Volvo 850 fuel pump relay: can I just use a universal relay?

[JimBee]

Apparently, not—and not "just because...."

I noodled this one with a O'Reilly clerk who surfed around when I asked him about aftermarket relays. He brought up the universal one which he had in stock, so got one on the counter.

The pins in the universal number perfectly align with the pins in the spare Volvo brand unit I had with me. But this savvy clerk said the universal number is simply on-off—that's not the same as the Volvo brand whose taller case is made that way because it has a resistor inside that modulates the signal going to the fuel delivery system. Really, how so? He explains: at idle, fuel delivery is only 2/3 of what it would be under acceleration; so doing a little math, he said, idling, you've got ~ 8 volts outputting, vs full 12 volts under acceleration.

But, of course, this brings up the question: So.....? Since the apparently last regulatory device in the fuel delivery stream is the fuel pressure regulator—and that's vacuum operated off the intake manifold), why wouldn't that regulate how much fuel is delivered to the cylinders even under full 12 volt activation?

Circling back to the original question then: what harm could come from using an off-the-shelf universal $16.00 relay? Is the Volvo design basically redundant?

Opinions?

[jmartin919]

Sounds like the Volvo one saves pump life.

[chrafael]

I seem to recall reading somewhere that there are different types of relays depending on the purpose. Diode, resistor, or none. The purpose of one of them is to dampen high voltage spikes feeding back into the computer. I'm sure there are people here a lot more familiar with electronic theory than me. Perhaps further reading is in order. The pin numbering is universal.

[BEJinFbk]

It's been a long time since I went down this rabbit hole,
but it's been looked into a lot over the years. I recall some
theories and repair posts from a number of sites, but the
general consensus is that there's a lot more going on with
the electronics within than a straight up relay, and a correct
replacement is the way to go to keep the car running properly.
https://www.ipdusa.com/products/22792/F ... vo-9434225

FWIW, here's a post that may help:

viewtopic.php?t=18071&start=10
A deeper dive here at MVS will find more on the topic.

Other resources:
SwedeSpeed still gets good traffic for x70's and 850's.
You may also find some legacy info at VolvoSpeed.
V70R.com used to be pretty busy, but I think it's dead now.
XC70.com is also pretty much a graveyard.

[JimBee]

BEJinFbk: thanks for looking this up. Today, I popped the casing off a red one that I know works, and I agree that "there's a lot more going on in there" than just switching the fuel pump on/off. The one I looked into has a complex circuit board with numerous components including one that actually looks like a CPU. The "green" ones I found in my collection are stamped 92 and 93, so probably the first generation. Upgrades look more sophisticated. I'm not messing with any of that and will just use OEM Volvo.

[BEJinFbk]

Got a part number off of that chip?
Just curious…

[abscate]

I don’t think the relay modulates the fuel pump voltage. Mr Al did a deep dive into this relay amd concluded there is 40 Hz safety signal that shuts off the pump if the engine stops , implemented through this relay

I don’t see a way to feedback engine rpm to this relay in the wiring diagram

[JimBee]

These numbers are on the case: 9434225 89 8797 000
The only part number I can see on the PC board is 034.

There's definitely an electro-mechanical relay, with pretty substantial copper spool winding and a thin metal finger at the end that's magnetically attracted to close a circuit. But no other markings are visible.

Re the 40 hz safety signal, what if something makes the relay susceptible (age and wear of the relay?) to activating that 40 hz safety signal while the engine is still running—effectively shutting off the engine due to fuel starvation. Could that happen?

I don't think I've ever bought a new fp relay for any of my Volvos, but have interchanged them without any issues.

[JimBee]

Oops, that last statement needs an update: "until recently". Last week when both the fp relay and the camshaft position sensor failed, the big event was the CPS. Fortunately, it threw a code and I had an extra one in the used parts bin.
I found an extra used relay and swapped that one in, fixing that little problem. I don't recall any other failing since I've been driving Volvos (he says while knocking wood).

[foggydogg]

My memory is now jogged. I found a Meyle listed for a good price a few years back, bought several. Turns out they're reboxed Stribel, and others did the research and found them to be the OEM maker for Volvo.
https://www.uspmotorsports.com/product/ ... pQEALw_wcB