AFM reostat board recoating

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micbrody
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AFM reostat board recoating

Postby micbrody » Sat May 20, 2017 2:16 pm

Hopefully the following will solve a problem all of we fuel injected owners have: The afm track wearing down.

Brief synopsis of my past history with my AFM:
1) Purchased car in late 2014; measured resistance on board; turned out many dead spots during sweep of arm. Per some videos on youtube, I bent arm to make contact with new parts of reostat board. The resistance measurements seemed to now not have any dead spots.
2) about 1.5 yrs later, started to get some funky starting issue; idle would waver(drop to 400-500) when coming to a stop right after deceleration. I checked everything out; the last thing I checked was resistance on reostat board.. Turns out many dead spots again. Even when I would manually press down on the copper loop contact, it still remained dead.

I took sweep arm off and just checked resistance of the the black surface. Seemed like most of the black surface had very bad conductivity; many dead spots. Not sure why so bad; though one thought was the "deoxy spray" I used a few years back on this surface might have been a bad idea. Also, there seemed to be new worn tracks; so maybe when I bent sweep arm, there was too much pressure which caused accelerated wear. (I did have an extra AFM which seemed to correct most of these problems. But in the spirit of passing on wisdom on te board, I decided to see what I could do to fix problem). My conclusion I needed a new contact resistance surface or new board.

I did a search (even going to web page of company that made board). Nobody selling (or making ) replacement boards. I then decided I would try to recoat the black part of board.

I went on Amazon and found some conductive paint : look up "Bare Conductive Electric Paint Pen 10ml ".
I masked off the sweep area of reostat board; I then applied 3 layers of paint. Each time I would apply, and gently wipe off excess. Let it dry 15-30 minutes; and repeat.

I let whole thing dry another 24 hrs.

I re-applied the sweep arm using the least pressure to assure contact throughout entire sweep. And Eureka!!!!
This time I test voltage. Very "linear" and smooth ad even; no negative fluctuations when going from rest to full sweep.

The only two question:
1) Does it actually work in car (I don't see why not); but can't test because lots of rain over last few days
2) How long will paint last before developing dead spots? One advantage of the three layers is that the paint layer has a "thickness" which might lead to more conductive contact area when the tiny contact projection starts carving into paint. The sides of projection should make contact. Worst case scenario: I re-adjust to have arm sweep over new part of board. If I were to do this again, instead of bending copper arm (which would affect contact pressure), I would remove board and elongate the the three mounting holes on board. This way I could just slide board to have contact arm meet new material (saw that on another AFM blog).

So the following are the photos:

Before painting: notice the wear near beginning of sweep surface....probably explains bad idle when coming to a stop
Image

The paint: ($10 on Amazon)
Image

Masking off of sweep part of reostat board:
Image

Applied bead of paint before wiping (a layer had already been previously applied, wiped , and dried)
Image

Finished with tape mask removed:
Image

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RRoller123
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Re: AFM reostat board recoating

Postby RRoller123 » Sun May 21, 2017 5:10 am

The resistance value of the thick film material is critical, hopefully the resistive paint is very, very close!
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Re: AFM reostat board recoating

Postby micbrody » Sun May 21, 2017 7:36 am

The resistive value of this material actually just has to be higher than the sum of the fixed green thin film printed resistors, but still conductive.
If you look carefully the first picture (unpainted), you'll notice that there are "conductive fingers" that are under the black part. These fingers conduct with minimal resistance into the fixed green resistor chain. Depending on where the swing arm is located, the current travels mostly to the closest finger, with some current going to the finger on the other side of sweep arm. As long as the black material has resistance (per linear mm on the black arc that is significantly more the total resistance of fixed resisters, then the current will only go through the resistor chain.

I did have a chance of using it the car yesterday and it worked great ! There was no issues with performance; and when coming to a stop, the idle went down to its normal idle, not overshooting.

I did open up the "working" AFM to see if there were any differences; and also tested voltages. There were small differences between starting voltage and ending voltage duriing the entire sweep. When I investigated, I found that some of the fixed green film resistor values were different between the boards. The AFMs actually had different part numbers , 280 202 023 - the one that came with car; and the replacement one: 280 202 018. Maybe they printed board differently (different fixed thin film green resistors) between those model numbers . Other explanation is that the green thin film resistors have degraded over passage of time
Last edited by micbrody on Sun May 21, 2017 7:58 am, edited 2 times in total.

