Lifting pump on M-25XPB getting voltage but not current.

[dhandzy]

I'm having an issue with my fuel lifting pump when trying to start the engine:

• The engine won't start from the ignition switch but I can get the engine started when I manually attach the lifting pump's power input wire to 12V. source while trying to start (from cold after changing fuel filters). In other words, I know the lifting pump is operational
• When I put turn the key to "run" on the pod panel, there is no clicking sound coming from the lifting pump, indicating it's not getting power. The lifting pump does click when I manually attach its power wire to a 12 volt source.
• The lifting pump power wire is normally connected to a two-wire (light blue) pair that appears to come out of the nearby solenoid (too many crowded bundled wires to be 100% sure).
• Those two light blue wires carry 12V when I put the key to "run" on the pod panel.
• BUT, when I put the key in 'run' position and I attach the lifting pump power wire to those two light blue wires, the voltage drops to 10V suggesting that there isn't enough current getting to the pump.
• I've opened the panel to get access to ignition switch wiring and there are no light blue wires but I have traced a purple wire from the key switch position to the solenoid near the lifting pump. All looks clean and un-corroded in the pod.
• Tracing the wires is difficult - I do not have a diagram for the electrical connections for the engine or panel.

Any suggestions as to what to do next? (I've already asked for a mechanic's help but they're swamped).

 

[Don S/V ILLusion]

Sounds like a bad (high resistance) connection not visible by simply looking at the wire connections.

 

[Stu Jackson]

In some Catgalinas, they wired the lift pump to only run when the glow plugs were energized (position 2 on your key switch), and then to resume running when the oil pressure is satisfied. I had a grossly long discussion about that here:

The reason your pump did not work when you turned the ignition switch, but NOT the glow plug switch on is covered in this very long and detailed thread: http://c34.org/bbs/index.php/topic,3347.45.html See replies #20 & #59 and the other posts I made to that thread. The newer M35 series engines have what I believe is an unnecessary change to the pump operation: switch on pump does not run, glow plugs on pump runs, glow plugs off to start the engine then the pump does not run, when and only after the engine starts and the oil pressure comes up to OK, then the pump runs again. Details covered in the link provided.

The short form with links is covered in Critical Upgrades:

CRITICAL UPGRADES - DO THESE OR ELSE!!! - Page 3

Otherwise, if your pump always ran before, it's either connections, bad ground or bad switch.

 

[JoeWhite]

Stu is correct. The ignition switch must be turned beyond the first position to provide temporary power to the lift pump to prime the pump. This position also provides power to the glow plugs to warm up the cylinders for easy starting.
Here is a simplified diagram that I redrew from my M35 B engine. I wonder if yours is the same.

Oil Pressure Switch | Sail La Vie

mei1970.org

 

[Tom J]

On the C310, M25xpb engine, the lift pump is controlled by the ground side of the circuit. It is grounded by the oil pressure switch. When you turn the key on to "run", there is obviously no oil pressure, so the low pressure buzzer will sound and the lift pump does not run (no clicking). When you turn the key all the way, the circuit provides a ground for the lift pump and energizes the glow plugs and you should hear clicking. Then you hit the start button. Once the engine is running and oil pressure is established, the lift pump stays on. It sounds like you might have a loose wire at the oil pressure switch.

 

[Tom J]

BUT, when I put the key in 'run' position and I attach the lifting pump power wire to those two light blue wires, the voltage drops to 10V suggesting that there isn't enough current getting to the pump.

Just to clarify, the voltage dropping from 12v to 10v indicates a large current, probably a short circuit.

 

[jssailem]

Could it also mean that the Glow plugs kicked in on his boat while his battery charge was a little weak?

 

[SJN]

When voltage drops from 12 to 10, Ohms law says current increases because resistance is greater. Bad connections/corrosion at same can cause the voltage loss.

