Lummer's right...most Volvo OD problems have an electrical cause. But your first step is to check the trans fluid level, as low fluid can result in poor OD operation and eventually a ruined trans. Next, get a wiring diagram. Haynes is OK, but if you plan to keep the car and DIY most repairs and maintenance, the Bentley book is the one to have.

With the wiring dgm you can see how the OD elec circuits work and how to bypass each individually to isolate the problem.

If all elec stuff and fluid is OK and OD still is reluctant to engage, shudders, drops out at lower RPM, etc. you likely have leaking piston seals in the unit itself - a common problem, and addressed in the FAQ's here.
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Bob (son's 81-244GL B21F, dtr's 83-244DL B23F, 'my' 94-944 B230FD; plus grocery-getter Dodge minivan, hobbycar 77 MGB, and numerous old motorcycles)