electrical wave

87 should still have a external regulator .Just replaced the one on my 88 Dak.

87. i had a 75' b-250 3/4 ton and a79 b-100 1/2 ton van also a 82' w-150 pick-up with a problem something like yours, all 3 had to much residant in the wiring goin' from the alternator thru to the battery, (bad wiring) changed out the wiring to a new thicker 12ga. and 10 ga. fixed the problem. check it out with a ohm meter to see if it's old or corraded wire the problem

mopar magic said:

87. i had a 75' b-250 3/4 ton and a79 b-100 1/2 ton van also a 82' w-150 pick-up with a problem something like yours, all 3 had to much residant in the wiring goin' from the alternator thru to the battery, (bad wiring) changed out the wiring to a new thicker 12ga. and 10 ga. fixed the problem. check it out with a ohm meter to see if it's old or corraded wire the problem

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you mean to much Resistance on the wire.

Check your cables for corrosion and the voltage regulator should be on the firewall if it is an external regulator ...... do you keep buying the alternator from the same place?
Maybe they are selling you the wrong alternator

depends on which alternator it has weather or not it is internally or externally regulated. There were three choices for that truck that year. If it has a 75 amp alternator then it does have an external regulator it would be mounted in the center of the dash panel. They also had problem with the wiring going from the alt. to the battery as many suggested. Especially if it has an amperage guage in the cluster. All the power has to flow thru the guage and they often burned up and would cause a low power situation due to the high resistance. Hope that I could help.