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Re: Air Pump? noise
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Posted by Dean (more from Dean) on Mon, 7 Feb 2005 08:55:31 Share Post by Email
In Reply to: Air Pump? noise, Tom N., Mon, 7 Feb 2005 08:17:57
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This an electric brake booster pump. It is controlled in part by a pressure switch on the firewall. Follow hoses and you will find it. (Some T5 implementations do not use a pressure switch, but let the ECU amd MAP sensor do the job with logic, in that case the ECU activates the relay.)

The ECU is not involved. The switch controls the relay #460 in the fuse box under the hood. The relay power input gets power all of the time via fuse #16 in same box. The relay coil gets +54 power via ignition switch relay and the pressure switch completes the circuit to ground. The pressure switch monitors manifold vacuum (not bake booster vacuum according to WIS). So the switch should be closed when engine shuts down. Now +54 voltage is controlled by the ignition switch relay 21 which is on the main relay board under the dash.

So the pressure switch cannot make the pump keep running unless the relay has its coil energized by igniton relay 21. If the igntion power stayed on, I think that you would notice many other things getting power. So I am inclined to pull the #460 relay or the #16 fuse to stop the pump, but driving in that state might be fatal. Pull the relay and pop the cover and see if you can see any reason that the relay might be sticking. Pits and weld on the contacts? When the pump stays on, open hood and fuse box. gently rap the relay with a hard object and see if that makes it drop out. The try hitting harder.

Double check relays and fuses against the legend on the fuse box cover. EPC lists relay 4947115 which replaces the old one which is discontinued. The list price for that is 8.20 $US

Do not ignore, I think that the pump might fail from continous duty. When the engine is running, and hot, blip the throttle and this will create a high manifold vacuum for a bit. The pump should not be running through all of that. If pump never turns off when the engine is running, then the switch is suspect... if the relay issue can be cleared up. I still think that the cause is the relay.
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