Repairing unobtanium potentiometers (worn out tracks)

[Silvas]

Hello guys,

I'm looking for a reliable method to restore worn-out traces in potentiometers. Please see the attached file. This is not an audio-related potentiometer but an automotive one that became impossible to get. What are the options here ? I have many of them that needs restoration if possible. Any leads here ? Techniques, ideas, products ? I hear you guys ! Thanks a lot.

 

[JohnRoberts]

Wow, that looks like more than a simple pot...  I am guessing the wiper fingers connects/shorts across the 4 traces, or perhaps shorts across two pairs of two.

The technology basically uses a conductive/resistive ink slurry, that gets screened onto the substrate to form the trace pattern.  Then that green ink gets cured in an oven to fix it and fine tune the bulk resistance. (Final resistance depends on time at temperature during curing.)

While in theory it might be possible to paint on fresh uncured ink, then cure with something like a hot air tool, but I wouldn't even know where to start, to source materials, or spec the slurry.

Before seeing the picture I thought to try to swap out the substrate from a similar pot, but that one looks pretty unique so I don't expect anything similar enough to repurpose.

JR

 

[Silvas]

Wow, that looks like more than a simple pot...  I am guessing the wiper fingers connects/shorts across the 4 traces, or perhaps shorts across two pairs of two.

The technology basically uses a conductive/resistive ink slurry, that gets screened onto the substrate to form the trace pattern.  Then that green ink gets cured in an oven to fix it and fine tune the bulk resistance. (Final resistance depends on time at temperature during curing.)

While in theory it might be possible to paint on fresh uncured ink, then cure with something like a hot air tool, but I wouldn't even know where to start, to source materials, or spec the slurry.

Before seeing the picture I thought to try to swap out the substrate from a similar pot, but that one looks pretty unique so I don't expect anything similar enough to repurpose.

JR

Thanks  for your reply John,  this piece is a TPS (throttle position sensor) for an old Bosch single point injection.

I was thinking about using toner and oven it to restore the traces...Has someone done something like that before ?

 

[JohnRoberts]

Is it possible a modern version could be adapted to work? They still use them in vehicles, but may use digital rotary encoder gray scales or some different technology.

Maybe a low mileage old part off a wrecked car.?

JR

 

[Silvas]

Is it possible a modern version could be adapted to work? They still use them in vehicles, but may use digital rotary encoder gray scales or some different technology.

Maybe a low mileage old part off a wrecked car.?

JR

I've already went that route, sadly not an easy part to get as you'll have to but an entire and used throttle body and you don't have a way knowing that the traces are in good shape. Other equivalents aren't compatible with the ECU even if they fit, they simply don't work as the original.  So i want to try and repair the traces, at least give it a try before giving up and installing a programmable ECU  (with all the expense and parts that comes with that).  Will try MG chemicals carbon paint, have a few worn out pieces so i can experiment a bit.

 

[JohnRoberts]

I don't feel lucky about DIY resistance elements.

Have you determined the form factor or configuration of the 4 resistance tracks?

How many are faulty? One looks a lot worse than the rest.

Is it possible to hang another pot off the same shaft and use it in place of the one or two faulty track(s).

Conductive ink might short across any open circuit dead spots, but will not track properly. A dead spot is probably worse than some mis-tracking.

Good luck... you are probably not the only person to deal with this.

JR

 

[home_listening]

Only experience I have with this is in my old BMW, an 1986 E30.  The mass airflow sensor is a similar mechanism, using a Bourns resistive element. No chance of repairing it, its a laser trimmed ceramic substrate resistor with complex looking curves.

Have you tried finding another vehicle that uses the same ECU? Its likely the throttle position sensors from other brands would be compatible even if part numbers are unrelated.

 

[Tubetec]

I read somewhere about restoring scratchy pots with B grade(soft) pencils ,graphite does indeed provide a resistive track even if drawn on paper , different kinds of B grade pencil have different resistances for a line of given lenght and width , try it yourself with the multimeter .I once had an electronics education kit that allowed you to make a crude oscillator ,the keyboard was several lines in pencil each emanating from a central  point , you touched the far end of each line with another electrode to complete the circuit and tune the oscillator .

For those tracks damaged in a very minor way at the very start a tiny drop of conductive silver paint might be enough ,
Maybe someone with 3d printing might be able to  layer up some new conductive material , finding the original resistance of the tracks on a working unit would be a good place to start , and if its log or lin your dealing with .

 

[pvision]

I've come across this problem on older Mercedes but in a throttle body not a TPS. The tracks wear at the rest position where the wipers spend most of their time

On a Mercedes throttle body there are two pots in reverse sense so one goes high resistance as the other goes low. I imagine the CPU derives a position from comparing both

I think a TPS might work using a pot driven from the pedal. It might take some finessing to get sufficient travel to get a suitable resistance from the pot but it may be doable

If the car is a popular model you may find an aftermarket supplier or repairer may have stepped in with a repair program

It would be worth checking with BBA Reman and other ECU repairers to see if this is something they can repair. Be careful to establish the cost if a repair is not possible - BBA Reman burned me quite badly on a throttle body repair and I won't use them again

Nick Froome

 

[Silvas]

Have you tried finding another vehicle that uses the same ECU? Its likely the throttle position sensors from other brands would be compatible even if part numbers are unrelated.