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Re: AFM reostat board recoating

Postby klweimer » Sun May 21, 2017 7:49 am

Pretty ingenious! Let us know how it works.
Kirk

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Re: AFM reostat board recoating

Postby micbrody » Sun May 21, 2017 7:55 am

It did work! (I added details to post just before yours)

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Re: AFM reostat board recoating

Postby RRoller123 » Sun May 21, 2017 11:59 am

That is fantastic! That problem has plagued us for years.
'80 FI Spider 2000
'74 and '79 X1/9 (past)
'75 BMW R75/6
2011 Chevy Malibu (daily driver)
2010 Chevy Silverado 2500HD Ext Cab 4WD/STD BED
2002 Edgewater 175CC 80HP 4-Stroke Yamaha
2003 Jaguar XK8
2003 Jaguar XKR
2021 Jayco 22RB
2019 Bianchi Torino Bicycle

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Re: AFM reostat board recoating

Postby PhillySpider » Mon May 22, 2017 7:21 am

what's the process to test resistance of the strip?

Thanks!

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Re: AFM reostat board recoating

Postby micbrody » Mon May 22, 2017 12:15 pm

Testing resistance of the board can be done a few ways. I inspected and test board components with an ohmmeter and needle probes.
For instance, the green thin film resistors can be tested by touching the silver exposed part on each side of resistor.
The black film can be tested directly by touching the probes on two different parts of board. There is a manufacturing test strip that is about 3/4 of a cm , just to the left of "Bourns" onboard . This is probably a quality control to test the resistance of black material over a given length x width. My two boards had different values (5000k and 7000k), but much higher than resistance chain (each green resistor is anywhere from 6 to 40 ohms...... this is from memory.....so I might be off.
To really test function of board, one should see how voltage in vs. voltage out changes with deflection of door. I connected with alligator clip leads the positive of 9v battery to the 3rd pin from right (when looking at board to read "Bourns"); negative to the 4th pin . I doubled up on third pin to one lead of voltmeter; other lead of voltmeter to 6th pin (this is the pin that is directly connected to large copper arm that eventually connects to copper swing arm.)
Then just move the door of AFM and one should see a gradual increase of voltage. Mine went from 2.3v to 6.6volt; if you move the 4th pin battery connection to the 5th pin, you'all start at voltages in the mid-range (I don't remember) and at full swing you'll get full voltage.
I believe they use two readings to get a ratio; that way, if your car battery voltage varies, the ratio will be constant between the two voltages, so it still knows where sweep arm is located (so it doesn't have to depend on a single absolute voltage). Also, using a ratio helps eliminate the change in resistance values (degradation of board; oxidation) of actual contact of copper to black resistive surface.) Only when there is "infinite" resistance (no conduction of current from swing arm to board)l-dead spots) will the data fron AFM be corrupted

I have a video of process ; but having trouble posting it

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Re: AFM reostat board recoating

Postby micbrody » Fri Jun 02, 2017 12:31 am

I have been using the replacement AFM (different, unpainted) for a few weeks; over last few days I noticed engine missing and poor performance when car is warm. (For instance, took car to farmers market about 4 miles always. Performed great. Spent 45 minutes there. On way home, noticed car not doing well during acceleration under load.
I was not sure of problem. I added pre-fuel pump fuel screen (100 microns) to see if my old tank was shedding flakings of rust. Noticed no debris (clear glass chamber). This problem seemed to repeat again after car warm (and on warm days). I then replaced AFM with my repainted AFM. Drove great (installed 10 minutes after engine issues; so car was warm.
However : the bad news: after about 5-10 miles instarted to notice lackluster performance( poor acceleration). I took original AFM; unmounted the reostat board; elongated the mounting holes. Remounted board slightly slid south so new reostat track. Car performed great. So unfortunate conclusion: my paint job probably not performing adequately. Either related to wear---- which could be related to AFM heating up from engine heat; or just quick degradation from copper sweep arm.

