or you can save a test lead (3 total) and just hook the (-) test probe lead to your car's ground. (You'll get different voltage readings, but all you want to know is whether the variation is smooth from bottom to top.)

OR... Maybe you could do this test in situ: Plug your AFM in. Hook the (-) probe of your mechanical meter to a chassis ground. Turn the ignition on, but don't start the car. Touch the (+) probe to the wiper assembly (the contact that hovers out over the top, and then observe the voltage as you move the vain. It might be tight, but you might have enough length on your harness to do it.

Edited February 6, 2011 by FastWoman

[Zed Head]

If you try to measure voltage at the AFM connector be careful, there is very little room in there for alligator clips, and 6, 7, 8, and 9 are all right next to each other. You might short your battery and smoke your test wire.

I measured my spare 78 AFM last night and got the numbers below. I have no idea if anyone can use this information to fix a funky AFM, just putting it out here to ponder.

1978 AFM

Pin 8 to 6 - 182 ohms

Pin 8 to 9 - 102 ohms

12.89 volts across pins 9 and 6

6.6 volts across 7 and 8 with vane closed

0.17 volts across 7 and 8 with vane open

Smooth rise and drop

Jenny, I can't give any details on whether my engine ran rich or lean with my bad 1976 AFM. All I know is that driving around the neighborhood, it would sputter along, occasionally smoothing out to give me hope, but then would buck, and pop back through the intake when I tried to give it throttle. I would guess that it was lean, since I've had rich-running conditions since then (AFR problem) and it ran okay, just stank like unburned gas through the exhaust. I adjusted and tuned everything to factory spec., from timing to valve lash, but it never got better. I did not do the voltage test on that AFM, I just went ahead and got the reman. It was my first Z and I did not know much about the engines.

[Jennys280Z]

Jenny, all I can tell you is that the resistance of two resistances in series is the sum of those resistances. This is the gospel according to Ohm. Praise be to Ohm.

Seriously, I am dead certain on this. I couldn't be more certain. It's as basic as the question of how many money you would have if I gave you $3 and Eric gave you $2. You wouldn't have $6. Although I'm a neurobiologist by training, electronics has been a lifelong hobby. Furthermore, I went to a very prestigious college (in the company of thousands of other nerds like myself), and I sailed through freshman physics (where this was covered) easily within the top half a percent of the class. Electricity and magnitism was one of my favorite subjects. So I'm really, really quite certain! I promise!

Ah, but people on the internet can blow hot air. Lemme find something more authoritative for you...

Here you go.... From Wikipedia...

http://en.wikipedia.org/wiki/Series_resistance#Resistors

So if you believe me (and you really should), there's an error somewhere in your measurements. It might have to do with the measurement point. It could be, for instance, that there is extra resistance from corrosion in some internal connection in the AFM, perhaps from the potentiometer board to the backside of the connector (if that's where you're taking your measurements). Wherever it is, there's an error. That might be the source of your metering problems too, ya' know!

I'm just trying to save you some serious money.

I'm not going to challenge that you know your Ohms and I know what you're talking about. I'm not going to challenge your theory here or your superior knowledge.

As for my limited background, we studied circuits in our 2nd semester Physics and I've had an undergrad electrical engineering class.

What I'm questioning is, whether you're summing the numbers up right. I know you looked at the circuits...what I'm wondering here is if some of these values are only point resistances and so a few of the values are summed differently, or if we simply don't have enough numbers to do a proper circuit analysis. I know you've looked at the schematic so you have me at a huge disadvantage here

But I see relationships here that add up very readily, like 126 + 73 = 199, exactamundo there, so says the ohmmeter. But presuming I measured 126 wrong would throw that value off.

But I've really tested every value of this thing at least six times, not three. Because I tested my AFM from the ECU three times last year, and at least three more times with the AFM removed. I know how to apply the probes, and apply #2 pencil pressure, and see the ohmage decrease at increasingly slower rates until it arrives at the correct value et al

Anyhoo, I still have to trust my bare hands and eyeballs more than your circuit analysis. Perhaps that it doesn't add up is indicative of it's failure?

I sauced the carbon sweep with contact cleaner at least three times. I didn't rub it or put any friction on it. The potentiometer looked immaculate when I saw it the first time and still does.

[Jennys280Z]

If you try to measure voltage at the AFM connector be careful, there is very little room in there for alligator clips, and 6, 7, 8, and 9 are all right next to each other. You might short your battery and smoke your test wire.

I measured my spare 78 AFM last night and got the numbers below. I have no idea if anyone can use this information to fix a funky AFM, just putting it out here to ponder.

1978 AFM

Pin 8 to 6 - 182 ohms

Pin 8 to 9 - 102 ohms

12.89 volts across pins 9 and 6

6.6 volts across 7 and 8 with vane closed

0.17 volts across 7 and 8 with vane open

Smooth rise and drop

Jenny, I can't give any details on whether my engine ran rich or lean with my bad 1976 AFM. All I know is that driving around the neighborhood, it would sputter along, occasionally smoothing out to give me hope, but then would buck, and pop back through the intake when I tried to give it throttle. I would guess that it was lean, since I've had rich-running conditions since then (AFR problem) and it ran okay, just stank like unburned gas through the exhaust. I adjusted and tuned everything to factory spec., from timing to valve lash, but it never got better. I did not do the voltage test on that AFM, I just went ahead and got the reman. It was my first Z and I did not know much about the engines.

