The diodes are there are to suppress voltage spikes on the relay coils when the relays switch off. The newer relays have internal flyback diodes.

 

 

Relay failure is usually the contacts getting burnt from switching. This is a perpetuating situation, as soon as the contacts have a scorch mark the contacts will arc more making the scorch worse and so on until the relay can no longer supply the current required to operate the load.

 

I wonder about upgrading these 2 relays to SSR (solid state relays). It might be the end of the issue. SSRs have internal clamping diodes.

 

SSR's do not need flyback diodes because they do not have a coil, as the voltage is turned off a coil it produces a voltage spike called back EMF, depending on the resistance and inductance of the coil this voltage can be in 100's or 1000's of volts from a 12volt source. AKA ignition coil.

The diodes in a SSR are actually a Triac and perform the switching, having the diodes reverse polarity means the switch is bidirectional.

 

Warren,

That's what I was implying, just not well put by me.

 

I would not bother with SSR's they are not infallible either, and are not a drop in replacement.

Good quality mechanical relays will last 20-30 years. All of mine are still going and my car has 270,000km on it.

 

well I tested the new relay, works fine , fitted and now the pump is firing up when the ignition is on.

but sadly still no start just a whiring engine and now a flat battery

 

See if it will run on a few squirts of ether (starting fluid). If it does, you know fuel is the trouble. If it does not, you know spark is the trouble - and maybe fuel too! But at least once it starts on ether, you can then concentrate on fuelling if the engine still will not start without ether.