KE-Jetronic potentiometer replacement

[chrisd87]

Hi everybody.

The air flow meter potentiometer on my 190E 2.6 is in need of replacement. There's been a problem of hesitation between about 1000-2000rpm when the engine is cold for some time now, but recently it's got quite a lot worse, so the time's come to sort it out. I'm 98% sure I've narrowed the problem down to the air flow meter potentiometer, as the resistance reading it gives is somewhat erratic/unresponsive towards the beginning of the sensor plate's travel. It's not due to the coolant temperature sensor, as I've checked that and it's all OK. OVP is presumably fine, as the ABS is working. Once the engine's fully warmed up, the problem goes away.

I was just wondering if anyone else had replaced the potentiometer, and had any advice to offer?

It seems like Bosch are no longer making this part, and the only (new) genuine one I can find for sale online is in America, and with shipping, import duty and VAT would cost about £200 - quite a lot of money for a variable resistor! Aftermarket copies that are available are very much cheaper, at £25 or so. Does anyone know if these are of acceptable quality? I wouldn't be expecting it to last as long as the original, but I would want it to work!

Many thanks.

 

[gbjeppm]

So will the original part last 8 times as long as the aftermarket? As it is 8 times the price.

For me its a no brainer, try the aftermarket part, and see how it goes.

 

[grober]

You want to take a look at this post Aiflow meter pot source - Bosch CIS - PeachParts Mercedes ShopForum
which details the old and new type [ with the small adjustable potentiometer] I think with the old type it was adjusted by physically setting the position of the unit in relation to the main body of the fuel metering unit. I'm guessing the new ones will be easier to fine tune/set up using the little adjustable pot but will need both??

basically its 2 metal wiper arms sliding across carbon tracks to alter resistance in proportion to the position of the air sensor plate. As the unit ages these carbon track surfaces get worn away causing erratic readings.


Scribe/mark the position of the old unit and take some resistance readings before removing the old one to help setting up the new one.

As has been said its possible worth trying a good quality pattern part first. There's not much that can go wrong.

 

[John Jones Jr]

It seems like Bosch are no longer making this part, and the only (new) genuine one I can find for sale online is in America, and with shipping, import duty and VAT would cost about £200 - quite a lot of money for a variable resistor! Aftermarket copies that are available are very much cheaper, at £25 or so. Does anyone know if these are of acceptable quality? I wouldn't be expecting it to last as long as the original, but I would want it to work!

I purchased one online, every possibly from eBay about 2-3 years ago which was a genuine Bosch as a spare and it was nothing like £200, from memory about £75-100. Spend a bit more time searching online, try eBay.de too. As for the cheapo one, personally I wouldn't bother.

Bear in mind when fitting the new one you need to get a certain resistance between two of three pins, sorry can't recall the voltage (something 0.5) and can't recall which pins either. Use the original location as a guide only because if it's been adjusted in the past, it's been adjusted due to wear on either or both the pot or throttle spindle, the new one will obviously have no wear.

 

[chrisd87]

Thanks for the responses. Wasn't aware that there were two different types of potentiometer - I'll have to see which type my car is fitted with. Probably the later one, since it's a mid-'92 model.

I've already looked at ebay.de, and all of the items there are aftermarket, too. Irritatingly, Amazon.de sells genuine Bosch ones for £135, but will not send outside of Germany. That is the cheapest I've managed to find anywhere by quite some margin. There are other sources that will send to the UK, but all are £200 or more.

There is a seller on ebay.de who offers an aftermarket part with a 2 year warranty for about £40, so that may be the best bet. I'd hope it'd last a bit longer than that, but in all likelihood this part is not going to need to last for the same 22 years and 150k miles as the original.

I have a PDF of the Mercedes manual, which describes the test procedure. The voltage at a warm idle needs to be set to 0.7v +/- 0.1v. This should be an interesting experience, since the idle is quite often not entirely steady on my car, and apparently the adjustment is very sensitive!

 

[grober]

TRY THIS BOSCH No:-
http://art.rexbo.net/bosch/pdf/f026t03021si0000cob200.pdf
keep trying ebay.de there are companies that will ship to the UK.

