CIS gurus: need help with 8V deceleration fuel cut-off - Saab 900 Bulletin Board - Saabnet.com
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CIS gurus: need help with 8V deceleration fuel cut-off
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Posted by Micah in NC [Email] (more from Micah in NC) on Mon, 29 Aug 2005 11:03:46 Share Post by Email
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Folks,

This may seem like a piddly little thing, but bear with me. In light of ridiculously high gas prices, I'm more interested than ever in fixing the deceleration fuel cut-off on my 1984 900S. (This is the relay-controlled valve which uses engine vacuum and an electric solenoid to open a bypass hose around the airflow sensor plate, causing plate to drop down and cutting off fuel to injectors during deceleration--and it's supposed to help emissions, too.)

Tom Townsend told me the decel solenoid valve wasn't working when I had the car in for other service a couple years back. Said it wasn't detrimental to engine performance, but could help get me 5-6 percent gas mileage if I replaced the valve.

I tinkered around and used my Bentley 8V manual and found that the valve does not work (duh, Townsend told me that!) nor does the decel relay provide ground for the valve's solenoid (so valve couldn't work even if it WAS good).

The solenoid valve plug has 2 wires: red and white. Battery voltage is supposed to be present after warm-up on the white wire (12V +). Blipping the throttle to any speed above 1600 rpm and letting it snap back is supposed to have the decel relay ground the red wire on this valve's solenoid while the rpms are above 1600. But the relay doesn't give the ground connection.

The battery voltage which should be present at the solenoid's white wire is there, but it never receives the ground through the red wire (through the decel relay). Grounding the red wire directly makes the solenoid click (I probed a jumper wire into the rear of the decel solenoid connector plug while plug was attached to solenoid), but this click doesn't affect the engine at all. A vacuum guage teed into the solenoid valve's vacuum line reads 18" Hg at idle, so I don't think the valve's diaphragm is leaking. But the piston inside never moves, even when I grounded the solenoid directly! It seems like the valve's piston is stuck shut. (I had stuck a second decel valve in place in the airflow sensor bellows/bypass tube without vacuum hose or electrical plug connected in order to test the other decel valve I had. Neither's piston works when vacuum and voltage are supplied.)

Just for kicks, I carried out all the Bentley tests on the decel relay (except I didn't have long enough probe leads on my digital volt-ohm meter to check if that one particular relay terminal has continuity to pin 12 of the K-Jetronic engine computer). All other tests were OK, except for the signal from the ignition system. It's supposed to be 3 to 5 volts AC. I got a reading of about 26.2 volts AC, if I remember correctly!!!

Is this why the decel relay doesn't provide ground to the solenoid--the ignition system input voltage is out of parameter? How would I fix that? By replacing the ignition amplifier?

Also, I have seen precious little in way of replacement decel relays or decel valves. Any ideas?

--Micah in NC
'84 900S, 297K mi.

posted by 65.1.173...


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