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Knee Pads are Overrated!
 
Rawknees'Turbo's Avatar
 
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Alternative to Yellow, Overboost Relay?

The overboost relay in my car only works 100% some of the time (sometimes it does not prevent the fuel pumps from running with just the key on, and it never allows for the quick, half-a-second run/prime of the pumps at the initial turn of the key), so I would like to replace it and relegate it to my spares kit (a huge kit, that these cars unfortunately require ).

A little over a year ago, I bought a Witren branded one (OEM supplier and supposedly made in Germany) - it only worked for about 300 miles, and Pelican took it back for store credit. I've tried to order another one, or another brand, since then, and Pelican does not carry them (even though they are still listed on the online catalog) . . . the only one that "might" be available is the Porsche branded one (probably a Witren that is re-badged), but it is $245.00 for that thing (totally nutz!).

Soooooo, has anyone figured out an alternative, perhaps using a modern relay, that functions in a similar manner?

PS - I've already taken the cover off my current relay and checked it the best I can for problems - am no electrical expert, though not a complete numbnutz.
Old 09-27-2017, 11:05 AM
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I guess I got lucky. I bought a Wittren branded relay from Pelican a little over a year ago and it has worked up to now. I also repaired my original and kept it as a spare. My observation about these electrical replacement parts seems to be that they either fail almost immediately, or they last forever. There doesn't seem to be much of a middle ground.

My first thought is intermittent problems like you are having are more of a connectivity issue rather than being outright busted, so maybe go through that again to make sure you are getting good contact on all connections?
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Old 09-27-2017, 12:02 PM
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Knee Pads are Overrated!
 
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Good point (different than nice tip ), Jeff - will check the existing one out again.
Old 09-27-2017, 01:20 PM
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RT- good thread. I like the idea of solid state "replicas" for those of us who drive our cars and don't worry too much about the authenticity of these little boxes. Solid state would really hold its own on roads like the Cross Bronx Expressway which loosens fillings not to mention what is does to solder joints on 30 year old relays. Starting to smell a group buy if a vendor is out there. I'm pretty sure that's what I'm smelling…
Old 09-27-2017, 02:06 PM
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Knee Pads are Overrated!
 
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Will, since Jeff has been in this thread, there's no telling what yer smelling!

But yep, I either need to take steps to upgrade relays (don't care one bit about the originality/authenticity of it, like you mentioned) or figure out how to wire in a bypass switch that would enable me to supply power to the fuel pumps even when one of the many relays goes bad - so that I could re-power the pumps on the fly (I think Jim Fairman did this but never saw details of how he went about it). It really sux azz to have the engine shut down while zipping along one of the nose-to-tail, crowded expressways around here.
Old 09-27-2017, 02:17 PM
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Nose-to-tail....Heck nevermind

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Old 09-27-2017, 02:48 PM
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Knee Pads are Overrated!
 
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Nose-to-tail....Heck nevermind

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That has a special meaning in ol' Cali!!!
Old 09-27-2017, 02:49 PM
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Its a knight club,where anything goes...

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Old 09-27-2017, 02:57 PM
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Hey there knees...

Mine is a euro '84, and I have no "yellow relay" from hell nor any form of Lamda....so why not just wire yours like a euro model?

Mark
Old 09-27-2017, 03:03 PM
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Who the hell is Raw Knees?? Must be some farkin' noob...
Old 09-27-2017, 03:04 PM
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Knee Pads are Overrated!
 
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Originally Posted by full quack View Post
Hey there knees...

Mine is a euro '84, and I have no "yellow relay" from hell nor any form of Lamda....so why not just wire yours like a euro model?

Mark
I need to look into that - sounds like a good idea, Mark.



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Who the hell is Raw Knees?? Must be some farkin' noob...
I'm yo' Daddy!!!



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Sounds kinky - nice!
Old 09-27-2017, 03:16 PM
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Hey Knees,
Why does the euro turbo not have the yellow relay.?
My cars been running fine. I pulled the yellow relay out.
Rub the contacts to make sure they're clean . and replaced the fuses.
The white one at the bottom was broken. now that the heat is gone.. running fine.
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Old 09-28-2017, 10:19 AM
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Just FYI for all you folks that were wondering WTF is with the yellow relay.

Copied from over on Rennlist by PorscheMD....


Okay I have the answer to "why Porsche put the magic Yellow relay in the fuel pump circuit instead of a regular red relay". For starters the red relay and the yellow relay pin connections are the same so a red relay will work. As you know, in the yellow relay there is a circuit board with diodes and transistors on it. I enlisted the help of a great friend who is an EE to tell me what the circuit board does. Between my understanding of the fuel pump circuit and his understanding of the circuit board I think I have the answer. As many of you know when you have the key in the on position and the engine off, if you push the sensor plate down or disconnect the plug at the plate the pumps will run. This switch on the plate is a very unstable "switch" in the world of electronics being it is mechanical and moves very sudden given its environment. So, there is potential for the plate to make contact or bounce against the contact and very momentarily cut the fuel pump off if the plate is swung violently up. (high revs, high engine load, quick shift type of scenario). Within the circuitry of the yellow relay there is a de-bounce function on pin 85 which is the ground potential coming from the plate switch to the tune of about 282milisec. What that means is if the plate for some reason is swung up and makes contact with the switch and completes the ground path to pin 85 of the yellow relay while the engine is running there is a delay of approx. 282milisec. (which is about two blinks of the eye) before it completes the circuit and cuts the fuel pump off. This potential for cutting the pump off in a turbo motor even for a short amount of time or if done repeatedly could theoretically be very damaging to the engine as we all know. So, the red relay will work but you run the risk of cutting the fuel pump off momentarily if you use one. Perhaps its a little overkill on Porsches end but very well thought out I think...



