Many auto radios have a "trimmer" capacitor to match the antenna to the AM tuner. The standard auto antenna is 1/4 wave at mid-band of the FM band, but it is far off of an optimum AM antenna. Check your radio manual for the AM tuner adjustment if it exists.

Back in the day when I worked on this stuff, you set the radio to a weak station around 1400Khz, then tuned the trimmer for best reception.

Ken,
The 77-78 chassis wiring diagram I have shows the radio ground as being through the radio chassis, not a separate wire, so it gets ground from the dash frame it is mounted to. It also shows the dash is grounded by a separate wire direct the battery Neg (-), NOT to the engine block.

You failed to follow that wire any farther at the battery. That wire continues on through the battery connector and to a bolt on the engine Block (engine ground). We frequently call that continuation cable the negative battery cable.


Since the antenna shield is grounded at the antenna (on the body) and also at the radio, the radio chassis is connected to the body ground. Therefore as-built there is already a ground loop in place.

While the above is partially true, the antenna connector shield side is NOT DC connected to the radio frame itself. SO there is NO connection between body and engine ground there

If the engine to body ground straps fail, the antenna cable shield will carry the current. If the battery to engine block ground wire fails, again the antenna coax will complete the ground.

That connection inside the radio does not exist. If it did we would have many, many reports of smoked antenna coax cables and blown radios when the braided jumpers fail or were forgotten to be replaced on engine and transmission swaps. The engine to frame crossover is located on the back of the trans near the rear mount. I have replaced a few of them on other people's coaches.

My Farm-boy mechanic and 45+ years in mobile radio communications tells me that separate dash ground wire from the battery should be disconnected from the battery and grounded at the engine.

Agreed, but see previous comment. GM figured why not do it their way and it has worked just fine (other than corroded or loose connections) for a long time. The dash is still hooked to engine ground just by a slightly longer route on a 2 or 4 ga. cable.

When GE came out with their first microprocessor controlled, programmable radio we were having endless troubles with instability of the transmitter frequency. They finally sent an Engineer out to look at a new Police car with this issue. He promptly told me I had connected the radio power wrong, the ground wire had to go directly to the battery. I disagreed arguing the radio ground had to share the same ground as the radio case (mounting point) and the antenna ground, therefore be grounded to the vehicle body. But he insisted so we wired the radio ground wire to the battery. It did not fix the radio's instability, but as soon as the engine was started some starter current flowed though the new radio ground wire from the battery and out the case of the radio onto the body ground, through a grounding wire from the body to the engine block. This current was enough to burn a PCB trace from the radio ground wire connection to the radio chassis (case). This caused the 5 volt regulator to loose its proper ground reference and smoked the radio's microprocessor. The Police Chief was not amused and the Engineer was dumbfounded.

GE never did fix the issue as they claimed it worked fine on the bench (into a dummy antenna load) Trouble was when connected to an antenna the RF field when transmitting would upset the radio's voltage regulator powering the Voltage Controlled oscillator (VCO) causing stability in the VCO and therefore transmitter frequency. Once again... works as designed, but not designed to work!

Just my experience