A few years ago my engine would start sometimes and other times not. I took the instrument pod cover off, hooked up a long jumper wire, and tested resistance for every wire still attached to engine parts and panel. Several were in poor condition. In every case, the problem was the connectors at the engine end either corroded or loose. I used better quality connectors as replacements and added water proof heat shrink tape.

 

[Stu Jackson]

In every case, the problem was the connectors at the engine end either corroded or loose. I used better quality connectors as replacements and added water proof heat shrink tape.

WIRING HARNESS CONNECTORS. Since, like, forever...

Critical Upgrades http://c34.org/bbs/index.php/topic,5078.0.html

 

[dhandzy]

Stu is correct. The ignition switch must be turned beyond the first position to provide temporary power to the lift pump to prime the pump. This position also provides power to the glow plugs to warm up the cylinders for easy starting.
Here is a simplified diagram that I redrew from my M35 B engine. I wonder if yours is the same.

Oil Pressure Switch | Sail La Vie

mei1970.org

Thanks - I should have added that I did put the key switch to 2nd position (glow plug) for 30 seconds (it was cold after the winter) but I did not hear the clicking of the lifting pump even then.

 

[dhandzy]

Just to clarify, the voltage dropping from 12v to 10v indicates a large current, probably a short circuit.

Ya, good point. It would be helpful if I had my facts straight.

 

[Tom J]

Could it also mean that the Glow plugs kicked in on his boat while his battery charge was a little weak?

Yep, very likely jumping the wires energized the glow plugs.

 

[JoeWhite]

Handyz, if I had to guess, I believe the oil pressure switch is bad. To test this unplug the 2 wires that is connected to it and connect them to each other. Then try to start the engine. If the engine runs fine, then you have located the problem.
Remove the switch and take it to your favorite auto parts store. There is nothing special about it. The switch is normally open contacts and the contacts are closed above 5 PSI.Make sure the threads match. Or you can get the original Universal part. It takes a deep socket to install it, avoid using vice grip.
You can download the owners manual from my website that includes parts list and the wiring diagram you are looking for.

Sail La Vie | 1999 Catalina 36 MK II

mei1970.org

 

[Tom J]

Ya, good point. It would be helpful if I had my facts straight.

It's difficult to troubleshoot these circuits without the wiring diagrams or schematics. The owner's manual has very comprehensive diagrams, etc. If you don't have the manual, it is available on this site from s/v smitty.

Handyz, if I had to guess, I believe the oil pressure switch is bad. To test this unplug the 2 wires that is connected to it and connect them to each other. Then try to start the engine. If the engine runs fine, then you have located the problem.
Remove the switch and take it to your favorite auto parts store. There is nothing special about it. The switch is normally open contacts and the contacts are closed above 5 PSI.Make sure the threads match. Or you can get the original Universal part. It takes a deep socket to install it, avoid using vice grip.
You can download the owners manual from my website that includes parts list and the wiring diagram you are looking for.

Sail La Vie | 1999 Catalina 36 MK II

mei1970.org

Unfortunately, this test is not definitive for the C310 since, in most cases, the engine will start and run fine without the lift pump working. The fuel tank is mounted higher than the engine, so, with about a half tank of fuel, gravity is enough to supply fuel to the engine.

 

[dhandzy]

Stu, Tom J and Joe White: thanks for the informative comments. I went through the OM (and parts manual) and now see the wiring from key switch through solenoid, fuses, oil pressure switch and lifting pump. I'll start by looking if there's a lose wire at the oil pressure switch and work through each failure point to find the problem. You've all been very helpful and I truly appreciate the guidance. I'm now well prepared for next weekend when I'm next on board.

One comment on gravity/fuel tank: I had a problem with the fuel pump a few years ago and learned that the engine will indeed start without the lifting pump and it will even run, at idle, in gear that way. But, even a little throttle will then stall the engine as it's not getting enough fuel. So I imagine I can perform the test JoeWhite suggested but I'd need to rev the engine and test it at higher throttle than idle.