Matt, --------------------------------- You said: "...it's also not obvious WHY the RPM relay only should activate for "about a second" when the key is on the ON position. The service manual clearly states that it will, but why? What controls that? If my frequency valve is buzzing and my fuel pump is running when I put the key in the on position for more than a second, perhaps this is a failed component somewhere (capacitor in the RPM relay?)? Do you have a schematic for the RPM relay?" --------------------------------- Operation of the RPM relay, also known as the impulse relay in Volvo publications, can be determined by reverse analysis. In addition to the *duel relay contacts there is a voltage sensor and timing circuit. When the ignition system is turned on it receives a current pulse similar to RPM tach pulses. This is why the relay may activate when you turn the ignition key to the "run" position prior to starting. Because the "run" on pulse has different characteristics than tach pulses and depending on other conditions, the relay may not activate when the ignition is turned on. In some cases, again depending on different electrical conditions, the relay may cycle several times after the key is turned to the ignition "on" position. Finally, concerning a schematic diagram; outside of a block diagram I know of no component diagrams that exists. This has never been of real concern because if you remove the relay cover all of the components are in clear view. Basic circuitry and component knowledge makes it obvious how this device works. *some versions of the RPM relay have a single contact for the pump and Lambda relay. DMC Joe -----Original Message----- From: Matt Spittle [mailto:supermatty@xxxxxxx] Sent: Monday, December 27, 2004 10:13 PM To: dmcnews@xxxxxxxxxxxxxxx Subject: [DML] Re: Fuel pressure & RPM Relay --- In dmcnews@xxxxxxxxxxxxxxx, "endotex23" <endotex23@xxxx> wrote: In this example all the owner had to do is study the diagram to answer his own question. Learning how is not difficult and basic auto electrical theory is something anyone who wants to work on cars should learn. So Greg, I studied the diagram to answer my own question, and I wasn't able to; not even with a background in electrical engineering. Part of my original question was if the frequency valve was supposed to run while the key was in the ON position. So I look at the diagram...it looks like the frequency valve is powered by the lambda relay, which is powered by the RPM relay. If this all happens, then whether the frequency valve turns on or not is determined by pin 15 of the ECU. Do you have a schematic for the ECU, because it's not obvious to me what it does? Second, it's also not obvious WHY the RPM relay only should activate for "about a second" when the key is on the ON position. The service manual clearly states that it will, but why? What controls that? If my frequency valve is buzzing and my fuel pump is running when I put the key in the on position for more than a second, perhaps this is a failed component somewhere (capacitor in the RPM relay?)? Do you have a schematic for the RPM relay? So this is why I asked the list. Please correct me as to where I should have looked if my "homework effort" was inadequate before posting. thanks, Matt #1604 To address comments privately to the moderating team, please address: moderators@xxxxxxxxxxx For more info on the list, tech articles, cars for sale see www.dmcnews.com To search the archives or view files, log in at http://groups.yahoo.com/group/dmcnews Yahoo! Groups Links <*> To visit your group on the web, go to: http://groups.yahoo.com/group/dmcnews/ <*> To unsubscribe from this group, send an email to: dmcnews-unsubscribe@xxxxxxxxxxxxxxx <*> Your use of Yahoo! Groups is subject to: http://docs.yahoo.com/info/terms/