I have an FT-8900R with a new Diamond K400s mount and a Diamond CR8900A antenna.
The radio is acting like I have a ground loop issue, but I can't rectify it. On 10m and 6m, AM mode, I get very bad alternator hum and when my climate control fan or engine cooling fans are running, they also cause a hum. All of the noise goes away when the antenna is disconnected. With the radio manually placed in AM mode, the problem does not exist on 2m or 70cm while the antenna is connected. It only happens on 10m and 6m.
I moved the antenna and transceiver from the back of the vehicle, to the front. I bolted the antenna mount to the front fender, just inches away from the battery, and then grounded both the fender and the mount to the negative battery post with a 1 inch wide ground strap that was only 7 inches long. I connected the transceiver power lines directly to the battery, with both the negative and positive leads under 18 inches in length. The issue still exists.
I thought it may be a bad alternator, but my son is an auto tech, bench tested the alternator as well as load testing it in the vehicle. He said everything looks fine and there is only a 0.02v AC ripple; well withing normal tolerances. I tested the ripple voltage myself and found the same results. Also, a bad alternator would not explain the two electric motors also causing a hum.
As a side note, neither of my other two HF AM radios (Ham and CB) experience this issue. Also, if I hook up a proper 1/4 wave whip, it gets worse, including testing with a different mount... implying the antenna and mount are not the issue.
Any advice on further testing and possible remedies would be greatly appreciated.