Yes ! I did - didn't work well. I mean, it somewhat works but engine's behavior is erratic :-(

 

[home_listening]

Damn! Bosch ECU is so popular its a shame the parts can't crossover more. Good luck!

Yes ! I did - didn't work well. I mean, it somewhat works but engine's behavior is erratic :-(

 

[Silvas]

OK...How about spraying "Graphite-33" or making a compound powder (which has to be baked i guess) made with toner and graphite to reconstruct the tracks ? A precise mask has to be made for application of the spray / compound of course !

Take a look of a couple worn-out TPS pieces i have.

 

[12afael]

Mmm I might drift a bit thinking in a solution but maybe would be possible to make an encoder there and the control transistors maybe. I see more easy to print a pcb with the pattern. I don't know the resolution needed.

Does omeg make custom pots? Probably for one is not cost eficient but I'm thinking if this is so rare it could be  a side business.

 

[Tubetec]

I saw one guy make a resistive paint with a mix of graphite powder  and wood glue , thats going to soften from moisture or solvents though . Years ago here there was a product available in locksmiths , Lock eze I think was the name , it was pure very  finely powdered graphite ,  maybe some carbon added could be used to get the appropriate resistance , a masking layer sounds good , then you might be able to polish off any excess you end up adding without touching the original layer .

I found a good article once about fader production at Penny+Giles , cant remember who wrote it or where , but its goes through the whole process , granted they will have had special machines ,where you'll need to do the job by hand , its the materials and method you want to find out about .

The two other wiper plates you showed look in better condition , If you want to experiment try mixing graphite with various glues , solvents until you get something that can be painted on with a tiny brush that dries tough and can be sanded or scraped down flat and level , only experiment on an actual unit once you've found a good mix of glue ,graphite etc , in pencils the hardness or softness of the tip in determined by the amount of fine clay added in the graphite mix ,  it makes a difference to resistance also .

Theres a trick from guitar repairs for filling cracks dents, baking soda and super glue , a drop of glue is placed on the area then the powder is sprinkled in the gap ,  the process is repeated until the dent is over filled ,then its gently sanded back level , maybe a similar approach might work here ,im not saying super glue will do it  , maybe try clear nail polish or cellulose based 'Dope' . Kapton tape might make a good mask . could also be advisable to experiment on carbon tracks from old potenitometers to make sure the Sh!t (slurry) sticks  ;D

Maybe this can help ,

http://www.dpaonthenet.net/article/29764/Hall-effect-throttle-position-sensor-for-motorsport-applications.aspx

 

[Gus]

I seem to remember reading about people somehow offsetting the wipers so they touch a not worn section of the tracks.
If I am remembering correctly it was at some BMW forum over 10 years ago.

 

[Silvas]

I seem to remember reading about people somehow offsetting the wipers so they touch a not worn section of the tracks.
If I am remembering correctly it was at some BMW forum over 10 years ago.

Thanks Gus ! Sadly, this seems not to be an option here because of the multi-brush, very wide wiper design. Also, tension on the wiper is critical (has to be kept incredibly low as not to cause extra wear) and i guess that even attempting it will damage the wiper which is an integral part of the lower throttle body :-(

 

[JohnRoberts]

Yup I looked at the wear patterns and don't feel lucky about a simple repair.

Wipers wipe,,,

Surely this is not an isolated problem so somebody smarter than me is working on a solution. Keep digging, answer may not come from musos (no offense).

JR

 

[Silvas]

Surely this is not an isolated problem so somebody smarter than me is working on a solution. Keep digging, answer may not come from musos (no offense).
JR

You're absolutely right ! However, this is proving to be a very educational research regardless of the outcome ;-)

 

[cyrano]

Do you have a precise parts number?

A friend of mine specialises in Mercedes conversions and restoration. He's converting vans to mobile homes and mobile workspaces and restoring vintage cars. He's sitting on a pile of parts.

 

[Silvas]

Do you have a precise parts number?

A friend of mine specialises in Mercedes conversions and restoration. He's converting vans to mobile homes and mobile workspaces and restoring vintage cars. He's sitting on a pile of parts.

Hello Cyrano !

Yessir ! Thanks for the help. The TPS itself as far as i know was never available as a separate spare but the whole lower throttle body (the one that have both the TPS and the wipers) is OEM part # 3435201597

It comes from Bosch single point injection, car is a Renault Clio 1.4 with Energy 8V K7 Engine. But it was a pretty common injection system at the time AFAIK.  Used also on the R9 and R19 amongst many other models sharing the same engine.

TPS itself has moulded a "9285" number and it comes in both black or brown color.

Any leads would be greatly appreciated !