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Question re supplementary air valve function


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Hi all

2.7s1976 MFI

im nearing the point when the car will be run 2 1/2 years after staring a full restoration.

I have a question

the supplementary air valve and the warm up regulator are fed via the same red / white wire off the fuel pump relay but is the warm up reg a sort of choke and what does the supplementary air valve actually do.

perhaps I should know and I do have someone I can ask but thought I’d put the question on the forum

regards

keith

 

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WUR controls the mixture by varying the fuel pressure to the fuel distributor.  Yes, it's a sort of choke but it also maintains the control after warm up (despite its name).

AAV lets in extra air during warm up.  rotating air cut off door inside that is temperature controlled.

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Carrera 2.7 MFI? We need pictures, its one of my dream cars I need one in my life

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Does a MFI have a WUR and AAV, I'm thinking they are CIS. The MFI stopped in 75 with the carrera 2.7? I'm not sure, in any case, you can check how they should be wired using a electrical wiring diagram in the factory workshop manual. The purpose of the wire to the WUR is to richen up the WUR for cold running, it then drops to a higher pressure (which is leaner) as the  WUR heater has done its bit. 

If you have any specific CIS problems, then we should talk about those, the WUR/AAV are just one component of the whole system. For sure setting the fuel pressures is a key part, but not the only set up bit. The factory manual is a really good read for CIS set up and checking over. I would not work on the CIS without reading it first   

Ive not read through this thread since,  but 5 or 6 years ago I downloaded the factory manual here: 

 

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there are 76 2.7 carreras  ( MFI 210 bhp) I think, Ive seen em. 74-76.

Edited by axel
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Sorry I’m having a brain storm

the car is a 76 2.7s CiS not mfi

my reason is I’m connecting in the WUR and air valve which both are fed from the fuel pump relay.

Basically once the ignition is on the fuel pump runs, the energised fuel pump relay sends a feed to the WUR and Air valve, 

the WUR and air valve are these energised continuously and self adjusting . 

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Note

that I do not have a 14 pin connector block, this was discarded as badly broken and I have a good quality connector block neatly installed behind the rear fuse cover

the loom is not original Porsche as this was in poor condition prior to the restoration and I have made the loom myself.

everything works well and correct and looks good, maybe better than an original would look.

its now just a few final wiring points to sought out

I do have copies of the original Porsche circuit diagrams 

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On 26/01/2024 at 10:20, Ivelosttheplot said:

 Basically once the ignition is on the fuel pump runs. 

On the earlier cars yes, but on the later cars (certainly on the SC), the fuel pump only runs when the engine is cranking.

 

 

Just now, Jonny Hart said:

On the earlier cars yes, but on the later cars (certainly on the SC), the fuel pump only runs when the engine is cranking.

 

 

Essnentially, the air flow flap switch prevents the fuel pump running when no air flows through but when the key is in 'crank' position the fuel pump always runs.

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On 26/01/2024 at 10:20, Ivelosttheplot said:

Sorry I’m having a brain storm

the car is a 76 2.7s CiS not mfi

my reason is I’m connecting in the WUR and air valve which both are fed from the fuel pump relay.

Basically once the ignition is on the fuel pump runs, the energised fuel pump relay sends a feed to the WUR and Air valve, 

the WUR and air valve are these energised continuously and self adjusting . 

Have you looked at the wiring diagram in my earlier post? Just follow that. 

Johnny, is on it! Essentialy the WUR and the other device take power from terminal 30 on the fuel pump relay. 30 is also the power to the fuel pump. The fuel pump relay changes between 87 and 87a depending on whether 85 is grounded. The ground for 85 is controlled by the air flow plate, there is a sensor at the back. From memory this wire does not go through the 14 point connector, is is a seperate thin browish wire near there....but it has been a while, and i only have a 79 ROW SC to look at.

The wiring diagrams only show 76 US models, but for the CIS that will be close enough. If you havent already done it, go up a few posts and grab the manual...there is a link there. At least to the post where they talk about the download. The wiring page quality isnt great but the rest of the manual is indispesiable. Im sure with a little google you can find a better 76 wiring diagram. Im sure pelican parts used to have them, just do a search,.

 

 

Screenshot 2024-01-27 184446.jpg

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Sorry only just read you do have the factory wiring diagram, in which case these answer your question of : " of the WUR and air valve are these energised continuously and self adjusting "

The answer is yes these constantly have power from terminal 30. The WUR heater heats up an then essentially "stays hot" (leans out), it only starts to cool down and reset once it stops receiving power. Same for the air valve. The circuit diagram confirms it.

A quick test of your fuel pump relay circuit....with ignition on push up on the air flow plate by hand, you should hear the fuel pump run. If you pull an injector before hand you can look at it spray (into a bottle), When setting up your CIS with some of the procedures. You basically "hot wire" the fuel pump relay, by removing it, and directly connecting 87a to 30 (with a "bridge"). You can also do this, if you brake down one day and you know for sure it is the fuel pump relay. I once worked on a 76 911, where this had been set up like this permentantly, and I had to unbodge it to set it up back to the factory method.

Luke

Edited by Strictly
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Thanks for all this info

yes I have original circuit diagrams and can confirm the WUR and air valve are fed from terminal 30 but if the fuel pump only runs when cranking then surely the pump would stop running when not cranking. So I assumed the fuel relay was fed from the run terminal on the ignition switch which stays on until you switch off

so turn on the ignition, pump runs.

turn off ignition pump stops

keith

 

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You are right, you were not given a complete answer earlier

 i didnt help eirher by mis intrepreing what you needed to know.

