Volvo 7.4 GL fuel pump diodes.

[harmony]

Can someone help me with the location of the two diodes in the Volvo fuel pump circuit. One is in the "light" lead from the alternator that supplies 12 vdc to the fuel pump relay while the engine is running and the other is in the starter solenoid circuit that provides 12vdc to the relay while the engine is cranking.

Where is the ground point for the fuel pump and relay primary circuit? I have an intermitant problem with the fuel pump shutting down while the engine is running. Unless I miss my guess this points to the circuit from the "light" terminal of the alternator, a marginal fuel pump relay or bad ground.

The fuel pump itself is apparently OK as I have jumped out the relay and the problem disappeared.

Any help appreciated, Thanks.

 

[Reel Poor]

Re: Volvo 7.4 GL fuel pump diodes.

A diode isn't going to be intermitant. It's either good or bad. I'm not positive, but I believe the ckt ground is on the flywheel cover stud.

 

[Don S]

Re: Volvo 7.4 GL fuel pump diodes.

I'm with Reel Poor on the diode, they are't intermittent, they either work, or they don't.
The wiring on the other hand does't work like that. I've found corroded wires (even the connection of the diode to the wire corroded and became intermittent for a short time before they quit completely. Same the the ground wire. While it hooks onto the flywheel cover, it's a large wire with numerous soldered connections along it's length. Opening and following the length of the wire is about the only option.
Usually the intermittent problems are nothing more than faulty relays or corroded relay terminals.

 

[harmony]

Re: Volvo 7.4 GL fuel pump diodes.

Thanks;
I realize that diodes that are "blown" are open, but I did not know if they were plugged in or hard wired. If they were plugged in there was another point of a possible poor contact.

I did clean the relay socket contacts and check the resistance on the primary side of the relay. The Volvo manual lists 70 to 100 ohms as the acceptable range on the primary side of the relay and I am showing 130 ohms., but I have come to the conclusion that the problem is in the 12 vdc feed to the relay through the lead from the alternator or in the ground circuit.

There was some kind of "potting compound" on the underside of the relay socket that had made its way onto the contacts, but after it was cleaned off of the contacts the problem still existed.

As fas as the ground circuit I have not had any other indications that there might be a bad ground, but will thouroughly check it out.

Thanks for the information, much appreciated.

 

[b00tstrap]

Re: Volvo 7.4 GL fuel pump diodes.

for what its worth, relays tend to fail more than a solid state device like a diode. You mentioned something on the contacts of the relay? How did you clean the contacts, are they pitted? Also, the coil part of a relay can fail intermittently.

 

[harmony]

Re: Volvo 7.4 GL fuel pump diodes.

for what its worth, relays tend to fail more than a solid state device like a diode. You mentioned something on the contacts of the relay? How did you clean the contacts, are they pitted? Also, the coil part of a relay can fail intermittently.

 

[harmony]

Re: Volvo 7.4 GL fuel pump diodes.

for what its worth, relays tend to fail more than a solid state device like a diode. You mentioned something on the contacts of the relay? How did you clean the contacts, are they pitted? Also, the coil part of a relay can fail intermittently.

The relay contacts I was refering to are the contacts between the relay and the socket. The relay itself is potted.

The diodes are used to protect the alternator whebn the engine is being cranked and the relay is receiving 12vdc from the starter solenoid and the other diode keeps the starter from cranking the engine when it is running and the relay is getting 12vdc from the alternator "light" terminal.

I am still trying to determine the location of the two diodes in their respective circuits.
thanks

 

[bjcsc]

Re: Volvo 7.4 GL fuel pump diodes.

Don't know the year of your engine. These are from the Volvo manuals and correct for LK models, but I don't know if they will apply directly to yours. Let me know when you're done with them so I can remove them to save space.

(removed)

 

[harmony]

Re: Volvo 7.4 GL fuel pump diodes.

Thanks;

I do have the Volvo manual and your schematics line up pretty close. It is a 1995 7.4 GL. 750 CFM Holly. The reason I was wondering about the location of the diodes was that in case I determine this weekend that the one in the alternator circuit is in fact "blown" or an intermittant open I will not have to unwrap the entire wire bundle. I have 2 diodes just in case. Try to do the "thinking" during the week and the "doing" on the weekend.

Thanks

 

[bjcsc]

Re: Volvo 7.4 GL fuel pump diodes.

Try to do the "thinking" during the week and the "doing" on the weekend.

me, too...