RE: [DML] Re: Fuel pressure & RPM Relay

["DMC Joe" <dmcjoe@xxxxxxx>]

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







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