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Replacing a water pump


BlueStringPudding

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39 minutes ago, Tony Brooks said:

If you put the meter cable into the amp socket then even if the meter is set to volts and the cables connected to the wiring you woudl get smoke from the meter and the amp scale may no longer be reliable or even working. This is a very common mistake and is the reason why all the training meter had a 5 amp fuse fitted into the lead. Some meters have a fuse inside but often its only for the  mA scale.

 

The way the voltage drops looks more like a flat battery to me although as the motor slows as the pressure rises the current would increase so any volt drop on the wiring would also increase. However The initial voltage readings on each test suggests the batteries are OK, especially the with engine running ones.

 

It may be easier/quicker to run a new cable to the pump of more than adequate size and replace any switches or at least run a test cable from battery to pump and see what happens.

Thanks. But it's a 65ft boat. I can't afford thick electrical cable and certainly can't cut through the bulkheads of each cabin, emptying cupboards and going through tiles and fire surrounds and the like to run it from front to back. That's months of work and upheaval and hundreds of £££ to pay someone to do it, all while I live aboard. I can't do that.

 

What I don't understand is that it is the same wiring that the old pump ran adequately with. It didn't judder. The water flow didn't pulse high and low. So why is it doing that with this pump? Nothing else has changed but the pump.

 

It seems overkill to pull the boat to pieces and spend hundreds on changing infrastructure when the real problem surely has to be the only thing that has changed: the new pump? ?

 

I'll retest the voltage tomorrow. 

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If it is volt drop and it seems it may well be then assuming the cable is large enough then you need to start at the batteries and work towards the pump checking everything that can cause volt drop. This includes the master switch, fuses, inline connections, and any switches. This includes the negative "bus bar" that is often close to the fuses/breakers or in a fuse box.

 

What current did your last pump draw? was it identical to the new one? No on can say why this has happened until the problem is solved. my suggestion that you could run a temporary cable up the boat is intended to save all the work you think permanent cabling involves. you just drape it up the walkway or even outside the boat and through windows if you must. Another way is to remove a battery and carry it close to the pump so you only need a length of cable.

 

To check for volt drop run a length of thin cable up the boat (walkway). Clip one end to the battery negative and the other end to the meter set to volts. The connect the other meter lead to the negative terminal at the pump.Turn the pump on and the meter should read very close to zero volts. The higher the voltage the greater the volt drop. Repeat but this from battery positive to pump positive. If you find any volt drop you can start moving the battery end from connection to connection stepping towards the pump and running the pump at each step. When you find the volt drop has come back or goes away you will have located the area of the volt drop.

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