So you were probably running lean and you started doing good stuff in good order to tune the car, and wound up running rich...until you replaced your AFM? Is that correct?

AFR? You mean the air regulator?

Your AFM is in line with the Bible. When the Bible says "approximately" it means within 5 or 6 ohms, so it would seem from the numbers...

Edited February 6, 2011 by Jennys280Z

[Zed Head]

Sorry, AFR was a typo, I meant FPR, Fuel Pressure Regulator.

I ended up running rich months later after I had the Reman. AFM installed and the engine was running well.

Recap - my original AFM had test values of 126 and 226. I tuned and adjusted everything I could, but it was not even reasonably drivable until I got the Reman AFM. Months later, after many miles, the engine started running richer and richer and I traced it down to an FPR that let the fuel pressure get quite a bit higher than spec. when it was hot. That was my experience with a "rich" mixture. Smelly exhaust, terrible gas mileage, black spark plugs but still very drivable.

I installed a new FPR, immediately got better mileage, the exhaust stopped stinking, and the plugs cleaned up and look very good now.

In summary - with the out-of-spec. AFM, the car stumbled, bucked and popped back through the intake. Installed the Reman AFM, all those problems went away, passed emissions testing, got registered, drove it around all spring and summer. Months later I had the rich mixture problem.

The AFM I tested above is an extra one that I have. But I also tested the resistance (but not the voltage sweep) of my remanufactured AFM, that is on my car now, and it came in at 100 and 180.

Those are all of my AFM tuning experiences. I have had issues with the fuel pump contact, but that was just a poor adjustment by the remanufacturing company.

Edited February 7, 2011 by Zed Head

[Jennys280Z]

Sorry, AFR was a typo, I meant FPR, Fuel Pressure Regulator.

I ended up running rich months later after I had the Reman. AFM installed and the engine was running well.

Recap - my original AFM had test values of 126 and 226. I tuned and adjusted everything I could, but it was not even reasonably drivable until I got the Reman AFM. Months later, after many miles, the engine started running richer and richer and I traced it down to an FPR that let the fuel pressure get quite a bit higher than spec. when it was hot. That was my experience with a "rich" mixture. Smelly exhaust, terrible gas mileage, black spark plugs but still very drivable.

I installed a new FPR, immediately got better mileage, the exhaust stopped stinking, and the plugs cleaned up and look very good now.

In summary - with the out-of-spec. AFM, the car stumbled, bucked and popped back through the intake. Installed the Reman AFM, all those problems went away, passed emissions testing, got registered, drove it around all spring and summer. Months later I had the rich mixture problem.

The AFM I tested above is an extra one that I have. But I also tested the resistance (but not the voltage sweep) of my remanufactured AFM, that is on my car now, and it came in at 100 and 180.

Those are all of my AFM tuning experiences. I have had issues with the fuel pump contact, but that was just a poor adjustment by the remanufacturing company.

Hmmm based on that, our driveability symptoms are quite different despite our identical readings at the AFM. Here I am adjusting my AFM 5-teeth lean for a noticeable improvement and not getting any backfires at all. I noticed no gassy smell, no smoke out the tailpipe either. With the way yours was running with your AFM though, it sounds like the last thing you would have wanted to do is adjust yours leaner when you're backfiring out your intake. And my car is definitely running a lot better than the way you're describing yours. It just adds to the mystery here. The more information we share the more questions I have.

I still have to believe I have a rich running condition that I'm compensating for by leaning out the mix through the AFM, either because of the AFM itself, or something else. Like the coolant temp sensor, leaky cold start injector.

The FSM says the CSV should kick in for 8 seconds and then it's done with its business, unless there's a problem with it, and it's failing/leaking. If it's failing I would presume it wouldn't add fuel for as long as it should, if it's leaking maybe it adds fuel incessantly?

My car starts well enough. Fuel injection...gotta love it. Even when I'm having air/fuel issues, I still love it.

It's been said my fuel pressure is a bit high, the car starts fast every time but I'm not holding pressure in my rails for much more than an hour. Dunno...maybe my FPR could also be contributing to a rich problem.

Some guy on youtube said that a Nissan mechanic told him that if he tapped on his FPR enough the pressure would build up to like 40-something psi. Sounds like a terrible idea to me, but he was running a 280ZX turbo so maybe the rules change in that case.

[FastWoman]

Jenny, I had assumed you'd have a schematic in the '76 manual, but anyway here's one from my '78 manual (see thumbnail).

There may be minor differences between our AFMs, but I bet the boards are largely the same. If you've had E&M and a bit of EE, then I'll trust you know how to read the schematic and sum the resistances. But as you can see, it's all in series.

Another contact you might not have considered is the one between the wiper assembly and the little contact point that hovers over the assembly. I don't know if that's clear. I honestly missed that one myself and didn't think about it until today.

