Water pump

Issue seems to be resolved by curing another problem. Last year the site we were on had a power failure so relied on our leisure battery. It only lasted half an hour before everything ‘clicked off’. Investigating, I found the Sargent panel was set to Pug. as a base vehicle rather than Fiat .... resetting panel to Fiat means the ‘switch off threshold voltage’ was much lower and battery maintained habitation electrics for much longer but not as long as I expected. Performed a ‘load test’ on the notionally 110Ah battery (original to van thus 8 years old) and it only had a capacity of 24Ah so replaced it with a new, good quality 120Ah unit. Can now find that sweet spot on the pump adjustment and it operates normally both on and off EHU.
Thank you all for the helpful and considered responses. 
Peter M

petermcd wrote:I adjusted our water pump to operate correctly when the van is connected to EHU (ie volts = 13.8v), BUT it  then misbehaves (ie does not switch off) when not on EHU (ie volts = 12.6V).
Peter M

Congrats for solving the issue Peter. For the first time, the same problem happened to us last week when on a camping rally. The site had no EHU. Everything had worked ok previously when I filled up with water at home and on EHU. But without EHU the pump would not turn off ie. once the pump was activated at the control panel and taps turned on, with water flowing, then taps turned off, the pump continued to run continually. Checked for leaks and adjusted the pressure switch away from its usual 'sweet spot', still the pump would not turn off. Got my thinking cap on and went through [in my head] all the advice I have received from this forum regarding this long standing problem. The conclusion I came to was: the pressure switch might be faulty, perhaps an air leak or blockage in the housing for example. I loosened then tightened the screws on pressure switch. Tried again allowing water from taps to run for a longer period of time whist tweaking the adjustment screw on the pressure switch. This time when the taps were turned off the pump stopped...at last. Job done hopefully. The idea the problem might be from the L battery was not considered, it was almost fully charged. However it is 6 yrs old so might be on its way out. So thank you Peter for identifying one other possible cause for a long standing problem with a badly designed water pump system.

My theory is : as a battery ages the sulphation causes internal resistance to develop. This causes the battery voltage to drop when a load is applied ... my error was to measure battery voltage open circuit. The pump, being an inductive load, caused the voltage to reduce quickly thus there is a large difference between the voltage  when ‘on charge and off load’ and ‘off charge and on load’ .... the pressure switch is unable to cope with these voltage differences.

Reminds me of when I picked up my last van. I pitched just down the road from the dealership but was bamboozled that nothing would work until the EHU was connected and then everything worked fine. 

I had to go back to the dealer the next day to have a boiler fitted and mentioned the issue to them. They couldn't understand it because both batteries under the driver's seat were showing healthy charge. There was much head scratching and mumbling until it was realised that the two batteries under the driver's seat were both cranking batteries and the leisure battery slot, under the passenger seat, was empty. 

I hadn't helped because my previous van had been a SWB Transit and it did, indeed, have one cranking and one leisure battery under the driver's seat. I was to learn later that Ford's fitted two cranking batteries as a matter of course to any Transit that was MWB or larger. Apparently the second battery kicks in when the starter is operated and kicks out again when the engine has fired. The idea is to provide as much oomph as possible when starting.

All of this messing about didn't help me though because none of the leisure batteries the dealership had in stock were low enough to fit and it was 4pm on a Bank Holiday Friday. I drove 70 miles back home on an M6 crammed with vehicles, with the prospect of having to drive 70 miles back again the following week to have the battery fitted.