'93 E-350 7.5L. Chassis battery reads 12.8 volts at rest and 14.4 volts while van running but auxillary battery will not charge even if I drive it for hours on the highway. When I charge the battery manually all the interior lights work great. There is no reading at the auxillary battery on the voltmeter though with the vehicle running. Could this be a relay problem? Where is it located? and how do I test it?
Thanks

 

Have you checked the main fuse? I have a 94 e-350 7.5 and it has a power dist. box under the hood below the master cylinder. I had the same problem and found the fuse had popped, check you onwer manual for the location in the box. I as i remember it was #18, 40 amps and called aux. battery charging.

 

Thanks for the idea. Finally gave in and took it to a technician. Turned out to be the "battery float charger" circuit board in the converter. Some of the solderings had rusted. Replaced the part for $80 now it works fine.