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Re: Spark No Fuel
Posted by tom0826 (more from tom0826) on Fri, 16 Aug 2002 23:19:22
In Reply to: , Kok Chen, Sat, 17 Dec 1988 12:00:00 

Just wanted to thank everyone for their replies to the fuel pump relay
problem on my 89 900.  It was indeed the ECU which was not sending the ground
signal to the relay.  From the replies, it seems that 89 was a bad year for
ECU's in the 900.  So I checked a few junkyards and could get a used one for
about 150 bucks.  A rebuilt unit costs 250 on up to around 400 bucks
depending on where you go.

My 900 has over 300k miles, but still ran great before the ECU failure.  Even
though I'm willing to shell out the money on a used or rebuilt ECU, given the
mileage,I'm wondering what your opinions may be for bypassing the fuel pump
relay signal from the ECU to the the ignition switch.  It could save money
since I wouldn't need a new ECU since all other functions seem to be working,
but may potentially be a hazard in an accident if the fuel supply or return
line is ruptured since the pump will run as long as the ignition is
on...although I think there are cars out there like this.

The first obstacle is to activate the fuel pump relay with +12v from the
ignition.  The problem is that the ECU sends a GROUND(-)signal to the relay.
It's very hard to find a (-) signal from turning on the ignition anywhere in
the wiring.

But the fuel pump relay (a common 15amp Bosch relay) can be rewired to be
triggered with positive +12v instead.  Saab wires terminal 85 as the negative
trigger from the ECU and terminal 86 is supplied keyed(ign on) 12v pos from
the relay just next to the fuel pump relay.  But this terminal supplies keyed
+12v for a few minutes even after shutting down the car, so it won't work to
trigger the fuel pump relay. If you did, the pump would still be on for a
while after you shut off the ignition.

For the new wiring: terminal 86 should now be grounded, and 85 should be the
pos 12v trigger to activate the relay.

Will it work?  Is it safe?

Tom

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