To help close it, the fuel pump runs under two scenarios.

Power to 87 which is when cranking. Terminal 50 on ignition switch

Power to 87a which is ignition on, terminal 15 on the ignition switch, but slightly indirectly.

-----

Just follow the wiring diagram back to see the interaction. While not explained on the wiring diagram, it is layed out and once you understand the German din terminal numbering it all makes sense. The attached Will also help.

 

Screenshot_20240128_091311_OneDrive.jpg

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My picture is a little blurry. Go to this link. Scroll right to the bottom. There is a download link to download the original pdf article which explains the wiring diagram layout and the terminal numbers. So you help understand what the numbers tell you.

https://www.motor.com/magazine-summary/understanding-european-din-wiring-april-2003/

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1 big error in what I said, the air flow sensor breaks ground to control relay. I had it in my head that it made ground to earth 86 to 85 and switch relay but it Is the other way around.

Then when the air flow plate lifts it de energises the relay, so that 87a goes 30.

From the Bentley. Although this is for an sc...the main premise of the fuel pump relay using the air flow plate to control 86 to 85 Is the same.

The Bentley manual should be I'm the download post discussion link from earlier.

 

 

 

 

17064371391031694004549746272155.jpg

17064371598541209751212432306102.jpg

17064371894847803891377392361311.jpg

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Your going to love this. My car is stripped to a bare shell so I can't check. The Bentley and the factory porsche manual contradict each other. Look carefully at the pictures. The Bentley says fuse 16 goes to terminal 87, but the factory manual says fuse 16 goes to 87a.

I reckon this Bentley diagram is wrong as it says elsewhere that 87a and 86 should have power with the ignition on.

 

17064381451277162025295227537235.jpg

Screenshot_20240128_103057_OneDrive.jpg

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2 hours ago, Ivelosttheplot said:

Thanks for all this info

yes I have original circuit diagrams and can confirm the WUR and air valve are fed from terminal 30 but if the fuel pump only runs when cranking then surely the pump would stop running when not cranking. So I assumed the fuel relay was fed from the run terminal on the ignition switch which stays on until you switch off

so turn on the ignition, pump runs.

turn off ignition pump stops

keith

 

Keith, 

Again I've taken you on a tangent. To answer your direct question a 76 should not have the fuel pump running with the ignition on. Only when the air flow plate is moving. See above posts for more info on that.

See this link for discussion on the fuel pump not running with engine off.

If I had a car I could make it work, not having one and trying to do it from memory doesn't help. The glaring mistake in the Bentley above...does not help cement understanding.

http://forums.pelicanparts.com/porsche-911-technical-forum/817274-fuel-pump-relay-1976-pump-runs-when-key-position.html

Screenshot_20240128_103920_Chrome.jpg

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Ps. I've drawn the diagram differently, essentially to make the fuel pump relay work you must use the air flow plate.

When plate at rest 86 is grounded to 85...but contrary to the diagram as far as I understand it 87 goes to 30 waiting for voltage at 87 from 50 on the ignition switch...to lift the plate which then breaks the connection between 85 and 86 and changes the relay to be 87a to 30.

This contradicts the relays in the wiring diagram bit it's logical and the diagram has a diode which changes how the relay appears. I reckon anyway.

The 85 grounds to d- This may have something to do with it, and you should replicate the full harness wring as per the factory diagrams.

Or just forget about the fuel pump relay and wire like a 73.5 to 75 cis car.

 

17064407806526057320242515831871.jpg

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Wow so much info I’m again confused .

the trouble is I have completely remade a loom to what I believe correct, everything works as it should

I have followed the original circuit diagrams but not 100%
im not sure of the sequence of the starting circuit but have gained some info from your posts

fortunately for me I know an  auto electrician who has worked on air cooled Porsches for years and as soon as I can get him over to run over these final points the better but him being retired he don’t go out when it’s cold so I got to fit in with him, which is how this restoration has been throughout 

I know I’m in the final throws now though just about everything is done

ill keep you informed 

thanks again

Keith 

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It's quite simple I could talk you through it one step at a time. The only thing maybe Is the earth to D- and if that has any influence on the earthing of 85 

If it all works, then just be sure that 

1. The cold start circuit works.

2. You have fuel pump on both when cranking and ignition on.

3. The same parts are fused as would be originally 

The early cars did not have a fuel pump relay, so you dont have to have it but it Is a safety feature and better to have it.

I've seen used engine looms on ebay for £50 to £100, not to late to get one.

Or just don't worry about it. If it all works then as long as the wires are good quality and all go to the right place it will all be fine. You will find out for sure if you have starting problems.

Happy to help, just give us a shout for specific points.

Luke

 

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Thanks luke

I was once an electrical engineer which helps although it’s different to auto electrics.

ill check out what you have said thanks

the loom I had was tatty to say the least and as I’ve made one for my gt40 I thought I’d give it a go 

Porsches are a bit different from most older cars, as I’m finding out but I like a challenge .

 

 

all wire, relays etc are all new 

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I dont blame you for rebuilding the loom. I will do exactly the same when I need to. Once you get your head around it the porsche injection is quite easy. Same tech as a Mk1 vw golf. Setting up your full fuel system will be the next steps, but get it running first. If you hit any snags post up and we will see what you can figure out. Ive had a 911 from every year in the 1970's, so ive gotten to know the fuel systems! Never owned MFI, only carbs or CIS

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  • johndglynn changed the title to Question re supplementary air valve function

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