No high gear
 

I have a 1991 300D with 386,000 miles. The engine is strong, however it won't shift into high gear. It pulls reverse fine. In drive it starts fine, and shifts from 1st thur a few grears then when it is ready to shift at 40-45mph and 3,000rpm on shift. I have changed the filter and fuild but no change. Any ideas?? With almost 400,000 miles not sure how much it will cost and if it is worth it? It is a great car I hate say good bye but 45mph is slow going. Thanks, Jim

have you checked the modulator, or it might be time for a trans, good luck

No I have not checked the modulator. I'am not sure how to check it? Can I test it or do you just have to replace it? where and how?

check vacuum line to make sure it sucks air and check mod by sucking to see if it holds vacuum, a hand vac. pump with a gauge is the easiest way

IF the 1-2, 2-3 shifts are good then the problem ISN'T the modulator. Modulator problems will affect ALL shifts the same.

Most common failure on that 722.4 trans is the K2 clutch pack & the teflon seals that that drum rides on! WITH that many miles look for a good used trans.