Mark
Old 09-28-2017, 11:45 AM
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Knee Pads are Overrated!
 
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WOW, Mark - that explains a lot - thanks for posting it!

So one possibility, to simplify things a bit and also avoid buying a ridiculously high priced relay, is to use a normal, round relay, and use an inertia switch instead of the air-metering arm's fuel pump stop switch (eliminating the "bounce" possibility of the air metering arm switch).

Something to consider.

Also, I did not know that a regular relay would work there in a pinch - great information for sure.
Old 09-28-2017, 12:01 PM
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I guess you really have to ask yourself...what are the chances that the engine will still be running after you pile the car?

I'm thinking 99.9% of the time the engine will stall at some point during the event, thus no airflow across the metering plate, thus the pumps shut off anyway.

If the car tears itself apart bad enough in the crash to catch on fire {remember this is not Hollywood where everything bursts into flame at the slightest provocation}....I'm thinking you have bigger problems than the possibility of the pumps running.

So it all comes down to your perceived personal safety threshold, where do you draw the line...where do you risk it?

Secondly, I've driven my car pretty hard over some questionable roads, including some rather abrupt partial lane deviation bounces and never have experienced the potential "metering plate bounce" scenario as outlined by Porsche. So I'm going to call that "over engineering" and just ignore it completely.

Mark
Old 09-28-2017, 01:27 PM
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Quote:
Originally Posted by full quack View Post
I guess you really have to ask yourself...what are the chances that the engine will still be running after you pile the car?

I'm thinking 99.9% of the time the engine will stall at some point during the event, thus no airflow across the metering plate, thus the pumps shut off anyway.

If the car tears itself apart bad enough in the crash to catch on fire {remember this is not Hollywood where everything bursts into flame at the slightest provocation}....I'm thinking you have bigger problems than the possibility of the pumps running.

So it all comes down to your perceived personal safety threshold, where do you draw the line...where do you risk it?

Secondly, I've driven my car pretty hard over some questionable roads, including some rather abrupt partial lane deviation bounces and never have experienced the potential "metering plate bounce" scenario as outlined by Porsche. So I'm going to call that "over engineering" and just ignore it completely.

Mark
And that's exactly the issue with disconnecting the airflow plate safety feature. With it disconnected and the ignition still on, if the engine stops the pumps are still running. So, if your fuel line is severed anywhere aft of the first pump you are pumping fuel out at a pretty good rate unless you substitute something else for that safety feature (like the fore mentioned inertial switch).

Debouncing a switch is usually something you run into when you are using mechanical switches with digital circuits, its pretty common to use a Schmidt trigger or some TTL logic with a resistor and a small cap to do that. Without it a digital circuit would see multiple on/off events. I'm not sure it makes sense in this case as the on/off time would be so small and there is nothing digital in the system. Additionally the fuel in the line has some inertia so the fuel doesn't stop flowing instantly and when it does stop it isn't a binary on/off event but fuel pressure/flow will taper. Even if fuel did instantly stop moving did how much would a few miliseconds affect mixture? I suspect not measurably, not that I've tested it.

However as I recall that relay also runs the pumps for a second or two when you first turn the switch from ON to RUN to bring up fuel pressure. The red relay won't do that so you may experience hard starting issues.

Last edited by flightlead404; 09-28-2017 at 04:39 PM..
Old 09-28-2017, 04:33 PM
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Quote:
Originally Posted by full quack View Post
I guess you really have to ask yourself...what are the chances that the engine will still be running after you pile the car?

I'm thinking 99.9% of the time the engine will stall at some point during the event, thus no airflow across the metering plate, thus the pumps shut off anyway.

If the car tears itself apart bad enough in the crash to catch on fire {remember this is not Hollywood where everything bursts into flame at the slightest provocation}....I'm thinking you have bigger problems than the possibility of the pumps running.

So it all comes down to your perceived personal safety threshold, where do you draw the line...where do you risk it?

Secondly, I've driven my car pretty hard over some questionable roads, including some rather abrupt partial lane deviation bounces and never have experienced the potential "metering plate bounce" scenario as outlined by Porsche. So I'm going to call that "over engineering" and just ignore it completely.

Mark
Well said, Mark. And the the earlier post you shared from PorscheMD....I had never read that one before. Had no idea about the prevention circuits for those potentially 282 milliseconds of pump-offage.
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Old 09-28-2017, 05:14 PM
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I still have the airflow plate sensor connected and only have a red relay in the rear fuse/relay panel and nothing else {see picture}, plus no frequency valve, no lamda stuff, no O2 sensor.

I have owned the car for 11 years, put 63,000 miles on it. Never once had a starting issue caused by this topic. If the engine stalls or stops for any reason, the pumps stop immediately!

The car still retains the factory several second run on delay when you turn of the engine...all the normal stuff, just none of the headaches.

Sure if the engine is running and a fuel line gets severed, it's going to keep pumping until low fuel pressure shuts down the engine.....but the yellow relay won't save you there anyway. The yellow relay has nothing to do with crash protection. The only crash protection we have is our feet on the pedals and our heads not up our ass.

Mark

Last edited by full quack; 10-02-2017 at 05:55 AM..
Old 09-28-2017, 05:23 PM
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I have never run a yellow relay. Just the red one as above. No issues.
I have also hit a concrete wall on the track - hard. Hard enough it needed a complete new front (north of 70mph prob). And the engine was still running.
Regards
Alan
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Old 09-28-2017, 07:40 PM
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WOW.

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Old 09-28-2017, 08:22 PM
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