What's the adjusting switch pot for in the 055 DME?

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DRPM635CSi
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What's the adjusting switch pot for in the 055 DME?

Post by DRPM635CSi »

I've just removed and opened my 055 DME (M635CSi) for the purpose of photographing it for the design of a piggyback board to be mounted inside to allow dual chips (one stock for passing road worthiness tests + the performance chip with DFCO null - taking a leaf out of VW cheating cleverness), with an external case mounted toggle switch for quickly and easily switching between the two programs.

On removal, I was surprised to see there is already a factory-spec hole in the case filled with a plastic grommet seemingly perfect for me to use for this exact purpose. On closer inspection I can see the hole lines up with a blue adjusting pot mounted on the top board looking like a mini-bullhorn PA speaker. It looks a lot like this can be turned with the right tool inserted through the grommet in the case.

Now, I'm not one for messing with things I don't understand, nor for trying to fix things that aren't broken in the first place, so before I go experimenting with this adjustable thing on the DME, can anyone enlighten me as to what it does or what it's for? It's the blue thing in the corner of the top board in the photos below.

Photos of open 055 DME
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Brucey
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Re: What's the adjusting switch pot for in the 055 DME?

Post by Brucey »

it is a 'fuel quality switch'. IIRC its workings are explained in the workshop manual. Up to eight positions, I think, so that various combinations of fuel properties can be accommodated.

Write up for contemporaneous Porsche type Bosch ECUs (which are similar) here

http://www.911chips.com/fuelsys.html

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DRPM635CSi
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Re: What's the adjusting switch pot for in the 055 DME?

Post by DRPM635CSi »

Nice, thanks for that. At least I know now what it is so I won't be tempted to touch it! :D
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Stoffie
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Re: What's the adjusting switch pot for in the 055 DME?

Post by Stoffie »

Would any of those settings be useful to compensate for how different gas is these days as compared to when our cars were designed?

I have a friend who did some adjustments to his AFM te compensate for lean running with modern gas (after going through the fuel injection system and verifying with a wideband O2 sensor) througout the rpm range but I am not technically inclined enough to weigh these two against one another.
The M88/3 was all about power, no cats. At idle the raw fuel from the exhaust will make your eyes water like an old V-8 powered muscle car's exhaust did.
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Re: What's the adjusting switch pot for in the 055 DME?

Post by philm635 »

It allows for mixture adjustment and timing adjustments to compensate for different fuel qualities.
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DRPM635CSi
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Re: What's the adjusting switch pot for in the 055 DME?

Post by DRPM635CSi »

Stoffie wrote: Mon Aug 20, 2018 8:49 pm Would any of those settings be useful to compensate for how different gas is these days as compared to when our cars were designed?
Very doubtful from what I've subsequently been told.
The blue switch (not a potentiometer) is what's referred to as a FQS, or Fuel Quality Switch. In a time before knock sensors, it is what dealerships could use to select from up to 6 pre-set adjustments, depending on region, local fuel quality, and the like. … Unfortunately it isn't setup to add any ignition, just reduce it. Generally, but not always position 1 is the standard -0/-0 setup. Unfortunately the code only references memory locations -- which the processor sets up upon startup -- and not actual values, so it's a bit tough to tell exactly what the settings do without running an emulator or in-line programmer to capture the actual signals. … In later iterations of Motronic, multiple map-sets were included with each tune (up to 6 different map-sets, echoing the original 6 positions of the manual FQS) that were selected automatically depending on a number of sensors' feedback that could, among other things, estimate perceived fuel quality, or could be digitally selected from a comparator-style switch the dealership could activate through the diagnostic port. Then, with the addition of knock-sensors, in the early 90s, all of this went straight out the window.
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Re: What's the adjusting switch pot for in the 055 DME?

Post by cjarvis »

I thought it was for altitude adjustments, but fuel quality makes more sense. or both. Here is a diagram for BMW specific ECUs. (if we are talking about the same thing...)
DME siwtch.jpg
DME siwtch.jpg (39.34 KiB) Viewed 5962 times
Craig in MI
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DRPM635CSi
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Re: What's the adjusting switch pot for in the 055 DME?

Post by DRPM635CSi »

Wow, thanks for posting that FQS matrix. That's good information to keep on file. Where was the source for that if I may ask?
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Re: What's the adjusting switch pot for in the 055 DME?

Post by cjarvis »

DRPM635CSi wrote: Tue Aug 28, 2018 12:41 am Wow, thanks for posting that FQS matrix. That's good information to keep on file. Where was the source for that if I may ask?
Now that is testing my memory! Short answer is I don't know . The date on the file is from 2012, so not that long ago. I had years ago searched and downloaded a lot of stuff. I would say the top sources at the time were:

- Eric (hitmanx) on bimmerforums, E23 forum, He has a ton of material and a ton of knowledge.
- Scottie Sharp's 745 site (not sure it is all there anymore)
- Jarmo's M106 boosting site
- there were others, just cant remember...

It's amazing how much was out there in the first place, and how much is no longer there...

Best!
Craig in MI
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Jono B good
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Re: What's the adjusting switch pot for in the 055 DME?

Post by Jono B good »

Thanks I'm saving that handy tid bit for sure. I vaguely remember something like this diagram on mye28.com 12+ years ago! This is s good reminder to me and maybe all, to copy your stuff to a CD JUST IN CASE! ](*,)
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