LED conversion issue: with headlights on tach turn signal indicators stay on

[step4d]

I've reviewed several posts/threads on the electrical gremlin issue of converting all indicator lights (except headlights) to LED, and can't find a straight resolution to my problem.
Car is 1970 911T Targa.
I converted my gauge lamps to LED, courtesy of CarMagic, and with Englebert's help, got everything to work properly.
Then I installed LEDs on the exterior lamps, including replacing the original flasher to Novita EP26, modified to ground pin C2. But when I turned on the headlights both of the tach turn signal indicators came on permanently. The turn signals worked on the outside as expected, but the interior tach turn indicators stay on permanently. Turn off the headlights and things are fine.
When I replace the original exterior dual-filament incandescent turn signal bulbs things are back to normal with the headlights on. When I replace only one exterior dual-filament incandescent bulb, one tach indicator stays lit (the one with the LED exterior turn signal), the other goes off and works normally (the one with the incandescent exterior turn signal).
From all I've read it seems like this should be a grounding problem, but I'm needing help in how exactly to diagnose and repair that at the exterior turn signal socket location.
Any help would be greatly appreciated!
Thanks all,
Gary

[spoke]

Here is a simplified schematic of the turnsignals similar to the 911. This basically is a '70-'73 914 schematic. The 911 of the same era are similar.

Notice in this schematic that headlights are nowhere to be found. I assume you have grounded C2 internal to the EP26, correct?

If C2 is indeed grounded that means that the upper terminals of the dash indicators in the schematic are also at ground voltage.

Note that the lower terminals of the dash indicators are not connected to each other through any wires except for the 4-way switch which is not energized in your testing.

Also note that the lower terminals of the dash indicators are connected to external turnsignal LEDs. Thus the tach indicators should be connected directly across the external lights. Tach lights on; external tach lights on.

You wrote:

when I turned on the headlights both of the tach turn signal indicators came on permanently. The turn signals worked on the outside as expected, but the interior tach turn indicators stay on permanently. Turn off the headlights and things are fine

So in your testing with headlights on, the tach indicators turned on. This means there is voltage across each indicator. If the upper terminals are grounded, then that means the lower terminals have voltage across them. This also means there is voltage on the external LEDs which you say isn't happening. That doesn't make sense.

Try this test:
1) With ignition off, pop out the tach so you can access the spades of the tach indicators.
2) Turn on ignition (don't have to start the car but you can if you want)
3) Turn on headlights
4) Measure all 4 tach indicators with respect to chassis ground. What are these voltages?

Then try this test:
1) With ignition off, turn on headights. The headlights should turn on. My '86 930 does this.
Question: Do the tach indicators turn on?

[PeteKz]

When you get weirdness like that, especially with LEDs, the first thing I would check is good grounds to the instruments, and then at the turn signal assemblies.

[step4d]

Thanks, spoke and PeteKz.
Spoke - yes, C2 is grounded. When you say "upper terminals" and "lower terminals" - will that be obvious when I remove the tach? (I don't remember now the configuration behind the tach)
I will check out your recommendations and report back.

[spoke]

Originally Posted by step4d

Thanks, spoke and PeteKz.
Spoke - yes, C2 is grounded. When you say "upper terminals" and "lower terminals" - will that be obvious when I remove the tach? (I don't remember now the configuration behind the tach)
I will check out your recommendations and report back.

I meant the upper and lower terminals in the schematic picture. When you do the measurements, it won't matter which ones you do. Just do all 4 spades and record the voltages. This will give some insight to what is going on.

[step4d]

Getting back to close out this thread.
With the help of John Audette, he has updated his exterior (turn signal) flasher LEDs to CANBUS-compatible LEDs, which have higher impedence and look like an incandescent bulb to the the relay, which is what caused the problem
John has sweated the details on this, and followed up until the problem was solved. Here's a brief discussion about it: https://tinyurl.com/2dm5hs4vss=r
Thanks, John!