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

[cozye]

Actually, as I recall, your AFM did not pass the testing with flying colors. But you did some work on it, and put it back on your car and said that it ran great now so the problem was fixed, even though the resistance readings were out-of-spec..

I remember because the resistance numbers on your AFM were the same as mine had been when I got my car, and my engine ran terribly until I got a rebuilt AFM. I remember being surprised that mine tested the same as yours but mine would not run the engine well at part-throttle at all, but yours did.

I also remember because the impression was that I had led you on a wild-goose chase, which I had not intended to do. So your AFM might be working fine, but the test resulkts of it were not "in-spec.", at least at that time.

Just trying to help you keep things clear. You have a lot of things going on. My car ran terribly with a bad AFM but yours might be fine.

which resistance values were out of spec? I missed that part.

[Zed Head]

Here is the original thread - http://www.classiczcars.com/forums/showthread.php?t=37299

I can't remember the pin numbers but what should have been 100 was 126 and what should have been 180 was 226. My car popped and backfired through the intake when I had those numbers. A rebuilt AFM cleared it up. From what I know now, I think the whole fuel curve was shifted lean, with those high resistance numbers. I think the ECU just runs 5 volts through the potentiometer and uses the split to determine enrichment.

Worth a double-check probably. SBlake01 said he had seen some AFMs come back to life after cleaning like Jenny did, but not all (in the thread).

[Jennys280Z]

Here is the original thread - http://www.classiczcars.com/forums/showthread.php?t=37299

I can't remember the pin numbers but what should have been 100 was 126 and what should have been 180 was 226. My car popped and backfired through the intake when I had those numbers. A rebuilt AFM cleared it up. From what I know now, I think the whole fuel curve was shifted lean, with those high resistance numbers. I think the ECU just runs 5 volts through the potentiometer and uses the split to determine enrichment.

Worth a double-check probably. SBlake01 said he had seen some AFMs come back to life after cleaning like Jenny did, but not all (in the thread).

But I got the exact same values on two AFMs. When you said your numbers were the exact same as mine, that told me that my AFM was fine. Particularly because here's a third AFM, again with the exact same values. Maybe that is wrong. Maybe these AFMs go bad and then all arrive at the exact same numbers (126 and 226) ?

Did you do anything else to your car at the same time you had the AFM off which might account for the difference?

For some of the measurements, my backup AFM was reading higher in ohms until I cleaned it. After cleaning it, it is now within 1 ohm of my original. I could put that AFM on the car and see if there's any improvement

The EFI Bible lists "continuity" for some of the expected results. For instance pins 7-8 is listed as "continuity" yet I get exactly 200 ohms on two independently owned AFMs. So if 200 ohms is considered "continuity" then I'm guessing 126 is close enough to 100. Or, if 200 ohms is NOT continuity, then what? All bad AFMs go bad the exact same way? Ok fine but why isn't that in the Bible?

If anyone with a '75 or '76 280 wants to drop their ECU and test their working AFMs, or have already done so and can produce their results, that would be cool. If they get the same results, that will be the 4th AFM with the same values. I think that'll prove conclusively that my test results are really flying colors.

The two tests are with an ohmmeter testing pins 6 to 8, and 8 to 9.

Maybe I should be following the Bible militantly and expecting to get exactly 100 and 180 ohms or else it's gone bad?

And now for the biggest question, if the AFM is possibly responsible for this, does the AFM generally go bad all of a sudden?

Edited January 27, 2011 by Jennys280Z

[Jennys280Z]

Pins AFM 1 AFM 2 (in Ohms)

6-8 226 226

8-9 126 126

7-8 199 200

6-7 73 75

[Zed Head]

Sorry Jenny, I meant that you had the exact same numbers as my bad AFM, the one that didn't work for me. When you said it was working for you after cleaning it up I left the subject alone. I did not do anything else to the car, I had to send the old one in for the core, and it was the last thing left for me to try. I tend to do things one at a time if I can so that I get the true effect of one change. For the record, with the bad AFM, my car revved fine with no load, but popped, spit and backfired when I tried to drive it.

I had the same thought that maybe some component in the AFM goes bad and they all end up at the same 126/226 values. All I know is that my rebuilt one measured 100 and 180 and the difference was night and day between the two for me.

The FSM shows continuity at the ECU connector as the quick test, then actual resistance measurements of the AFM off the car as the in-depth test, so you might get your money back if it's not the problem.

