[Erik Å]

It sounds very reasonably that the "non genuine" relay may not work with all different brands of ECU:s. They might use different "key sequencies" on the control line..
I bet the relay manufacturer tested in one car and since it worked fine everyone assumed it would work in all other model years as well since Volvo used the same relay in all of them

Furthermore, if you don't have the original Volvo requirement spec, you don't know the tolerances on for example clock frequencies or logic levels. Consequently the part you develop and test might work well in some car individuals but not necessarily in all..

[Stefan96]

Yeah, who knows what that manufacturer based the relay off of, or how it got tested...

I got around to soldering the LED "code reader" today and got the obvious 3-4-3 fuel pump relay signal fault code (what a surprise, I know).
However, no matter what I tried I wasn't able to clear the code. The instruction I followed said that one connects the terminals for 5 seconds then disconnect, then the LED should come on, then connect for another 5 seconds, disconnect and once again for 1 second. After that it should display the 1-1-1 (no errors). However, no matter what I tried it wouldn't come on after the first 5 second connection, so I must be making some stupid mistake. Or maybe the ECU prevents the code from being cleared if the component is still faulty (or not installed).

I also had another look at the Volvo 850 manual from MVS (since that should still apply to a large degree) and checked the voltages & resistance on the relay terminals again. The manual says around 0 Ohm between the relay ground & ground (I measured at the battery), which was around 2 Ohm, so not great but not terrible. I'll investigate some more in that regard...
They also give a specification at the relay control terminal of around 3 V which was only 0.2 V in my case, but they don't specify which ECU was used for that manual. Also since it's probably some sort of serial interface I don't trust that too much - but then again my V70 did show around 3 V...

Otherwise I'm hoping to get the relay tomorrow or Wednesday.
Semi-worst case question: if I have to swap the ECU, do I need to get that recoded or not? Since the immobiliser is a separate unit... and I found some conflicting info online regarding this. Some say one can swap the Fenix over without recoding, some don't. Same deal for most of the other ECUs, except the '99 & 2000s of course...

-Stefan

[Erik Å]

However, no matter what I tried I wasn't able to clear the code.
.. The manual says around 0 Ohm between the relay ground & ground (I measured at the battery), which was around 2 Ohm, so not great but not terrible.

My car is a later generation equipped with early OBD-II so I don't know if this works on yours:
Remove the plus cable from the battery and wait a few minutes before you reconnect it. That should clear any active fault codes. Stored codes remain though..
It is difficult to measure low resistances with standard measurement probes, the contact resistance at the probes might be larger then the wire resistance you try to measure. Handheld multimeters usually don't support 4-wire resistance measurement, that's the proper way to do it ..
By the way, did you see the other thread where we just found the location of th main relay for the injection system? That relay happens to supply power to the fuel pump relay, the injectors and the oxygen sensor heating..
viewtopic.php?p=1594064#p1594049

[Stefan96]

I didn't check that yet, but I know that disconnecting the battery to clear some codes works on the Motronic 4.4 in my V70 - but that actually has OBD-II...

Yeah, I don't trust measurements in that regard, especially not with the super cheap meter I have at hand...

I didn't see that yet, I'll give it a read later. Thanks!
Although I don't suspect that to be related, given how it runs beautifully if I bridge the relay. But once again, car electronics are weird so maybe it is

[Stefan96]

Got the replacement relay & ECU today, and got it running with that. Turns out it was the relay. I also had the ECU out to measure the relay control line properly under load etc.
So what have we learned today:

1. Despite the old relay working in my V70 (2.5T with Bosch Motronic 4.4) it didn't in the S70 (2.5 10V with Siemens Fenix 5.2). So even if the relay you have is new, try an additional one, and a genuine Volvo one at that!
The tip of just measuring the voltage (tip from the 850 manual, should be roughly 3V) at the control terminal of the relay seems to work for the Motronic, but not the Fenix. But the relay uses some type of pulse modulation signal anyway. Maybe I'll have time to probe & decode that one day out of curiosity, but it's also kinda useless because bridging the relay contacts is almost identical to having it in there.

2. Swaping the Siemens Fenix 5.2 (numbers right underneath the Siemens logo: S103955411B and the barcode on the right side of it: P1275476) works without recoding it to the cars immobiliser.
I didn't drive it, but it ran fine for at least a minute before I turned it off myself. If it wasn't fine I'd expect the car to not even start or shut off right away, so there you go. So I the immobiliser probably just sends a generic enable signal to the ECU.
I hope I helped someone who googles this in the future
If I'm reaaaaaaally bored one day I might probe & decode that, but I don't know if that could get me in hot water, so we'll see...

Drove it a bit and checked it over otherwise, now I know what else to fix sooner or later! And try to find a garage in my area
Thanks for all the help and trouble shooting ideas!


- Stefan