1998 v70 Secondary Air Pump Internals (Pics)

[cn90]

Below is the diode mod DIY...

SAS removal walkthrough(No CEL!)
https://www.matthewsvolvosite.com/forums ... =1&t=31536

[MrAl]

Hello again,

Thanks for the link.

Does anyone know exactly what that diode is doing there? Diodes conduct in one direction only, for the most part, so that means it provides current from the anode side to the cathode side, with a small voltage drop of maybe 0.5 to 0.75 volts.

I wouldnt mind getting it to work with all the right parts first then later adding the diode. That would be nice too.

PS
I have so many of those diodes on hand already, as well as a ton of transistors, micro controllers, resistors, capacitors, etc., so this would not cost me a penny as long as it works

PSPS
Hey, is this fix 'legal' ?

[cn90]

I started a thread a while back on the theory of the diode mod...

SAS Delete: the Math and Physics behind it...
https://www.matthewsvolvosite.com/forums ... hp?t=58391

[MrAl]

Hello again cn90,

Very interesting set of posts there.

Without taking anything from that informative and probably very useful set of posts, i do have to add a little as to the technical nature of the fix.

First, having worked in the industry for many years (now retired) on complicated electronic equipment both design and repair, i can tell you with certainty that there is no way to connect a diode in series with any circuit on earth without at least cutting one wire or lead or something so that the diode can be inserted between the ends of the cut leads and thus re-bridging the gap. That's the only way to make a series circuit out of an existing circuit.
The only exception would be if one of the leads was already opened, and when the diode was connected, it bridged the gap and thus made a series circuit.
If it was a series circuit, then the posts with the voltmeter diode and resistor would be definitely true and accurate, because the diode would cause between 0.5 and 0.7 volt drop depending on ambient temperature (lower drop for higher temperature, higher drop for lower temperature, coefficient roughly -2.2mv per degreeC).

So what this means is that the diode must be being placed in parallel with the existing circuit. This can happen without cutting any wires, and since i dont see any wires cut i have to believe this is the more accurate representation.
This can also cause a voltage drop, because the diode can divert some current away from the original sensor, which makes it less sensitive. So if the sensor puts out 0.8v normally, placing a diode in parallel with that will cause some current to flow around the sensor so the voltage at the sensor lead will still drop. It depends on the actual voltage already present and the current available, but if it was really 0.8v then the diode should bring it down to around 0.5 to 0.7v, and if the current is really low then it should be at the lower end of that range.

So in the end it will still drop the voltage, but it is because the diode is in parallel. The equivalent circuit would be a battery in series with a resistor, in series with the sensor (second resistor), and with the sensor resistor dropping 0.8v to start. Then when the diode is placed across the sensor (that looks like the case here) some current is diverted to ground and so the voltage drops to 0.6v perhaps. The system will however find it very hard to raise the voltage above that, because the diode can be a very low impedance (on the order of 1 to 10 ohms).

One of my questions would be what actually drives that Ox sensor?
Depending on what drives that sensor the diode may load it too much. If it has worked in 100's of car already though then it must be ok.

Also, does it have to be a diode? It looks like the cathode lead goes to ground (a little hard to tell from the schematic) and if so and there is no negative supply in the ECU, then a resistor might also work.
Is there any way to get a complete schematic of the ECU board? If so we can look into this further.

It you'd like to discuss this further in PM's that might be good too.

[erikv11]

The diode is installed in parallel. A resistor won't work.

The diode was Lucky's idea (formerly of IPD, now runs ARD tuning http://www.ardideas.com/) on the heels of trying other methods like a potentiometer, the original discussion is here:
http://volvospeed.com/vs_forum/topic/132158-sas-delete/

And then here:
http://volvospeed.com/vs_forum/topic/13 ... ckys-idea/

And finally the best (IMHO) original pics of how to do it right can be found here:
http://volvospeed.com/vs_forum/topic/14 ... with-pics/ and here

[MrAl]

Hi,

Thanks for the links.
I've been reading a little bit more on this now.
I have also read that the fix is illegal but the States dont enforce it that strictly probably because they understand the problem as well as we do. Except in CA, im not sure about that State, maybe because they are stricter on air pollution.

[precopster]

Where on earth is this pump located? Is it under the battery tray somewhere? My 98 V70R has the sound of a air pump kicking in every minute or so then it stops then kicks in again.

I would be interested in doing this mod if the air pump is fitted however my SAS relay in the engine fuse compartment isn't even present! !

[erikv11]

Yes, it is mounted under the battery. Follow the hose from the air box down to the pump!

[precopster]

Thanks Erik. I definitely don't have one. The hose on the airbox is blocked and I don't have that valve on the exhaust.

Now to work out the strange and eerie sounds under my bonnet!!

[MrAl]

Hi,

You dont have a secondary air injection system pump?