Measuring AFM output

[jaxtaterror]

Is there any way to measure the output of the AFM without an ocilliscope? I think my MAFT is not working correctly, but I'd like to see what is being reported to the ECU to verify.

I do not have any codes, the engine will start fine, but will die within 3 - 10 seconds. Applying the accelerator will increase revs, but engine will still die.

I've checked boost leaks, fuel pressure, and wiring. No codes.

Don

 

[3p141592654]

You can scope the KS signal from the AFM. It is light-green/red on my car. Best to use the E2 ground on the scope lead, it is the brown wire at the AFM plug. You can also use the frequency setting on your dmm if you have one.

If you don't have AFM codes it's unlikely to be the problem.

 

[CajunKenny]

A lot of Fluke Meters measure frequency. Fluke 87 for sure (that's what I have) and I think the Fluke 79. Other brands of DVM's measure frequency too; but, Fluke is the best. IMO anyway...

I agree with the above post too. If no code, probably not the problem.

 

[jetjock]

Although I have better I sometimes use my old HP 5325B counter. There's just something about nixie tubes

 

[CajunKenny]

He he... No arguing that one Jet!

 

[jaxtaterror]

OK - thanks, guys. Just ordered a dmm capable of frequency and I'll measure.

I've tested almost every component I can think of that would cause the problem - just seems like fuel isn't being metered correctly. I have nothing to base that on, though.

Don

 

[3p141592654]

Have you put a scope or noid light on an injector?

 

[jetjock]

3p is correct in that it's unlikely a bad AFM will cause that particular symptom. Try unplugging it. The engine should still start and run if everything else is OK.

 

[jaxtaterror]

3p is correct in that it's unlikely a bad AFM will cause that particular symptom. Try unplugging it. The engine should still start and run if everything else is OK.

OK - it won't run unplugged either. If I unplug it, it acts the same, except it will then throw a light.

If it's unplugged isn't it just using default fuel map? I have 550s with elevated fuel pressure, so if it is just using the base map, its getting LOTS of fuel...

No - I haven't checked anything with the injector(s). What is the procedure?

The incident that started this whole mess was a wire on the efi fuse rubbing and grounding out on the chassis. The short wasn't bad enough to blow the fuse, but put such a high load it literally melted the fuse and fuse box. That's why I'm kind of expecting some scrambled electronics.

Don

 

[figgie]

.....

If it's unplugged isn't it just using default fuel map? I have 550s with elevated fuel pressure, so if it is just using the base map, its getting LOTS of fuel...

umm

say that again? Please tell me that this is not the entire fuel mod list?!?!

 

[jaxtaterror]

umm

say that again? Please tell me that this is not the entire fuel mod list?!?!

Not at all - MAFT transalator (not pro) with 3" GM maf, AFPR (don't remember where I have it set, but above stock pressure), 550 injectors, upgraded supply/return line, MKIV fuel pump.

MAFT was tuned and working well with injectors and fuel pressure. I haven't made any changes there.

My point was by unplugging the AFM (MAF), the computer will be using the base maps expecting 440 injectors and stock fuel pressure. All my translation would be lost while unpluged because the MAFT has nothing to translate.

Don

 

[figgie]

Not at all - MAFT transalator (not pro) with 3" GM maf, AFPR (don't remember where I have it set, but above stock pressure), 550 injectors, upgraded supply/return line, MKIV fuel pump.

MAFT was tuned and working well with injectors and fuel pressure. I haven't made any changes there.

My point was by unplugging the AFM (MAF), the computer will be using the base maps expecting 440 injectors and stock fuel pressure. All my translation would be lost while unpluged because the MAFT has nothing to translate.

Don

no

the computer (TCCS) does not care what sioze the injectors are. Be it the stock 440cc/min or 1000cc/min.

The fuel pulse will be the same. Even in limp mode, the signal to the injectors will be in a given reference from the TP angle. A dead or partially working AFM (which can't happen as it is a freq) will throw a code, as you saw. But that said, the Maft Translator, you are not using the stock AFM so more than likely, a wiring issue.

 

[jaxtaterror]

no

the computer (TCCS) does not care what sioze the injectors are. Be it the stock 440cc/min or 1000cc/min.

The fuel pulse will be the same. Even in limp mode, the signal to the injectors will be in a given reference from the TP angle. A dead or partially working AFM (which can't happen as it is a freq) will throw a code, as you saw. But that said, the Maft Translator, you are not using the stock AFM so more than likely, a wiring issue.

I'm probably splitting hairs here, but as you said the injector pulse will still be the same, so with 550s and 20% more fuel pressure, you will get 45% (or so) more fuel for the same pulse as compared to 440s and stock pressure. So with a non-functioning air measurement device, the amount of fuel injected will be 45% more than with a stock fuel system.

But all that aside, I'll take the advice and start looking elsewhere as well. I ordered a mutlimeter last night that can measure frequency, so I can test the signal once it arrives.

I appreciate all the help, and if there are any other subsystem you can think of I should be checking, please let me know.

Don

 

[3p141592654]

You are confused. Without the MAF the ECU has no idea of how much air is going into the engine and calculation of the injection duration cannot be done.
So it defaults to an extremely primitive map consisting of 3 ignition/fuel values that covers starting, idle and off-idle. The engine will run but barely. In addition maximum engine speed is limited to 3000 rpm.

 

[jaxtaterror]

You are confused. Without the MAF the ECU has no idea of how much air is going into the engine and calculation of the injection duration cannot be done.
So it defaults to an extremely primitive map consisting of 3 ignition/fuel values that covers starting, idle and off-idle. The engine will run but barely. In addition maximum engine speed is limited to 3000 rpm.

Thanks, 3p. I guess I'm still confused - if the car is using those 3 primitive maps, they are based on a stock fuel system. Since the default pulse widths from those base maps would inject 45% more fuel based on my modded fuel system, I would think the engine would be unable to run on those base maps.

I guess I'm assuming those base maps assume the amount of air entering the engine from the tps angle, and the amount of fuel based on a stock fuel system. Since I've only changed one side of the equation -fuel (without the computer's knowledge) I would think the result of the base maps would be to overfuel the engine.

Again , thanks for all the help guys. I am taking all of your advice and will be searching elsewhere.

Don

 

[jaxtaterror]

Maybe this will explain my thinking a little better. I'm assuming those default maps are kind of like:

if 1% open throttle, pulse injectors 10ms
if 10% throttle, pulse injectors 100ms
if 50% throttle, pulse injectors 500ms

and so on.

So, the computer would see the tb open a %, and look that percentage up in the table and return an injector pulse.

With my fuel system providing 45% more capacity, that same pulse width would inject 45% more fuel than the computer expects.

 

[3p141592654]

Yes, you are correct on the 45% point. The operation is more primitive than you describe, because it does not use the TPS angle input. ECU looks at the STA and IDL inputs to decide which of the 3 operating points to use. It will be running 45% richer based on your mods to the fuel system.