70 Tach Dash RUN-only Power appears not to be clean

[Midlife 814]

Working with Rich Ackerman, he discovered that where-ever he probed for RUN-only power on his tach car, it had continuity with ACC power.  I believe we have traced it to the alternator indicator lamp.  This lamp compares RUN-only power with a 15 ohm resisted ACC line that connects to the alternator/voltage regulator line in the engine compartment.  When the engine is not running and the key in RUN position, the lamp will be lit and voltage transferred across the bulb.  When the engine is running, both sides of the lamp see 12V and the lamp goes out.  Continuity will still be there, albeit across a light bulb (typically about an ohm or so).

This was a new wiring system for Mustangs, beginning this year.  Past years, the comparison was from the same line from the alternator/voltage regulator and tied to ACC (15 ohms) but compared to ACC itself.  This prevents feedback from the ACC system into the RUN-only lines.  Other seemingly crappy electrical designs for the 70 include the no-ground system for the side marker lamps, among other things. 

This problem only appears with alternator indicator lamps and NOT in ammeter-provisioned Mustangs.  Rich was searching for RUN-only power for some aftermarket gauges and was concerned that they would be operational with this feedback problem.  I don't think that will be the case, as the amount of current allowed will be choked off by the 15 ohm resistor.

Anyway, an interesting situation, one I had not considered before.

 

[Mach1 Driver 560]

Boy, that marker light circuit in the 70 was somebodies wet dream.

I admit to some mild curiosity, but since I don't have a car with an alternator warning light, I don't have the interest to wade into this to see whats actually happening. On the surface the light appears to be in a series/parallel circuit with the regulator/alternator. You would have to know all the resistances (even the bulb) and what is going on in both the regulator and alternator.... and I don't wanta . I would rather do something more interesting, like watching season 2 of Unidentified tonight.

Mid, I may be oversimplifying this, and I know this is easier said than done, but if it is a problem for his gauges, couldn't he convert his car to the 69 alternator warning light circuit?

[Midlife 814]

I'm not sure that it will be a problem for his gauges.  Converting to the 69 lamp circuit means replacing one of the pins on the dash cluster connector, but that then will really screw up the circuit card.  That RUN-only line handles all of the indicator lamps, so there's some serious problems there.  I could easily handle it with the hard-wired dash cluster harness, but that's expensive.

By the way, Ford went back to the more standard configuration in the 1973 Mustang. suggesting that there were some issues found earlier.