500E reference resistor? - PeachParts Mercedes-Benz Forum

seacoast_benz · #1 05-10-2002, 11:40 PM

Was browsing the archives and read with considerable interest the threads which talk about the R16 "reference resistor" to retard the spark on certain of the older US-spec gasoline engines.

Given that this car (93 500E) will spend most of its time running on 91-octane California dishwater premium, I'd normally just leave well enough alone. But for now, I'm back East and the car has a tank full of Sunoco 94 Ultra, and curiosity is rampant. I'd really like to see what this engine feels like without the reported de-tuning for low octane.

Took a look on the AC return line by the firewall but didn't see the resistor as it's been described on other models. There was a comment to the effect that OBD-II cars don't have the R16 resistor, but I don't recall off the top of my head if OBD-II was in effect for the 93 model year.

Can anyone offer any guidance on this one? Is there a resistor, and if so, where would one find the beast? Thanks!

s/b

walker4953 · #2 05-11-2002, 12:00 AM

s/b,
On the driver's in front of the shock tower on the opposite side of the heat shield from the shock. It has BRIGHT green lettering & a big "2" on it.
Jay

rainmaker · #3 05-12-2002, 04:48 AM

The only thing I could find with a big 2 on it in the general area of the location you specified is a round plug with a single pole connector that looks like a button. Is that the one?\

So I just unplug it and I'm good to go?

walker4953 · #4 05-12-2002, 10:22 AM

That's it. It should just pull straight off.
J

seacoast_benz · #5 05-21-2002, 01:52 AM

Jay -- excellent instructions! Thank you very much.

I actually did read your post while we were still out on the road, but I didn't want to trifle with something as important as spark advance while dealing with rough US-interior high-speed touring and wildly fluctuating octane levels from state to state and station to station (this ethanol/MTBE scam is really annoying stuff).

Back home and able to experiment a bit. Removed the resistor today. The car does indeed seem a bit sprightlier at part throttle, and no hint of detonation at all. However, I'm averse to anecdotal evidence, and I'll want to be trying to quantify the improvement, either by dyno (preferably) or by run timing.

s/b

Michael · #6 05-21-2002, 09:36 AM

These motors WON'T ping-there's a knock sensor in each cylinder bank, and long before you can hear predetonation it's already dialed the timing back to prevent it

seacoast_benz · #7 05-21-2002, 01:54 PM

Michael, I'll defer to your experience here, but have a question: why would MBZ have even gone ahead and installed the advance reference resistors (or, in Europe, the advance adjusting potentiometer), if the computer is consistently good enough to anticipate knock via the sensor, and pull back the spark to halt it?

Would that have just been engineering conservatism on their part, or a way of ensuring a hard limit if a sensor snuffs it?

I have driven a number of high-performance vehicles with knock sensors, and almost all of them still do have audible detonation on low octane fuel.

Some of this may be due to choices made by the engineers who laid out the firmware maps for the engine control system -- one design option is to instruct the computer to pull back the spark a little bit when rattling is sensed, and another is to instruct it to execute a serious whoa-Nelly spark pullback. Obviously the latter will eliminate any hint of ping or rattle, but at the cost of responsiveness.

One thing that I have never seen, even in turbo exotics, is an indicator on the instrument panel which would gauge the severity and frequency of knock. That's very important info for those of us who prefer to be better informed than with idiot lights. I'll have to get right on that when I become a prima donna sportscar designer :-)

s/b

Michael · #8 05-21-2002, 03:00 PM

Excellent question SB. Regrettably, I can shed no light on the logic that brought about this thing in the first place.

Do these resistors exist in the 500s abroad?

run.exe · #9 12-03-2002, 03:25 AM

Is it true that variable-settings (i.e. - "dial-a-value") R16 (500E/E500) resistors exist in Europe?

I tried (in vain) to take apart my R16 resistor - to no avail.

Next, I'm going to buy some extras and hack them apart to rebuild them at different ohm/Kohm levels...

Thoughts, anyone?

rainmaker · #10 12-03-2002, 08:17 AM

Yup, the euro ones are variable. I've seen it at my tech's computer...too bad his printer's busted so I can't get a printout. I'll get you the part number though the next time I'm there.

run.exe · #11 12-03-2002, 12:23 PM

If you would be so kind as to send 15 or 20 of those to my work address, I will hook up our American 500 friends!

...assuming the parts in question cost $1-$5 (USD), that is.

RSVP / thank you!!!

rainmaker · #12 12-03-2002, 12:26 PM

Scott,

I could get you the part numbers real easy but the parts will still have to come from Germany. Remember, there are only 2 to 3 E500/500Es here so the local dealers wouldn't have that part...