I will investigate with ohmmeter in near future; but wanted to write this post to prevent people from painting the board ----- might be unrelated -----but I don't want other people to try this until I can confirm either way if the paint was a good or bad thing tondo

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Re: AFM reostat board recoating

Postby micbrody » Mon May 21, 2018 1:11 pm

Stay tuned........
I just ordered a more resilient conductive paint.
I will repaint board and try again

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Re: AFM reostat board recoating

Postby focodave » Mon May 21, 2018 4:28 pm

There HAS to be someone, somewhere who supplies new replacement circuit boards for these AFM's.
If you think of all of the Bosch L-Jetronic systems that were sold back in the day, and how many are still "on the road", and the fact that you can get remanufactured AFM's -- isn't there a source for these boards somewhere?

I am lucky -- I don't need a new board -- but I sure would like to have one in my parts inventory if I need to replace the thing years down the road.

If any of our go-to suppliers are reading this, can you please chime in?
Vick Autosports, Auto Ricambi, Allison's???
One of you must have an idea of where we can get these circuit boards.

I'll buy one from you if you need to make a profit, rather than telling us where we can buy these directly. (not trying to be snarky -- just keeping it real)

Does anyone know where we can get these??
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Re: AFM reostat board recoating

Postby RRoller123 » Mon May 21, 2018 4:37 pm

I expect that Bosch has no interest in selling the individual internal part, as the assy is a little tricky for Jack and Jill Sixpack. They would rather just sell the whole AFM. It is really too bad, because I bet the only real problem that ever develops with these is the track wear (other than maybe a backfire damaging the door).

Pete
'80 FI Spider 2000
'74 and '79 X1/9 (past)
'75 BMW R75/6
2011 Chevy Malibu (daily driver)
2010 Chevy Silverado 2500HD Ext Cab 4WD/STD BED
2002 Edgewater 175CC 80HP 4-Stroke Yamaha
2003 Jaguar XK8
2003 Jaguar XKR
2021 Jayco 22RB
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Re: AFM reostat board recoating

Postby focodave » Mon May 21, 2018 5:03 pm

I am quite sure Bosch would rather sell a complete AFM, than to sell just the circuit board.
I still believe there has to be a place to get the boards.
Where are the AFM re-builders getting the boards? Are they simply making adjustments to the arm position on the vertical shaft to re-locate the arm tracking location -- or -- are they supplying new circuit boards in their units?
Someone out there must know...
1980 Spider 2000 F.I. (my hobby)
1970 MGB GT (my other hobby)
2008 Ford Expedition (daily driver)
2019 Harley-Davidson Electra Glide Standard
2019 Harley-Davidson Iron 883 Sportster

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Re: AFM reostat board recoating

Postby micbrody » Mon May 21, 2018 11:36 pm

I did specifically ask Fuel Injection Corporation if they sold new boards. They responded no. And also added that :
“No such thing as "new". If the traces on your boards are worn out, they will be replaced. All of our products carry an 18 month unlimited mile warranty.”.

I interpret this as they do NOT have a source for new boards; they either have surplus supply of AFMs and cannibalize other AFMs; or they recoat the board as I have (and hopefully do better) tried

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Re: AFM reostat board recoating

Postby RRoller123 » Tue May 22, 2018 7:06 am

To properly recoat the wiper board requires a capability in thick film resistive printing (TFR), which usually exists commercially only in semiconductor and hybrid electronics mfg facilities. I don't think that modern AFMs use this technology, for obvious reasons, it wears out too easily. I think they use a hot wire anemometer approach, but don't quote me on that. My guess is that Bosch has farmed out the legacy technology to a small semiconductor/hybrid shop somewhere that makes the few spares needed for the aftermarket in AFMs, but there is not enough demand/profit to sell them separately. It would be interesting to know how FCI "replaces" the traces. :?:
'80 FI Spider 2000
'74 and '79 X1/9 (past)
'75 BMW R75/6
2011 Chevy Malibu (daily driver)
2010 Chevy Silverado 2500HD Ext Cab 4WD/STD BED
2002 Edgewater 175CC 80HP 4-Stroke Yamaha
2003 Jaguar XK8
2003 Jaguar XKR
2021 Jayco 22RB
2019 Bianchi Torino Bicycle


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