Cruisair PR8X Pump Relay Fuse Blowing When 2 Units Operating

[Nick in Manitou]

New to Boating. New to Forum.

1984 53' Extended Deckhouse

Using 2 Cruisair systems onboard for heat. They seem to work fine individually. Both operate the same seawater pump when their compressors are running. Everything seems to be fine unless we try to operate them at the same time. If we do, after a while we realize we are getting cold and that the seawater pump is no longer working...the 20 amp fuse in the PR8X has blown.

I have put my hand on the pump after it has been running for hours and it is just warm - not at all hot.

I don't have a lot of experience with AC systems, but it seems that if a pump is running and another unit calls for that same pump to turn on, that there would not be a significant change in the power being consumed by the pump circuit.

What might be the cause of the fuse blowing? It has happened 3 times so far, but if we run only one unit at a time, we are going on more than two days now without a blown fuse.

Any thoughts? Any experience with this problem?

Thanks for any help!

Nick

[Boatsb]

Voltage drop from a second unit running.

As the voltage drops the pump gets browned out and blows a fuse.

[SEVEN]

Welcome to the forum. I’ll take a guess because I’m not there in person. The fuses are old and weak. Try switching them out. If that’s it, it’s a cheap fix. Question, what power do you have going to the boat? 50/125/250?

[Photolomy]

Where is this fuse? The water pump has a breaker, no? Also, the controller only turns on the water pump once, that I am aware of, no matter how many AC units are turned on. I wasn't aware that it speeded it up as more units are turned on. As far as I can see, the pump feeds a manifold that then distributes the water to all of the units equally, regardless of whether they are running or not.

[Nick in Manitou]

We have 240V to the boat.

The fuse is a 20amp SFE (slow-blow?) fuse that looks like the old automotive size/style fuses. Box was onboard when we arrived 2 weeks ago and appears to be new.

The fuse holder cap sticks out of the top of the PR8X box.

I assume that the pump would operate at a fixed speed no matter whether one or more units call for it to be operating. The pump feeds water to a manifold that supplies water to all the Cruisair units.

I must admit that I assume that there is a breaker for the pump, but I have not specifically looked for one (yet).

Nick
(in San Francisco...it is cold enough to want the heat to work!)

[Photolomy]

I see. I haven't had to look at mine, but the PR8X is a set of relays that turn on the water pump if any AC unit is on, but the fuse in it supports the relays, not the pump. The pump is supported by a breaker in the helm. My guess is that the fault is in the PR8X, not the pump. Maybe one of the relays is bad, or the controller circuitry is bad and burning out the fuse. You could test which by turning on the other two AC units and seeing if that also burns out the fuse. But I don't think the water pump has anything to do with this issue.

https://citimarinestore.com/en/parts-displays-accessories/7230-cruisair-pr8x-pump-relay-115-230v.html#/1776-control_triggers_included-four_115v_triggers_115

I have the same box and might take a peek inside this weekend.

[Nick in Manitou]

I just spoke with a fellow at Dometic in product support for Cruisair.

He said that the problem is probably the fuse holder itself. He said that the spring in the fuse holder is weakened by heat over time and ends up not holding the fuse with enough pressure to maintain a good connection, building up heat and causing the fuse to blow. He said that they eventually eliminated the fuse from the design as the pump is protected by a breaker.

Perhaps this makes sense, as I was getting a brain cramp trying to figure out how we were blowing a 20 amp fuse in a circuit that is supposedly protected by a 15 amp breaker.

He suggested I just bypass the fuse holder and depend on the breaker to protect the circuit. I must admit that this makes me a little nervous. (I hate removing protection from a circuit where the wires are bundled up together with lots of other wires for other circuits...!)

Nick

[Nick in Manitou]

Brian,

When I first tried the system after installing the circuit breaker I started the galley/v-berth system first and saw the issue with the pump turning over very slowly. I eventually gave up and tried the stateroom system and had no trouble. Later I added the galley/v-berth system to the mix with the pump already running (started by the stateroom system earlier).

I have not experimented further this evening because since we have heat, I didn't want to mess with a good thing, plus since we may lose power due to a significant storm in the San Francisco area (many areas are already without power) we figure let's get it warm and keep it warm and experiment tomorrow when things calm down a bit.

I like the idea of swapping the triggers to see if the problem follows the trigger!

Nick

[kelpy]

Does the same issue occur when running on the generator?

[rustybucket]

I would almost suspect a motor capacitor on the pump. A bad capacitor would satisfy all the symptoms (as I am interpreting them). An amp spike when the motor starts causing the motor to struggle, especially in a voltage drop situation (such as when both units on) but able to overcome it when the systems are staged on. Lower voltage equals higher amps, possibly the voltage drop is just enough to push the amps over the trip level of the fuse/breaker.

You could clamp an amp clamp on a lead to the motor to see what kinda amps it's drawing at start/run. Is pretty easy to check the cap(s) on it as well and a VERY cheap fix if that's the issue.

I'd be tempted to measure amps on the motor at start/run. Then also voltage of the system as the units are turned on (this will take two people). If you see an amp spike on the motor and a voltage drop when everything is powered up then I would pull the capacitor and test it. Or if you have a spare handy just plug it in and give it a test.

A second consideration. Is the pump a higher amp pump than what was originally installed on the system?