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I expect the starters are 48V (ish) as there are two in series, no?
What is the rated voltage of the coils on S1 & S2... 72V? If so, according to the Albright bumf the drop-out voltage is 16V. We can tell from the way the engine accelerates despite the interruptions that the battery voltage and internal resistance must be at least fairly OK and would not cause the voltage at the coils to drop anywhere near 16V otherwise it simply wouldn't turn over, and/or the solenoids wouldn't engage. So I think we can rule out that end of things.
That is not to say the voltage at the coils is not falling to 16V. There can be many interfering factors but no factor is as important as the supplied coil voltage in deciding whether a contactor holds or drops out. OK, strictly speaking the current, but for a quick check we can look at the voltage and accept that the resistance of every contactor that's been tried is not going to jump up and down rhythmically in the same way. If need be, we can then look at the current to confirm a diagnosis.
As yet, I don't think we have actual figures at S2's terminals at the moment it first pulls in, when it drops out and when it pulls in again. IMHO, trying to diagnose without knowing these, is working somewhat blind. If the coils are only getting 20V when S2 drops out, then whatever the difference is between S1 & S2, they are operating completely outside their spec and are entitled to do whatever they please and be influenced by any and all external factors. If it is 60V, then the problem is definitely with the physical position and load in the S2 position as it should be holding in perfectly well.
What is the rated voltage of the coils on S1 & S2... 72V? If so, according to the Albright bumf the drop-out voltage is 16V. We can tell from the way the engine accelerates despite the interruptions that the battery voltage and internal resistance must be at least fairly OK and would not cause the voltage at the coils to drop anywhere near 16V otherwise it simply wouldn't turn over, and/or the solenoids wouldn't engage. So I think we can rule out that end of things.
That is not to say the voltage at the coils is not falling to 16V. There can be many interfering factors but no factor is as important as the supplied coil voltage in deciding whether a contactor holds or drops out. OK, strictly speaking the current, but for a quick check we can look at the voltage and accept that the resistance of every contactor that's been tried is not going to jump up and down rhythmically in the same way. If need be, we can then look at the current to confirm a diagnosis.
As yet, I don't think we have actual figures at S2's terminals at the moment it first pulls in, when it drops out and when it pulls in again. IMHO, trying to diagnose without knowing these, is working somewhat blind. If the coils are only getting 20V when S2 drops out, then whatever the difference is between S1 & S2, they are operating completely outside their spec and are entitled to do whatever they please and be influenced by any and all external factors. If it is 60V, then the problem is definitely with the physical position and load in the S2 position as it should be holding in perfectly well.