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An exhaust brake will work fine on a M939, check out the TM, as it is an optional installed component.Do not think a engine or exhaust brake will work on a 939 series because of automatic and the UN-locking t/c, what would be NEEDED is a RETARDER for the transmission like the 915s ect, have.
That won't work on a 8.3 in the 939a2s