Your blower motor may be drawing too many amps at higher spped. If it's old there may be some restriction (crud, rust, worn parts). I just replaced mine as it was seizing up. may need a meter to see what's going on.

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1987 300TD 147,000 miles- Palomino leather interior, 1995 facelift and body cladding, E350 wheels, Rebuilt suspension and sport springs, rebuilt turbo, New Monarch injection pump and injectors....and the list goes on and on...