 

[John Jones Jr]

Here's another reference which matches Grober's attchement above. :

Adjustment potentiometer, Jetronic Bosch F026T03021

Also check the mixture 3mm hex key required (& a light hand). Check the throttle micro switch is operational too.

 

[chrisd87]

Thought I'd update this thread for the benefit of anyone else needing to do this job. In the end I decided to go with an aftermarket part, which cost about £40. It came with a 2 year warranty, and cost 1/4 of the what the cheapest genuine Bosch part would have cost. Whether it was the right decision, time will tell!

The part came with a set of instructions, which are pretty similar (but not identical to) those produced by SG Motorsports here.

There are a few points not mentioned in the instructions which complicate the job somewhat. In this installation of the M103 engine ('92 190E 2.6) there are more parts that get in the way of the mounting screws than just the fuel pressure regulator. Both the wiring conduit and the idle control valve also have to be unbolted and pushed out of the way in order to reach the lower mounting screws with your screwdriver. See what I mean:




Once you've got the old pot off and the new one on, words cannot convey how fiddly it is to get it aligned properly. The instructions state that a small movement can make a big difference to the voltage reading (you need to get the voltage between pins 1 and 2 to 0.7v +/- 0.1v, at a warm idle). No kidding! I would guess that to get it within that range, you have at the very most 0.5mm of adjustment, probably much less. Perhaps the most frustrating aspect of this job is the fact that the voltage reading alters when you fully tighten the mounting screws, so just when you think you've got it right, you're back to square one again.

To make matters even worse, the adjustment has to be carried out with the engine running, yet to get at one of the screws, the fuel pressure regulator has to be disconnected from the fuel distributor and pushed out of the way. So you get the pot aligned OK with 3 of the screws done up, stop the engine, disconnect the pressure regulator, do the final screw up, reconnect the regulator and start the engine, only to find that it's now no longer within range. Garrrgh!

 

[chrisd87]

Just thought I'd update this thread for the benefit of anyone else who finds themselves needing to replace this part. The aftermarket part I fitted worked for a while, but about 14 months and 6000 miles later it started malfunctioning again. Interestingly the problem manifested itself somewhat differently than the 'dead spot' I'd been getting with the original part. This time the idle speed was very unsteady, causing the car to sometimes stall when you came to a stop at a junction. Other times the speed would dip very low and then recover, and even more rarely it would be too high. There also often seemed to be an excessive amount of engine braking when you took your foot off the accelerator at lowish speed.

My initial thoughts were that the OVP might be on the way out, but there were no other issues like cold starting trouble or ABS light on, so I began to suspect the potentiometer. I tested it and and sure enough it gave erratic resistance readings as the plunger was depressed and released (sometimes going completely open-circuit), so I decided to bite the bullet and buy a genuine Bosch replacement. Not cheap (about £130 delivered from Germany) but hopefully it will be the last time I have to replace it! Interestingly the genuine Bosch part was much easier to adjust correctly than the aftermarket one. With the aftermarket part, it took hours of fiddling to get the correct 0.7v +/- 0.1v, mainly because when you fully did up the mounting screws, the reading would change dramatically. Not so with the Bosch part - the reading only changed by 0.02v when the screws were tightened, which meant the adjustment procedure only took a few minutes.

Having taken the car for a very quick drive, it seems that the fault has disappeared, so I'm hoping it stays that way!

 

[toilet]

Just thought I'd update this thread for the benefit of anyone else who finds themselves needing to replace this part. The aftermarket part I fitted worked for a while, but about 14 months and 6000 miles later it started malfunctioning again. Interestingly the problem manifested itself somewhat differently than the 'dead spot' I'd been getting with the original part. This time the idle speed was very unsteady, causing the car to sometimes stall when you came to a stop at a junction. Other times the speed would dip very low and then recover, and even more rarely it would be too high. There also often seemed to be an excessive amount of engine braking when you took your foot off the accelerator at lowish speed.

