SI reset tool information - here's the goods.

["Aaron Bohnen" <bohnen@xxxxxxxxxxxxxxxx>]

Hi everyone,

Well, this SI reset thread has generated a ton of responses - most pointing 
me to the FAQ's, and a couple with some more specific information. And I 
was busy testing things myself today so here's all the latest...

I got out the multimeter and had a go at the car (E28, 15 pin 
connector) today to see what was going on at pin 7, the diagnostic reset 
pin in the connector. It stands at just over 5 V with the engine controls 
off, and just over 5.1 V with the controls to "run". That is correct, the 
system reset pin is 5 volts above ground potential. "Resetting" the SI 
lights consists of actually grounding this pin, allowing current to flow 
between the SI reset pin and ground. I measured the current flow through a 
jumper between the reset pin and ground at 0.34 mA. Basically not much 
juice at all.

It seems from all the information that I have received from the FAQ's, 
digesters, the Bentley manual, etc. that the reset mechanism is a short 
from the diagnostic reset pin 7 (on all models) to ground (pin 1 on 15 pin 
connector models, pin 19 on 20 pin connector models). 

The connection between the SI reset pin to ground is made for a short 
time period (2 to 3 seconds) to reset the oil interval and for a longer 
time period (8 to 15 seconds) to reset the service interval indicator.

On 15 pin connectors the engine can be powered off to terminate the 
connection time interval. Apparently you may have to remove the jumper 
manually with the engine still powered on after the specified duration on 
cars with the 20 pin connector.

So where does that leave us?

Well, I think it would not likely be a good idea to apply battery voltage 
to the diagnostic pin. Likewise it would not likely be a good idea to 
unintentionally apply battery voltage (or any other voltage for that 
matter) to the other pins in the connector. 

A resistor in the traditional jumper lead would help limit current flow if 
an incorrect connection were to be made. Alternately, a diode could be used 
if the jumper were made directional. Such a scheme would allow current to 
flow only FROM the diagnostic pin to ground, and never any current to flow 
the other way (ie: back into the diagnostic pin). A diode would not remove 
the risk of applying +5 V from the diagnostic pins to other things, 
however, so a resistor in series with it (after it most likely) would be 
helpful. More on the specifics of these a little further on below...

In order of preference, then, here are the options for a reset tool:

1. a fixed configuration reset tool a la Peake, BMW, or Bentley in the "BMW 
Enthusiasts Companion" book. Bentley makes his out of a 35mm film casing, 
but that most likely won't work on the 15 pin connector. I'll try to get a 
connector match and report back here on it... If I can get a mating 
connector lined up from some supplier I'll be sure to keep the digest 
posted.

2. a directional jumper lead with both a diode and a resistor. I'd suggest 
a reasonably hefty diode, not because of the admittedly low voltages and 
currents, but rather because of the durability issues involved. A suitably 
rated diode is very small and very flimsy in physical construction. In my 
opinion, more durability is desirable. 

My personal version of this tool uses a 600+ Volt 5+ Amp high speed fast 
recovery Motorola part (Motorola part number ZP MR 856 9350). I chose this 
particular one since I had a couple of extras left over from another 
project on my electronics bench and it is a good-sized, durable well-made 
part. 

I also have a resistor in series with this diode. I suggest a 470 Ohm 
resistor which will limit the potential current flow from the reset pin 
to any other pin of under 11 mA in case of a misconnection of the tool. 
In case of such a misconnection the resistor would be dissipating 0.05 W. I 
would however suggest a half-Watt rated resistor, again from a durability 
standpoint. The tool could conceivably also be misconnected to the battery 
pin and voltage applied to various other components. In this case the max 
current flow would be under 30 mA. 

Don't forget to label which end goes to which pins... The diode allows 
voltage flow FROM pin 7 to ground. If you hook up the directional 
jumper wire backwards it won't work at all. I suppose you could just 
reverse it and try again... :>

3. A non-directional jumper is my last choice of reset tools. The 
paperclip is the worst example of this. I suggest you build the one with 
the diode in it!!! But if you don't want to add the diode, in the fixed 
configuration tool I would also recommend a resistor. 470 Ohms will limit 
the max current to under 30 mA if it is misconnected, although it cannot 
protect against you putting back voltage on the service interval reset pin 
if wrongly connected.

So the recommended layout for the diode/resistor jumper lead is:

              pin - wire - diode -> resistor - wire - pin

and you put this <-- end onto pin 7. The other goes to ground.

I have a graphic of mine if anyone's interested.

Good luck and keep your eye out for more news on the mating connector...

best luck to all,

Aaron
___________________________________________________________
Aaron Bohnen                     email: bohnen@domain.elided
- -Ph.D. Candidate, Civil Engineering Department, U.B.C.
- -Technicraft Engineering Services

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