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Date:         Thu, 25 Jun 1998 23:27:46 -0600
Reply-To:     kenstich <kenstich@BEWELLNET.COM>
Sender:       Vanagon Mailing List <Vanagon@vanagon.com>
From:         kenstich <kenstich@BEWELLNET.COM>
Organization: Central Intelligence
Subject:      Re: digifant test adaptor improvement suggestions =>> Long
Comments: To: Darrell Boehler <midwesty@TAOS.MIDWEST.NET>,
          "Vanagon@VANAGON.COM" <Vanagon@VANAGON.COM>
Content-Type: text/plain; charset=us-ascii

Darrell et all,

Very Cool! -> I've fought with my L-Jetronic system trying to troubleshoot various problems. And, since it has no built-in diagnostics or test points - *^)(#$!#^&*%, I've ended up exposing the ECU connector contacts by removing the backshell an pin probing the contacts -> very kludgey at best - I'm very impressed by your homegrown test adaptor. I'm considering adapting your idea to my system. Being an aerospace engineer, somewhat familiar with test equipment and harness design, I thought I'd suggest some potential improvements and cautions:

1) Suggestion - Certainly, at least the power pins ought to be protected from shorts to ground by wiring in series resistors (in-line) to prevent shorts from having a devastating effect. A 1 thousand ohm (1K) or 10K resistor is about right. With a 1K resistor, the max current from a short is around 14 millliamps (approx 14 volts across 1K - ohms law applies) which will protect your 26awg wire (and internal Digifant circuits!) nicely. The resistors should be wired in inside the ECU as one of the failure modes is wire chafing through the insulation where it penetrates the case. These resistors are called current limiting or test point protection resistors. In the aerospace industry, we protect virtually all test points and still manage to fry some hardware. How much is a replacement Digifant ??? Resistors are cheap - OBTW - use resistor packs - usually 4 or more resistors in a single tiny plastic package - resistor power dissipation rating can be very low (i.e. 1/4 watt for 1K & 1/8 watt for 10K resistances) as power dissipation is low.

2) Suggestion - The wiring inside the case is subject to long term chafing/abrasion (due to vibration) and should be tack bonded "glued" to the printed wiring board every couple inches. The aerospace industry uses epoxy compounds - Radio Shack must have something for this purpose. This is also particularly applicable where the ribbon cable penetrates the ECU case.

3) Caution - One needs to use a DIGITAL multimeter on the test points to prevent excessive loading of the test points when measuring voltage. Digital meters have around 10 Megohm of input impedance resulting in virtually no loading of the circuit being measured. Digital circuits (i.e. injector control outputs) really need a oscilliscope or digital test equipment to "see" the signal. Some of the fancier digital multimeters have pulse width duty cycle measurement - way cool. One exception is the power leads, which instead of a LED indicator light, an analog voltage meter would be way cool =>> actually, a 6-way rotary switch with one meter would allow one to see any of the power voltages selectively => a few bucks more than the 6 led's.

4) Alternate design idea - If one could find mating male and female connectors, one could construct a "Tee" harness that one plugs into the ECU/Digifant and the vehicle harness that tee's off to a connector to provide test points while passing the signals/power through to the ECU. This is the usual method in the aerospace industry (we call them break-out boxes - the slang term is "sandwich box"). It has the advantage that it can be temporarily plugged in for troubleshooting and is not limited to one vehicle.

5) Question - Do you run the ribbon cable up to the driver area/dash so that you can see the measurements while driving ??

Many thanks for sharing your ideas - looks like a winner - Good explanation/illustrations in your web pages!

Thanks,

Ken Stich 80 Vanagon-L 57 Vintage Oval Bug 85 Prelude Conifer, CO


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