My initial thoughts were that the OVP might be on the way out, but there were no other issues like cold starting trouble or ABS light on, so I began to suspect the potentiometer. I tested it and and sure enough it gave erratic resistance readings as the plunger was depressed and released (sometimes going completely open-circuit), so I decided to bite the bullet and buy a genuine Bosch replacement. Not cheap (about £130 delivered from Germany) but hopefully it will be the last time I have to replace it! Interestingly the genuine Bosch part was much easier to adjust correctly than the aftermarket one. With the aftermarket part, it took hours of fiddling to get the correct 0.7v +/- 0.1v, mainly because when you fully did up the mounting screws, the reading would change dramatically. Not so with the Bosch part - the reading only changed by 0.02v when the screws were tightened, which meant the adjustment procedure only took a few minutes.

Having taken the car for a very quick drive, it seems that the fault has disappeared, so I'm hoping it stays that way!

Thank you for taking the time to reply to this back then as 8 years on, I'm benefitting from it! on my '91 300E with the m103, the car sometimes starts up instantly in the morning, sometimes it takes a bit longer to crank. Also the idle creeps up to 900 rpm as the car warms up, and then settles at 600 rpm, which seemed somewhat normal, but everywhere else I read it's supposed to go up to 1200 rpms. Another strange symptom was the car jerking when shifted to D. Not sure if related but everything else checks out. Now I've checked most of everything, and replaced all fuses recently (they were all original to car). This actually made the biggest difference in terms of drivability (somehow feels like I'm putting down more power, shifts are smoother too). Then I checked resistance at air flow potentiometer, and the results were exactly as you described, starts off normal, as plate is pressed, it then goes open circuit then comes back, but the numbers were rising gradually and coming back down also. However, I noticed that as they come down, the resistence would settle at a different number at idle. So overall it seemed erratic.
I'm planning to go with an aftermarket one also first, just to diagnose the problem, and if that fixes it will go full Bosch mode. Shall post updates too!

 

[optimusprime]

It is ok gong there , if all the HT system is up to date . When was the plugs leads distributor top replaced along with the rotor ?

 

[toilet]

It is ok gong there , if all the HT system is up to date . When was the plugs leads distributor top replaced along with the rotor ?

2 years ago in Jan '21, so kinda ruling those out for now.

 

[optimusprime]

Well your engine revs at 900rpm is the same as my m103 ,.Then mine drops down to 750 ish . With potentiometer fasults its like revs are there one second and then a dead spot before it picks up again right away. Did you clean out the inside of the -IAC valve ? and any hoses split replace asap

 

[toilet]

Well your engine revs at 900rpm is the same as my m103 ,.Then mine drops down to 750 ish . With potentiometer fasults its like revs are there one second and then a dead spot before it picks up again right away. Did you clean out the inside of the -IAC valve ? and any hoses split replace asapView attachment 143484

That sounds comforting actually if yours is doing the same. I did clean the ICV with brake cleaner. And yes there's a hose split on both ends, but it felt like it was able to seal pretty snug despite. I took pics, uploading.

With the potentiometer, I checked resistance readings between pin 1 and 2. When I deflect the plate, at certain points the resistance goes to infinity, then comes back. So I thought perhaps there are dead spots.

I also have a small smoke machine. Was wondering if you could tell me where I must inject smoke so that it tests ALL vacuum lines as well as the ICV? Will the brake booster vacuum line be sufficient, or should I introduce smoke through air intake.

 

[optimusprime]

Brake booster should be a good place to use smoke test. ,,I think you will find more bad hoses as you test it with smoke . Any vacuum leaks that you do find, must be eliminated for a good running engine. Tell me what your econo guage is reading in the dash board .

 

[toilet]

Brake booster should be a good place to use smoke test. ,,I think you will find more bad hoses as you test it with smoke . Any vacuum leaks that you do find, must be eliminated for a good running engine. Tell me what your econo guage is reading in the dash board .

My cluster doesn't have an economy gauge.
But here's an interesting update, I started the car after 2 days today, and it started instantaneously from dead cold. haha..
No fiddling in the past 2 days except to adjust the belt (unsuccessful attempts cause 19mm bolt just won't budge).
I don't know what's wrong and right anymore

Smoke test next to see if idle air control pipes are leaking.

 

[optimusprime]

Did you say you had a manual o this m103 ? Ist the bolt holding the tensioner or the adjusting bolt is it bolt 16 or 5

http://www.w124performance.com/service/w124CD1/Program/Engine/103/13-3450.pdf