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

[Jennys280Z]

Now I've got 3 AFMs all reading the exact same freaking 126 and 226 Ohms

If I have three good AFMs on my table and I'm wasting time money energy and sleep on nothing I'm going to blow a fuse. In fact this rebuilt POS reads higher resistance on the adjustable figures than either of my other two units. 12 month warranty? I wouldn't run my motor with this POS installed in it for 12 seconds.

Pins 8-6 226 Ohms

Pins 8-9 126 Ohms

Pins 7-8 251 Ohms

Pins 6-7 126 Ohms

That's MY frame of reference. A 1976 Z gets 226 and 126 on pins 8-6 and 8-9 respectively so say 3 different 1976 AFMs including one fresh reman. Any questions?

What's someone got to do in this world to get a good AFM to spec, or is the "EFI BIible" starting to look severely overrated?

I am past the point of SICK of F'ing around with this stupid thing I just want my damn car back.

Edited February 20, 2011 by Jennys280Z

[Zed Head]

Whoa. A shocking start to the morning (West Coaster).

Where did you get it? And, just to confirm, what brand of ohm-meter are you using? I had to buy a new one when I started working on my car, because my old (very) Radio Shack analog unit wouldn't cut it in the low resistance range.

Something is not right. Anthony G. in Post #201 got 186 and 103, I have a 76 and a 78 that both get ~180 and 100, and those are the specs. from the FSM.

If your ohm-meter is working right, at the least you have received an out-of-spec. AFM.

I bought my reman, AFM from MSA and it had a sticker on it from Fuel Injection Corp.

POS = Previous Owner's Stuff?

[FastWoman]

[broken record]

It's the voltages that matter, not the resistances. In fact it's possible that different versions of the AFM have different resistances but the same output voltages. I don't think your old AFMs are necessarily bad. (I haven't seen any evidence either way.) Also I doubt your rebuilt AFM is bad.

[/broken record]

The FSM's diagnostic criteria for the AFM do not specify tolerance limits for their resistance measurements, beyond which the unit would be rejected. This is in sharp contrast to items such as the CTS, which have rejection limits specified. I think these measurements are therefore approximate, and the resistances given in the FSM are only guidelines. If you read 500 ohms when you should be reading 100, then you might have a problem. However, 125 and 100 are perhaps close enough to verify that the resistor is largely intact, which might be all that's necessary. Remember that tight resistive tolerances are a more modern thing. Back in those days, the typical carbon resistor had a 10% tolerance specification.

CTS: You might be able to sand down the CTS connector to fit inside a deep socket. There's probably no harm in that. Be careful that you're not torquing the connector, rather than the base, when you screw the thing in. FAIW, you might be able to fit a crow's foot wrench around the base. You don't have to turn the thing very hard to get a good seal.

A general word of advice from the BTDT (been there, done that) camp: You might not be in the right frame of mind to work on your car right now. When you're frustrated and angry, it's time to set the wrenches down, before either you or your car gets hurt.

-------------------

Zed, I think I might know why our pennies vs. voltages measurements are somewhat different. I used pennies collected in a jar by an elderly gentleman. His might be older than yours. If so, they would have a higher copper content, while yours would be mostly copper-clad zinc (lighter). Perhaps we need a different standard than coinage. Either that, or the pennies must first be sorted by date!

[Zed Head]

I agree on the voltages. That's the purpose of the quarters/pennies procedure. f the AFM shows 50% of supply voltage closed and essentially 0% open, and a smooth change with vane movement then the electrcial circuit should be in good working order.

I don't necessarily agree on the acceptable resistance readings. As I suggested earlier, 25% is a large variation when you're talking about measuring how much air is entering the engine. As the voltage output experiments have shown, the AFM is just splitting the voltage in half, and, most likely, using one side to determine air volume. I haven't done any electrical calculations for years, but maybe you could determine and compare the voltage split of a 100/180 potentiometer to a 126/226 potentiometer. Then we would have the difference in voltage output of the two, closed, open and in-between.

