98 Mazda b3000 alternater fluxuating

[Peterferg]

I have a 98 Mazda B3000 with 400,000 kilometers on it.Just recently my alternator gauge started fluxuating but when I put the heater fan on it stops.My heater fan has a blown resister so it is other off or on full.Then the next day I was low on gas and when going down a big hill my gas gauge drops as usual but this time my amp gauge droped and stayed at about a quarter.I turned around and when I came up the hill and my gas gauge rised up again my amp gauge went back to normal.I went down the hill again and my amp gauge droped again,turn around and it went back to normal on the way up.I then filled up with gas and went down the hill and amp gauge stayed normal.Any ideas?

 

[1Aauto]

Thanks for joining, it's great to have you here

Tony