mass flow meter pins 2 and 3 ohms

[hcoles]

Car - 1989 3.2 stock - 79k miles - 6k after top end rebuild.

This is probably duh... but I figured since I had the motor out of the car I would check a few other things in the process of trying to find the noise/vibration....

The manual says pins 2 and 3 should have a smooth transition of ohms starting with closed.... I assume smooth means start at one value and move up or down in value until WOT....

well I checked it and it starts with .5kohms and then starts going up to about .9kohms and then goes down in the .8's and then back up to .9's and then back down in the .8's...

This doesn't seem right...should I unseal the cover and see if the reistat in good shape? What ohm range is there with a healthy mass meter? The ohms at the temp. pins (1 and 4) seem ok and correspond to the temperature in my garage at the moment.

-henry

[2.7RACER]

Henry,
Do a search "AFM gone bad?". Near the bottom Fastpat posted a site with all the info you'll ever need about testing an AFM.
Bottom line I checked my AFM at the ECM, pin22 and ground with the engine running. The voltage should go smoothly from near zero to 5 volts as you accellerate the engine.
The key is smoothly. As the potentiometer wears inside the AFM the resistance can vary wildly as the AFM door opens.
Essentially the resistance which creates the voltage as seen by the ECU should represent the position of the door in the AFM.
As the door is opened by the air flow the voltage should increase smoothly.
If the voltage jumps around the AFM thinks the door is jumping around and adjusts the fuel to the injectors based upon the voltage reported to the ECU by the AFM.
This causes flat spots.
My '85 failed smog a couple years ago because of a bad AFM.
I did the above test and replaced the AFM.
I don't think this will cause any noises.
An AFM is expensive. I would not replace it unless you are certain it is bad. That is causing hesitation, poor running, failed smog, etc.

[hcoles]

I must be a dumb ass but I can't seem to get to the link..can you give dirrections again or another way to find the link to fastpat's thread..thanks for checking your volts on the AFM..

-h

[Jeff Alton]

Here you go...

http://frwilk.com/944dme/afm.htm?session=b9FGjNcXME8ophyKIiu5YWUMnT

Cheers

[hcoles]

Read the link. Thanks. I think I'll see what my mechanic says re. that using an ohm meter doesn't work... I'll try the bench top test with a 9V battery...that seems simple enough...

In any case it does seem that taking the cover off is ok to do...

-h

[hcoles]

ok..update.
Took the top off and it is pretty much design exactly like the link and has the wire added update.
The link says that measuring with ohm meter doesn't work....we are not convinced. The circuit is passive. Took the thing into the lab. here and hooked it up. I also had an electronics engineer look over shoulder... I'm mechanical so I don't know about lectrical. ;-) Anyway we are getting ohm readings that are jumping around and will clean with special reistat cleaner tomorrow and then see what we got. Put the surface under a microscope...definately there is considerable wear but doesn't look worn through...you can see that much of the wear is at the beinging...as would be expected for just off idle and driving
Very interesting stuff...we should know in a couple of days.
Might need to shift the board to get the two contacts on fresh tracks, will also inspect the contacts...if they are too rough then may shift and run as long as possible...

[hcoles]

for 911's or SC's and 3.2's we could use a electronic fuel system forum heading...

[2.7RACER]

Henry,
It may be possible to move the wiper arm slightly and find a clean, unworn section on the pot. I've replaced two AFM's, because I couldn't find a good section on the pot.
The interesting thing for me is the same thing you have discovered. That is the thought you can't test the pot with an ohm meter and decide whether it is good or not.
Actually a simple concept, as the pot moves with the door the resistance changes, which in turn increases the voltage to the ECU.
To believe that you can't test with an ohm meter, defies the laws of physics.
Interestingly my 911 ran rather well when it failed the smog test. I tested the output of the AFM at pin 22 of the ECU as I said previously. The voltage jumped around as I accellerated the engine. To me this was simple, the AFM potentiometer was bad.
I changed the AFM, The voltage changed smoothly. Next day passed smog easily.
Sounds like you are curious about the operation of the AFM. I don't see it making any noise or vibration that you are looking for.
Please continue to post what you learn from your investigation.

[hcoles]

Doug,

Good..looks like you and I are thinking along the same lines.
The AFM should have smooth ohms and then smooth voltage change as the vane moves. Both should be smooth. We will clean tomorrow and test with ohms only and then apply voltage on the bench. As an electrical engineer reminded me today...an ohm meter reads by applying dc volts...so in fact you are applying volts in both cases. We will know more tomorrow, I got some special contact cleaner as instructed today at Frys. We also put the track under a microscope....there is no doubt that there is grove wear...but it doesn't appear to go through to the substrate. In some places if we press the wiper down the ohms changes....this indicates bad contacts or track wear or dirt/etc. It is too bad that this is made with 20-25 year old technology...not long life, air restriction, probably not that fast and accurate. The 911chips people sell a completely updated plug replacement but it is $2500.

Re. the vibration problem...this AFM issue is probably not related. ...but I did read that a bad AFM can cause the controller to oscillate

I want to start a DME only forum....I'll bet there is a lot wrong with fuel control systems on these cars but they still run pretty good as you indicate and the same with me.

Have you found a link that is good for step by step checking the inputs to the DME? E.g. how to set up the throttle stop, full and closed throttle switches, set idle rpm and CO adjustment under the AFM, which is just a air bleed around the vane.

