Fuel Pump Diodes

[Colin S]

I have the exact same problem with my 1995 Larson Volvo Penta 7.4 GL PHUS.
Only at idle the fuel pump turns off.
Everything replaced up to including the fuel pump relay. (TANK to RELAY) Looking into the signal that runs the relay coil.
Need to find the diodes and check the wire connections.

 

[GA_Boater]

You have your own thread now instead of a post in an inactive thread. Please read the Help Tip on top of the page.

You originally posted in http://forums.iboats.com/forum/engi...utdrives/174486-volvo-7-4-gl-fuel-pump-diodes

 

[Dave-R]

My son-inlaw had a similar problem with his 1996 Volvo penta 4.3 . The battery had been jumpered backwards, and it blew the inline diode from the alternator that feeds the fuel pump relay after the engine is running. They use the starter to power the relay when the engine is cranking, and when the engine is running the alternator supplies the power to the relay. The diodes that are in the wires from each supply, protect each other from backing feeding, and damaging the other. Diodes are cheep. My boats previous owner had the same problem, and the dealer had a hard time finding the problem. Hope this helps. Good luck Dave-R

 

[Colin S]

Thanks for replying. Tonight I looked thru all of the wiring and never found any smoking gun. I did find the diodes that are located in the wire harness running across the front of the engine. All of the wires are wrapped in black tape from the factory and the fuel pump diodes are under heat shrink. The ignition comes in on a larger light yellow with a red stripe, hooks onto a smaller yellow with a red stripe (that goes back to the starter ignition relay). Where they hook together is the diode and then exits to an orange wire which feeds the fuel pump relay coil. The green wire from the alternator has the diode in the same area under some more heat shrink and feeds the fuel pump relay coil with the orange wire attached under the relay socket.

Everything measured and looked good. The only other theory I have is... When I'm running at idle, the altenator is not putting out a good enough signal on the green wire?? When I'm at idle the alternator doesn't charge unless I increase the rpm's.

I'm going to check the signal with a meter using min-max on the alternator tomorrow...

 

[GoFastr]

Possibly the voltage regulator on the back of the alternator is bad?

 

[Colin S]

Tonight, I installed 4 Led's to display what's going on. 1 hooked to the output of the alternator (green wire/light). 1 hooked to the ignition crank wire. 1 hooked to the combined input of the fuel pump relay coil. 1 hooked to the output of the fuel pump relay positive contact. Boy did that really help troubleshoot what was going on.

Sure enough, my theory was correct. Not all the time, but once in a while, when you just turn the key over and start at idle, no signal out of the alternator. Then you can rev it up and it turns the pump on and back to idle and the pump turns off via losing the signal from the alternator.

Most of the time, it will put out a signal just fine and everything works OK.
It's the very few times that you are out on the water and it keeps you wondering why your boat dies when you are just idling along.

 

[bruceb58]

Time for either a new regulator or some diodes are gone inside you alternator.

Or just bite the bullet and get a whole new alternator.