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Old 06-11-2010, 08:45 AM   #1 (permalink)
Engineering first
bwilson4web's Avatar
Join Date: Mar 2009
Location: Huntsville, AL
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NHW11 pig-tail fixes OBD scanner issue

Thanks to w2co and friends, my new OBD pig-tail adapter works:

  • Passes only ISO 9141 signals K and L, ground and B+ to scanner
  • Connects Ts, Tc and chassis ground to miniature audio plug to trigger two-code flash or error code clearing

ISO 9141 Signals

The pig-tail keeps any extra signals from the NHW11 diagnostic plug from the OBD scanner. This reduces noise to the OBD scanner and protects the NHW11 from OBD signals that might spike or confuse the vehicle. The following table shows the signals passed:
4 | chassis ground
5 | signal ground
7 | ISO 9141, K-line, data signal
15 | ISO 9141, L-line, data is active signal
16 | battery voltage, 12 V
No other signals pass between the scanner and vehicle.

This pig-tail solves the problem of Auto Enginuity inducing false codes. It also stopped the mystery hangs.

Since the ZVW30 uses a different pair of signals, I'm planning to add a 4-pole, double throw, slide switch so it can work with both the NHW11 and ZVW30. It is very likely that the ZVW30 and NHW20 are using the same signals.

Code Flash and Clear

There are two signals used to trigger flash of the two-digit error codes; Tc, triggers flashing the error codes, and; Ts clears the code when tied to chassis ground. The mini audio jack connects to a momentary toggle switch, (on) off (on). This allows reading major error codes when an OBD scanner is not available or does not have enough support to read all codes:
4 | chassis ground
13|Tc tied to ground causes ECUs to flash two digit codes
14|Ts tied to ground causes ECUs to clear codes
These two signals are documented for the ZVW30 and probably work on the NHW20. This may be a universal code reader for all North American Prius.

I built this from my junk box but if anyone would like one, send me a PM and based upon current parts costs, I'll work up the price. Currently, these look to be ~$50-75.

Bob Wilson

2019 Tesla Model 3 Std. Range Plus - 215 mi EV
2017 BMW i3-REx - 106 mi EV, 88 mi mid-grade
Retired engineer, Huntsville, AL

Last edited by bwilson4web; 06-11-2010 at 02:37 PM..
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