So they report less than 16MB even without RAM cards? That's unusual. The entire range had 16MB soldered so that should be the minimum visible. What OS are you using? Does it report a memory problem at boot? Usually there's a message about "a problem has been detected with your memory" if there's an actual fault with the RAM.
A lot of Macs will have garbled or no boot chime if the onboard RAM is bad but then, if there's good expansion RAM available, will boot and use the expansion RAM while ignoring anything onboard; conversely, if there's a bad expansion RAM module, the machine will often boot normally (though initial power-up may take longer than usual) and simply report less RAM than you expected because the machine is simply ignoring the bad module.
Also, while physically similar, the RAM for the 190/5300 doesn't work in the 3400. It's possible that there were some universal cards built but the majority will only work in one model or the other.