Sorry. I don't know whether to hope that the AFM is your issue or not. They are expensive. If you could find one to borrow or find one in a junkyard that tests correctly, that might be the way to go. Some yards have a 30 day guarantee.

Edited January 27, 2011 by Zed Head

[Jennys280Z]

Sorry Jenny, I meant that you had the exact same numbers as my bad AFM, the one that didn't work for me. When you said it was working for you after cleaning it up I left the subject alone. I did not do anything else to the car, I had to send the old one in for the core, and it was the last thing left for me to try. I tend to do things one at a time if I can so that I get the true effect of one change. For the record, with the bad AFM, my car revved fine with no load, but popped, spit and backfired when I tried to drive it.

I had the same thought that maybe some component in the AFM goes bad and they all end up at the same 126/226 values. All I know is that my rebuilt one measured 100 and 180 and the difference was night and day between the two for me.

The FSM shows continuity at the ECU connector as the quick test, then actual resistance measurements of the AFM off the car as the in-depth test, so you might get your money back if it's not the problem.

Sorry. I don't know whether to hope that the AFM is your issue or not. They are expensive. If you could find one to borrow or find one in a junkyard that tests correctly, that might be the way to go. Some yards have a 30 day guarantee.

Yes I suppose it's possible my AFM is toast and it's possible I've already wasted money on a 2nd bad one. If I'm going to prowl junkyards I'll take my multimeter with me and give myself an extra guarantee.

Looking at my ECU test results and my recent results with the AFM in hand, my AFM tested identically the same whether from the ECU or directly. Both my ECU and AFM had exceedingly clean connectors. The ECU's pins were (are) immaculate.

I certainly hope that the AFM's not what the problem is because they are very expensive rebuilt parts.

[Jennys280Z]

I'm also wondering if these AFM resistance values in the EFI Bible ever changed from year to year of 280Z?

[AnthonyG]

I can check what my AFM readings are this weekend for a comparison.

also a 76 280z

[Zed Head]

The 100 and 180 values are for the 1976 AFM for sure. In general, the AFMs are not interchangeable except for a few years. I think that 1975 and 1976 are the same, maybe 1977 too.

It's possible that 126 and 226 will work. But the FSM says not. I'm not an electronics person so can't say exactly how these values come into play.

I don't think that high resistance comes from the connectors, it comes from the resistive elements that make up the potentiometer.

[Jennys280Z]

The 100 and 180 values are for the 1976 AFM for sure. In general, the AFMs are not interchangeable except for a few years. I think that 1975 and 1976 are the same, maybe 1977 too.

It's possible that 126 and 226 will work. But the FSM says not. I'm not an electronics person so can't say exactly how these values come into play.

I don't think that high resistance comes from the connectors, it comes from the resistive elements that make up the potentiometer.

How the ohmage effects the unit's function...that'd be nice to know.

Perhaps 126 and 226 are points that the unit intentionally rises to, to still be functional on the car, but yet also communicate a universal signal that it's gone bad and needs replacing/fixing.

The test you posted has the "standard" and "approximate" resistivity measures. 126 could be considered "approximate" to 100 depending on what kind of range in Ohms we're scaling with imho. It would be nice if we could form a validated consensus on this here. I'm still suspicious of units all falling back to 126 and 226 if gone bad, and if so, it's even more suspicious that this fact isn't in the EFI Bible.

Anyhoo, if three or four of you with good running stock 280s can all flash your 100s and 180s at me then I'll surrender, and go get the harness and straps because that's how I'm going to feel having to go through all this again.

AnthonyG: please do! If you could run at least the four tests that I ran in the table posted above that would be so good.

The 100 and 180 values are for the 1976 AFM for sure. In general, the AFMs are not interchangeable except for a few years. I think that 1975 and 1976 are the same, maybe 1977 too.

Zed...okay. I was thinking of the part numbers changing for different year 280s and then I read your post again...

Edited January 28, 2011 by Jennys280Z

[Jennys280Z]

Got the cap and rotor from MSA today. They are Daiichi brand parts. Not what they told me over the phone but they fit right. Installing those right now. I love how NGK numbered my spark plug wires for me. How thoughtful!

[Jennys280Z]

I don't think that high resistance comes from the connectors, it comes from the resistive elements that make up the potentiometer.

Yup I don't either. *sigh*

Zed, when your AFM readout like mine, did your car seem to miss when you drove it, even audibly at low RPM? Did it feel like it was at a loss of power, almost like it was running on five cylinders? Was it intermittent? Was it sudden? Did the problem come, then go, then come back again do you remember?