the way starter works, the starter button sources about 1A of current to the stater relay
the starter relay closes and it sources about 30A of current to the starter motor
so you see the problem with bypassing the starter relay, the starter button cannot source 30A directly, it would melt (but first it would blow its small fuse).
If the starter has been working with the replacement relays, its possible the starter is drawing too much current, but that should blow the main fuse (30A).
Check and make sure the main fuse has not been bypassed, for example, it blew out and someone wrapped it in alum foil for a road side fix, and never replaced the 30A fuse.
That is the only way I can think of that a bad starter would damage the relay, the fuse is there to protect it, and the wires.
You can get a clamp-on DC amp meter and measure the current in the starting circuit. They are expensive because measuring a DC current is more difficult than AC current. Its possible the starter is spiking the current for a second and its not long enough to blow the fuse, but it is damaging the relay.
or you just had bad luck with a couple of faulty relays? There is a thing with electronic parts called infant mortality: if a part is not assembled correctly, for example poor soldering or a poorly connected heat sink, it will operate for a few hours, then fail. Once a part has worked for several hours its past that infant mortality stage, and will probably work for 20 years.
If you are going to try to measure the current with an inline amp meter, you will need at least a 50A meter to see the 30A current, and if its shorting you will want a 100A DC meter, or the short circuit current will blow your meter out.
If you have not used an inline amp meter before, you have to disconnect one of the wires to the starter (or at the main fuse) and put the meter in series with the wire you took off, so the meter completes the circuit and all the current flows thru it.
If you connect an amp meter across the two starter wires, like you would to measure the voltage, you will blow the fuse in the amp meter, and maybe blow the meter out too.
Last edited by KCW; 07-06-2019 at 01:43 PM.