Sorry it's not working for you Jenny.

Re the pennies - I had pondered that. Best to get a balance and use known weights in the future.

[FastWoman]

Zed, 100/180 and 126/226 are roughly the same ratio (=.56), hence the voltages would be the same.

It's possible that that these resistances all drift the same way over time with heat and oxidation. If the resistive material all ages uniformly, the resistance ratios would stay the same, and hence the voltages would remain the same.

With regard to 25% being a large error: Well, yes, it would be significant if resistance were what is important. However, if we're talking about the ratio of voltages U/Ub (per what the FSM says), then the errors would be inconsequential.

Edited February 20, 2011 by FastWoman

[Jennys280Z]

Zed, 100/180 and 126/226 are roughly the same ratio (=.56), hence the voltages would be the same.

It's possible that that these resistances all drift the same way over time with heat and oxidation. If the resistive material all ages uniformly, the resistance ratios would stay the same, and hence the voltages would remain the same.

With regard to 25% being a large error: Well, yes, it would be significant if resistance were what is important. However, if we're talking about the ratio of voltages U/Ub (per what the FSM says), then the errors would be inconsequential.

I understand and concede your points about the voltage being the important measure. Perhaps all I really need is a water temp sensor and a well-tuned AFM to have a great running car. That's what I feel like doing at this point. Ordering the right temp sensor, turning my AFM back to stock, and enjoying the ride. That's the hope (fumes) I'm running on right now at least. sigh

[Jennys280Z]

Whoa. A shocking start to the morning (West Coaster).

Where did you get it? And, just to confirm, what brand of ohm-meter are you using? I had to buy a new one when I started working on my car, because my old (very) Radio Shack analog unit wouldn't cut it in the low resistance range.

Something is not right. Anthony G. in Post #201 got 186 and 103, I have a 76 and a 78 that both get ~180 and 100, and those are the specs. from the FSM.

If your ohm-meter is working right, at the least you have received an out-of-spec. AFM.

I bought my reman, AFM from MSA and it had a sticker on it from Fuel Injection Corp.

POS = Previous Owner's Stuff?

Actron CP7676. It works excellently and handles these low values easily. I understand how to get a good resistance reading and how the resistance reading decelerates down to its true value when working correctly.

POS = Piece of Stuff

I got mine from Python. Blech.

Going to just replace the CTS for now and see if I can get my magic mixture problems resolved. I suspect this to be one of the problems my car has been having. All of my symptoms point to the CTS. Yes I have an out of spec AFM perhaps .

Zed when was your '76 manufactured? Mine was in 11/75. If yours was made in 11/75 can you tell me for sure what model# AFM you have?

Edited February 20, 2011 by Jennys280Z

[Jennys280Z]

[broken record]

It's the voltages that matter, not the resistances. In fact it's possible that different versions of the AFM have different resistances but the same output voltages. I don't think your old AFMs are necessarily bad. (I haven't seen any evidence either way.) Also I doubt your rebuilt AFM is bad.

[/broken record]

The FSM's diagnostic criteria for the AFM do not specify tolerance limits for their resistance measurements, beyond which the unit would be rejected. This is in sharp contrast to items such as the CTS, which have rejection limits specified. I think these measurements are therefore approximate, and the resistances given in the FSM are only guidelines. If you read 500 ohms when you should be reading 100, then you might have a problem. However, 125 and 100 are perhaps close enough to verify that the resistor is largely intact, which might be all that's necessary. Remember that tight resistive tolerances are a more modern thing. Back in those days, the typical carbon resistor had a 10% tolerance specification.

CTS: You might be able to sand down the CTS connector to fit inside a deep socket. There's probably no harm in that. Be careful that you're not torquing the connector, rather than the base, when you screw the thing in. FAIW, you might be able to fit a crow's foot wrench around the base. You don't have to turn the thing very hard to get a good seal.

