For MVS’s 1500th (!) Volvo Repair Database entry, I bring you what is probably the most technical post in the VRD: jose456891‘s Fuel Relay #103 — Is it Really Necessary?
I had to lie down after I read it. And yes, I read it. And it was mildly fascinating.
A big thanks to all the technical guys here who understand this stuff, and through no fault of their own, help us laymen to understand what the parts in our Volvos do.
jose456891 » Recently my fuel pump relay went bad. The first instinct was to jam a piece of thin cardboard between the poles on the point of the coil to keep it closed. This is essentially the same as putting a jumper.
After replacing the capacitors 100µF, 22µF I plugged it back in the car. To my surprise it kept cutting the coil off and on erratically.
So I took it back to the lab and started to decode the function of the PCB to figure out what exactly this little bugger does, and is it really necessary.
…
Perhaps the reason behind putting a monostable vibrator chip behind the fuel pump is to modulate the duty cycle, thus perhaps reducing stress on the fuel regulator, and perhaps extending the life of the fuel pump. Perhaps the PCB monitors the resistance of the fuel pump via its power feed and cycles it to cool it off. (some one please chime in)
My Volvo was assembled on November of 1995, rendering this component 20 years old, its 21st birthday is in a month!. The capacitor replacement did not fix the relay, upon further inspection the coil’s resistance fluctuated upon flicking the sides. It fluctuated from 4 Ohms (good) to infinite (bad). It was a broken/melted core wire on the coil.
The only situation where I see this relay providing benefit to the fuel system integrity is when the operator leaves the can on, but the engine off. No need to overwork the fuel pump when the engine is off.
I conclude that replacing this relay long-term, or until next paycheck, with a jumper wire is OK, and will not hurt the fuel pump. While a jumper is installed, I recommend not leaving the car on without the engine running. I noticed that my Volvo was a lot peppier than before. I gained a couple of ponies with a jumper.
Don’t worry about the fuel pump running when you remove the keys, the middle “J” relay will cut the power off.
MVS Moderator Ozark Lee »
It also cuts off the fuel pump if the engine dies which is a desirable trait after you run into a tree and break a fuel line.
Fuel Relay #103 — Is it really necessary?
Last Updated on August 19, 2020
