A better test than a voltage test would be to measure the current drawn by each GP. This is more difficult because the wires have removed unless you have an inductive ammeter that you just place the probe around the individual GP wire.

If all the GP are drawing about the same current, about 15 amps, the GP are OK and do not need to be replaced.

I still think the problem might be how long the relay is turning on the GP and that should be investigated before you replace any GP.

P E H