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TOPIC: 1990 HVAC

1990 HVAC 21 May 2020 21:14 #21224


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i have a 1990 s4 127K miles i have a HVAC problem for no reason the solenoids will start open and closing real fast and can't get them to stop has anyone had anything like this happen before ? if anyone can help my email is This email address is being protected from spambots. You need JavaScript enabled to view it. my cell 818 599 9948 thanks
Last Edit: 21 May 2020 21:17 by jk1990s4.
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1990 HVAC 29 May 2020 16:26 #21238


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What solenoids are you referring to? What is the general location of the components/solenoids, and what is it they're doing that makes you say they're opening and closing? Are they making sounds of some sort?
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1990 HVAC 29 May 2020 17:36 #21239


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Hi they are the solidionds that activate the pods to open and close different vents be it the air flow going to the center vents or to the floor or defrost ect they have all the different vacuum hoses coming in so its in the lower center of the dash if you open the driver side panel you would see the vacuum line thats there it is and as I'm driving for no reason they would get activated and fire off real fast thus opening the pods i had the pull the brown ground wire that dos to that unit to make them stop and if I'm driving and i hit the brakes that makes them go off too . have you and idea what it could be?
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1990 HVAC 29 May 2020 20:37 #21240


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The most common cause of what you're describing is that the rubber diaphragms in the control actuators for the various doors in the HVAC ductwork have developed holes and are losing vacuum. Check out to following link which explains what's happening and how it can be addressed, and give Jeannie a call at www.928gt.com for the parts you'll need.
dwaynesgarage.norcal928.org/1987%20HVAC%...pair%20Procedure.htm
Last Edit: 29 May 2020 20:40 by y33trekker.
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1990 HVAC 30 May 2020 10:43 #21241


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ok ill check that out thank you very much
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1990 HVAC 31 May 2020 09:40 #21242


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Just wish I could have provided better news if it IS the HVAC vacuum diaphragms, since the job to replace them isn't the easiest project to complete on a 928.
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1990 HVAC 31 May 2020 11:22 #21243


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i was running some more test and with the car in drive park brake on it was making the solenoids run like cray i notice that the X relay was going cray and i traced it to the 14 pin connector on the passenger side of the engine compartment i found to almost cut wires that where hitting the air pump pulley i fixed them and rerouted the harness away from the engine ill see how that works now please let me know if you ever heard of this problem . thanks
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1990 HVAC 31 May 2020 19:36 #21244


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If it was the same solenoids that were running like cray, that could be explained by the high vacuum conditions created by having the car running (Idle) while in drive with the parking brake on. Engine vacuum is highest at that point, and only reduces once the engine moves off idle to higher rpms.

Your "X" relay is your ignition relay. If you think there may be something wrong with the relay itself, you can swap it with the XVI (Horns) relay and see if there is any change. If the swapped in relay acts the same and your horns work, the problem is elsewhere. (Be sure to check for any corrosion or oxidation on the terminals of the relays and clean before installing though).

I'd need a refresher myself on what harness runs through in the area of the smog pump, so until that happens, maybe someone else will chime in with that info. and what could possibly be effected by it's wiring possibly being shorted.
Last Edit: 31 May 2020 19:39 by y33trekker.
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1990 HVAC 01 Jun 2020 12:08 #21245


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i pulled the X relay the car started and ran fine the only thing didn't work was the A/C so i put a new relay in with that and the rerouting that 14 pin connector i have been driving it for 2 days and it seems to be working fine I'm not going to get excited for a long time because i think one day the gremlins will come back thats how long i have been dealing with this problem and thank you for your time to help me with this because if this doesn't fix it I'm going to have to get rid of a car i like a lot .so fingers crossed
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1990 HVAC 01 Jun 2020 13:45 #21247


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If I had to gues, I'd say it was your repair of the wiring harness wires that made the difference. The types of electrical gremlins in multiple systems that can be caused by an issue like that are not easy to figure out sometimes. Glad to have tried to help in any way.
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1990 HVAC 01 Jun 2020 16:16 #21249


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yes thank you very much for your hep hopefully this save my beloved 928 ill let you know if it comes back .
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