Also, I have a occiliscope and a prob. that should let me see the PWM signal going to the injectors...the plan is to set it up and read the scope while driving around.

[2.7RACER]

Henry,
I used the Bentley Carrera manual to troubleshoot my '85 911. I didn't get too deep once I found my problem.
I found the "Bosch Fuel Injection & Engine Management" book by Charles O. Probst very informative. Also a Bentley pub.
About the time you fully understand your system, you'll be getting a different newer car and the process will begin again.
Of course the more you study, the more you'll realize the shortcomings of the motronic system.
The new stuff just gets better and better.
Learning though is half the fun.
For me the other half is on the track.

[hcoles]

Yes short comings of the motrnic system and many other parts of the engine design... I like the Porsche, I like the way it feels when driving but I have to say the more I learn about especially the engine design and all the issues and upkeep the more I'm not sure it is a good track and drive to work car for me. I keep forgetting that this car was probably layed out in 1981 as Porsche was getting ready to preplace the SC and many parts have the same design as far back as 1964 when they were designing to replace the 356. It is probably pretty good considering how far back much of the design is.

As for my ability to get a better P-car or other trackable car, don't hold your breath, you don't know my wife.

[2.7RACER]

Henry,
Don't get me wrong. I'm not knocking the motronic system.
It's just as these fuel and engine management systems evolve they get better.
Things like a hot wire mass air flow sensor with essentially no moving parts.
I've got 165,000 miles on my '85 Carrera and it is very reliable. Like I said, it was running great before it failed smog.
My son's '86 Carrera has 240,000 miles and running strong.
I'm sure once you learn about your system, you will feel more comfortable with it.
My wife has gone through 4 cars since I got my '85 Carrera. Maybe I need to have a talk with her.

[hcoles]

well, we checked using a dc power source...selected 5vdc..the output vdc is nice and smooth...so I guess the guy was correct...you can't check it with an ohm meter....must be there needs to be a min. current level to get it to work properly...

we cleaned with a contact cleaner and got some old carbon off...I will let it dry overnight retest tomorrow and then seal it back up for now...

-h

[hcoles]

an interesting side note for people using run of the mill RTV, many types contain a corrosive outgassing cure agent. There is a special GE sealer that has electronics friendly cure agent...I forgot the pn but you can probably find on the web.

[hcoles]

I finally found GE RTV162 for $18.60 for 3 oz.

[lindemans]

Having a MSc in electronics, I support those who argue that it is a passive resistor circuit that you can check through resistance measurement. Yet through voltage measurement in conjuction with a battery will work too.

One critical note however: (digital) measuring equipment tends to show the average value (measured over a certain sample-period) on the display. If the contact-points are not always permanently making proper contact, it will not necessarily show on the reading.
Best way to check as with any potentimeter-like component is to apply voltage eg with a battery and measure the voltage using an oscilloscope. Any flaws in the contact while moving will show as spikes on the screen.

Compare it with the worn volume control of an old radio: you can still control the volume but while turning you get this scratching noise from the speakers.

[hcoles]

yes..we were very strong in the camp of passive resistor should be smooth...apparently there is something else in the circuit. Tomorrow I'll hook up silly scope and see what gives.. When I pushed the door slowly open I got erratic ohm readings...when apply 5vdc and pushed door open I got no noticable erratic reading using dvm...but as you say we should check with scope.. Having two of us with MS in EE and ME..and 60 total years of experience..not sure if that is an advantage or not...
I'll send in a report tomorrow... thanks for comments.

[hcoles]

Here is a picture of approx. what I have..this is 1984 vintage I have 1989 vintage...the two wipers are to eliminate bad or jumpy connectivity or so they say...the track seems to be made of something like carbon. I guess this was pretty slick back then...not now.

[jase007]

Henry:

I've used that site and the probst book and Bentley's and had the same results when using the 9V bench setup. This bosch(?) AFM is the same on a LOT of cars ... I did all of my testing on a 1990 BMW E30 325iX. I even picked up two used AFMs from junk yards and "investigated" their wear. They were pretty cheap if I remember correctly. I'd be curious is the Carerra, E30, ____other f'ern car AFMs are all the same and one could cross-shop for parts.[simple part # comparison]

As referenced in the frwilks 944 site ... there is a "non-serviceable" intake air temp sensor in these Bosch AFMs [pictured on the site and #6 in the pic you posted. These can and do go bad and can cause a lot of grief when diagnosing problems.

I have also seen several AFMs where the spring tension has been "messed with" [clock spring] and they never seem to work right. E30 guys like to swap the M20B25 [baby six motors] AFM guts into M30 [older larger senior six motors] AFMs [larger throughput] and increase injector size, as well as FPRs to try and gain some HP. Lots have done it with Mustang #19 injectors ... can't comment on how cars ran or how long the ECU / DME took to compensate [reduced duty cycle, etc...]

Keep us updated.

Cheers,

Jase

[hcoles]

After more messing with the thing in the lab. it seemed that I could make the ohms approach work better but not smooth. Using 5vdc I was able to get a smooth reading.

It would be nice to know the, I think it is called the beta factor the the NTC temperature device. I expect it is just a thermistor but as you know they don't have a linear response.

So I got some special GE RTV and buttoned the thing back up.

I ordered a breakout connector and box so I can easily monitor the signals going/coming from the DME..we will see how that goes.

I'm also thinking of making a custom breakout box with LED's/etc. showing how the inputs change while driving.