VW Golf3 pre heating system

[tonyoa78]

Hello!

I have a vw golf 3 engine code 1Z 1.9 TDi with pre-heating system 3 glow plugs.
Somebody knows this system how works?
Pre-heating relay ok, 3 "extra" glow plugs ok, engine coolant sensor(4wires) ok, wires (low and high) beetwin engine ecu-pre-heating relay ok.
This relay works, engine ecu should switch earth to relay, but not. What is problem? Engine ecu?
I have only 1 trouble code in ecu: alternator DF signal implausibly.

Thank you very much!

[tonyoa78]

Anybody doesn't know this common problem?

[bobolin4o]

Those 3 plugs have current about 10 amperes each.
So, if in the ECU have DTC about alternator...