low voltage at purge valve. ‘04 XC70 Help from experienced owners!

[abscate]

“ and found that a small blue capacitor on the ECM board for this EVAP solenoid valve line was resistive (to ground) and causing the problem. I cut out this small capacitor, which is a type of signal noise filter cap and I haven't had any problems since.”

I’d like to see a picture of this capacitor in the board if you could possibly get one. It would really help me with diagnosis. My electrical board experience is minimal, but enough to know I could screw it up pretty quick . I’ve ordered an ecm removal tool and figure on working on it before looking for a new ecm which might do the same thing. Thanks all for weighing in!

Your ECM is VIN coded to your car, so you can’t just swap them. Applies to Volvos from 1999- on

Removing noise caps from the ECM ? Oooof, board surgery often has complications. Not recommended.

[firstv70volvo]

Removing noise caps from the ECM ? Oooof, board surgery often has complications. Not recommended.

I didn't like having to do this myself but I wasn't actually able to replace the faulty cap, access was limited. I can confirm I haven't had any problems with the ECM since doing this in 2008. By backup plan if this didn't work was to purchase a new and very expensive ECM.

[Volvalive]

Ok, thanks. When my ecm removal tool arrives, I will test it and post my findings. Thanks again!

[Volvalive]

So I opened up my ecm today and couldn’t find anything blue. I figure that that must have changed the board for 2004. I also did a little more testing, and found that I get 12v from one pin of the evap purge valve and 4v from the other. This makes 8v between them. So apparently, there is 4v feedback that shouldn’t be there? Does anyone know why a circuit that should be grounded is giving voltage?

[jonesg]

I've read the neg side isn't really negative in the traditional sense.
so that 4v might be normal in a volvo, someone would need to compare for you or send the ecm to xemodex and them test it.
, elecs are beyond me.

[abscate]

Correct. All voltages are referenced to ground.

Check that you have B+ ( not 12 volts but the same voltage that exists across the battery plus and minus posts ) on the positive side of the purge valve

Almost all valves and solenoids are operated by the ECU pulling the ground pin down, with a fixed 12 V on the supply side.

The voltage to ground on the negative pin of the purge valve will depend on the state of the transistors in the ECU

[firstv70volvo]

So I opened up my ecm today and couldn’t find anything blue. I figure that that must have changed the board for 2004. I also did a little more testing, and found that I get 12v from one pin of the evap purge valve and 4v from the other. This makes 8v between them. So apparently, there is 4v feedback that shouldn’t be there? Does anyone know why a circuit that should be grounded is giving voltage?

The board design is different than the 2001 design, the capacitors on your board are the small rectangular light brown devices. The 4V measurement still doesn't look right to me so you could still check the capacitor for the A.9 EVAP pin. With the board removed measure the resistance across the capacitor connected to this pin, should be an open measurement. If you don't read an infinite resistance it would be best to remove the capacitor or at least one leg of the capacitor and measure it again to confirm if it's the capacitor or not.
To verify A.9 is the correct EVAP pin, with the ECM removed measure between the A.9 pin in the ECM connector to the pins on the EVAP connector, one pin should have continuity, should be the white wire according to my wiring diagram. On the backside of the circuit board you can see there are 13 of these capacitors located near the pins of the connectors, it will likely be one of these capacitors.

[abscate]

You need to be using ESD protection if you fool with circuits inside the ECU.

With everything buttoned up, do you have b+ voltage to the positive side of the purge valve, with the negative side of the voltmeter buried in the negative battery terminal?

If so, stop. The ECU will actually detect a current fault in control signal on the negative side and pop a code.

The voltage between the pins on the purge valve will not be 12 volts nominal , that’s normal

[Volvalive]

Right. So I am definitely getting a code on this. Currently looking into another ecm lined up in case I ruin this one. I’m going to do some careful (grounded) checking of the circuits and see if there is a fault in the resistor. Uncharted waters for me, but worth it to keep the car running! I will keep y’all up to date on this when I find something out, meantime thanks again for the info!