A general word of advice from the BTDT (been there, done that) camp: You might not be in the right frame of mind to work on your car right now. When you're frustrated and angry, it's time to set the wrenches down, before either you or your car gets hurt.

Bless you hunni your advice is always so good. :kiss:

Look I hear you and I agree with you of the possibility that there may well be nothing wrong with my AFM at all. If it's voltage ratio and all these values shifted together, that sounds accurate to me. But this further degrades the value of the EFI Bible doesn't it? It puts so much emphasis on resistance. I made a smaller point earlier that if these units "fail" the same way and arrive at these higher values 126 and 226 (why?) then this information should be common knowledge too!

[Jennys280Z]

I am so depressed about my car right now I'm sorry if I "let it out" on anyone. I was so excited about this weekend...sometimes I really should lower my expectations x

[Jennys280Z]

Zed, 100/180 and 126/226 are roughly the same ratio (=.56), hence the voltages would be the same.

It's possible that that these resistances all drift the same way over time with heat and oxidation. If the resistive material all ages uniformly, the resistance ratios would stay the same, and hence the voltages would remain the same.

With regard to 25% being a large error: Well, yes, it would be significant if resistance were what is important. However, if we're talking about the ratio of voltages U/Ub (per what the FSM says), then the errors would be inconsequential.

But Sarah is roughly the same good enough? Sorry I guess I'm asking what you just said. I'm just wondering how much small errors in voltage matter? How influential would 0.05V difference be?

Edited February 20, 2011 by Jennys280Z

[FastWoman]

No worries, Jenny. You have every reason to be frustrated right now. These old cars can get pretty funky, and they stay funky until you find all the problems. I think the best perspective to have is to keep tinkering, be happy when it runs, be thrilled with any improvements along the way, and keep tinkering. Eventually your car will be right again.

You know....

Eric might disagree with me about this, but I suppose an argument could be made that you DON'T have to have everything perfect before making the mixture adjustments that we've been recommending. This frustrating process has been a bit like Master-san training young Grasshopper mechanic, starting with wipe on, wipe off.

Maybe the more sane approach (that will keep your spirits up) is to fix everthing you see that obviously needs fixing, leave unknown stuff for the future, install a variable resistor in your coolant temp sensor circuit to adjust your mixture, and enjoy your car. Then whenever you discover and fix a new problem that might alter your mixture, simply readjust your mixture.

Contemplate this procedure:

1) Get a 5 kOhm variable resistor from Radio Shack. Set it to roughly 2.5 kOhms Attach two wires to it, and plug those wires into the CTS connector (the harness side).

2) Start your engine. Adjust the variable resistor for the smoothest idle.

3) As your engine continues to warm up, keep adjusting the variable resistor to maintain a smooth idle.

4) When the engine is fully warmed up, do a very careful adjustment of the variable resistor to achieve the highest engine vacuum. Then shut your engine down.

5) Measure the resistance of the variable resistor, and measure the resistance of the CTS. Calculate the difference.

6) Report back, and I'll help you figure out what resistance to wire where, in order to achieve a good idle.

7) Wire in your extra resistance, and give your car a test drive. You can adjust the resistance a bit, if necessary, to fine tune it.

8) Love Cars, Love People, Love Life. Enjoy the ride.

[FastWoman]

Honestly, when you're talking about fuel and air in a cavity, going bang, I think there's some room for error. You might find this hard to believe, but I was even able to start my engine without the AFM plugged in (forgot to plug it in!). It blew black smoke out the exhaust, but it still managed to keep running.

As I've said before, I think my car was delivering about 25% less fuel than it should have, when my local Z specialist declared that my engine ran pretty nicely. The PO felt the engine ran nicely when it ran even leaner than that. I didn't completely agree, but I thought it was reasonably driveable (enough to drive it a few hundred miles home), and I felt it had the potential of becoming a good engine (which it did). So there's considerable margin for error, IMO.

Edited February 20, 2011 by FastWoman