But again, I can't stress enough, it's the voltages, not the resistances. I bet the ECU's input impedance for this circuit is at least 10-20k, if not higher. I doubt any added contact resistance on the wiper assembly amounts to a hill of beans.

Anyway, if your AFM is off, but the potentiometer is still reading smoothly, you can fix it very easily by changing the resistance between 8 and 9. You can either add resistance between the board and the #9 terminal to diminish the U/Ub ratio, or you can add resistance in parallel to increase the ratio. Food for thought.

I'll shut up now. Good luck with it, whatever you do.

Edited February 7, 2011 by FastWoman

[Jennys280Z]

Jenny, I had assumed you'd have a schematic in the '76 manual, but anyway here's one from my '78 manual (see thumbnail).

There may be minor differences between our AFMs, but I bet the boards are largely the same. If you've had E&M and a bit of EE, then I'll trust you know how to read the schematic and sum the resistances. But as you can see, it's all in series.

Another contact you might not have considered is the one between the wiper assembly and the little contact point that hovers over the assembly. I don't know if that's clear. I honestly missed that one myself and didn't think about it until today.

But again, I can't stress enough, it's the voltages, not the resistances. I bet the ECU's input impedance for this circuit is at least 10-20k, if not higher. I doubt any added contact resistance on the wiper assembly amounts to a hill of beans.

Anyway, if your AFM is off, but the potentiometer is still reading smoothly, you can fix it very easily by changing the resistance between 8 and 9. You can either add resistance between the board and the #9 terminal to diminish the U/Ub ratio, or you can add resistance in parallel to increase the ratio. Food for thought.

I'll shut up now. Good luck with it, whatever you do.

Looking at the circuit design, it looks to me like my R0 is toast, and that is fixed, and independent of the potentiometer wipe, as verified by my unchanging values I tested.

My 8-6 reads too high as well, but I suspect R10 is the culprit there. I don't see there needing to be anything wrong with the variable resistance from 7, other than the ends are wrong. I don't think there's anything wrong with added resistance in the pot, I think that it's possible that R0 and R10 have knocked themselves up ~+25% for whatever reason.

I did see a smooth increase in resistance as I moved the wiper across the trace (on either side of 7). I supposed that was validation enough that the potentiometer is fine in that regard. Both AFMs I tested (actually this almost doubles the number of test results in that I fully tested two AFMs after pulling mine out).

From the design, yes they're obviously in series, and I'm sure my total between 6 and 9 would be higher than Anthony's and Zed's. I'll be 352 Ohms while, I should be expecting 280-290 Ohms.

Edited February 7, 2011 by Jennys280Z

[Jennys280Z]

Cleaning the air regulator, I've sloshed, soaked and spanked this AR with WD40 for 24 hours and am trying to determine if I got it to open any more. Maybe it appears that I have but I might be kidding myself. A local Z mechanic suggested I try the soaking bit because as he said these things get gummed up with residue over the years. I shook the AR in an arc a few hundred times till most of the WD40 came out, and little pieces of dirt or whatever were coming out in the liquid the first time. Another soaking overnight got me to this point....

xx

[Zed Head]

The critical fact is does it open and close consistently. It might have been stuck open, and you have adjusted your warm idle speed to compensate, giving a low cold idle speed. It is pretty easy to connect 12 volts to the two pins and see if it closes. Then disconnect the power and see if it reopens. Do that a few times and you'll be fairly sure it's working as well as it can.

[Jennys280Z]

Doubtful Zed. I never had a high idle problem that I had to dial down with idle adjustment. My car idles @800RPM for 30 seconds then drops to 550-600. It does this every time, so the symptoms are that it opens and closes consistently it just doesn't open enough. It will heat up and close eventually with the heat of the engine, and it does close as i tested with heat rather than voltage. As for the voltage heating the heater and closing it, I'm considering the possibility my car will behave better with the thing unplugged, as it would heat up slower and give me at least the 800RPM i start with for a little bit longer.

I'm thinking now that I can get it to "stay" open more by moving it with a screwdriver, but just handling it and warming it up with my body heat it seems to close it up again like the first photo, and then it doesn't open up any more than that until I prod it with the screwdriver again. So I suspect that I'll install this same AR and it'll work better than before but only the first time I start the motor and heat the thing. That is, when it cools again it won't open up that much on its own. So I guess in that, it isn't opening consistently, but it closes just fine.

I'll have it installed again this weekend...see if there's any improvement which I doubt. Then I'll flip a coin whether I want to buy another one. Will test the CTS (hot) and CSV from the ECU too and see if there's anything untoward going on there.

The local mechanic here who's worked on Zs for over 30 years said he's never replaced a single AFM in his career and told me I'd be wasting my money on a new one. I admitted that my car might run a lot worse with a rebuilt AFM installed and he's almost talked me out of replacing it....sigh so i dunno. Once I get the rear end reassembled and legal again I'll make the 30 mile drive out to see him if I can't diagnose the problem myself and that 60 miles will be the first real drive in a long time. I just hope I don't get stranded or something.