Get a manual to study the setup. My first inclination is just to provide a self variable voltage source to vary the idle.Or more likely set it where you want it.

It will just be not variable as fed from the computor that was in the 603 car. First establishing what was a normal voltage range when operated by the computor.

Otherwise you probably will have to incorporate the 603 diesel electronics. I suspect without really knowing it reads the rpm and automatically corrects the idle. Wait until others with superior knowledge come on your site.

Not only have I never worked or seen this setup personally. What I quote is just suspicion and guesswork at best.