Fuel Injector Solenoid: Insufficient Voltage

Hello, I drive a 1990 Audi 100 TDI with a 2.5-liter engine.
Here's the problem. The preheating indicator doesn't turn off completely; it continues to glow. This error initially occurred sporadically and usually resolved itself if the vehicle was parked for a certain amount of time. There was only about 5 volts at the glow plug relay. When I directly supplied the relay with 12V, the engine started immediately. Now, the error occurs constantly, and the turbocharger is no longer engaging. The vehicle can only be driven by directly supplying 12V to the relay and without turbo assistance.
Who has advice?
I am grateful for any suggestions.

Wishing you always a safe and pleasant journey:
Siegfried Weber.

Siegfried Weber wrote:

Audi 100 TDI, 2.5 l engine, manufactured in 1990. The solenoid valve was only receiving approximately 5 volts. When I directly supplied the solenoid valve with 12 volts, the engine started immediately. Now, this problem occurs constantly, and the turbocharger is no longer engaging. Driving is only possible by directly supplying 12 volts to the solenoid valve and without turbocharger assistance. Wer weiß Rat??

Hast Du den 5Zyl. Engine with VP37?
I suspect a transition resistance in the MSG supply could be the common cause. In the case of the 1.9 models, the main relay (109) is often the culprit.