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rsrmike rsrmike is offline
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Join Date: Jan 2008
Location: Birmingham, AL
Posts: 538
Bill, It's smart to think that you could harm your buddies control unit. You are safer to plug yours in his instead. Maybe not at all after reading on. First off, Which control unit are you talking about? There are 3 under the driver's seat. The big one says Jetronic and the wiring diagram refers to it as oxygen sensor control unit. There are two smaller boxes behind that one, one is the acceleration enrichment control unit and it mostly compliments the jetronic control unit by manipulating the frequency valve to change fuel pressures and thus change fuel mixture. The other is referred to as turbocharger control unit in the factory wiring diagrams. I think you mean this one? This is the one that gets a trigger from the ignition and consequently turns on the fuel pump relay. This function replaces the switch on the airflow sensor plate housing on the older cars. Instead of turning on the fuel pump when the sensor plate moves, it waits for a signal from the ignition, hence no spark, no fuel. I would assume this was more to satisfy EPA. ANYWAYS, I would be really careful about plugging your friend control unit in YOUR car. I don't see how it's working to be honest with you. Your car came with more of a Mercedes type ignition than the typical Motronic setup. Porsche was actually struggling with finances in this era(hard to believe now) Which is part of the reasons the cars still ran CIS, They evidently needed a new ignition system though and the Mercedes route was the most practical. The only problem for guys like us is that the TACHOMETER signals from the ignition are quite different. I checked both of them with a lab scope with the help of an electrical engineer customer and friend. The electromotive signal works fine with most porsche tachometers, although we determined that the electrotive signal would not work well with this control unit. Nor should it in yours. Who did the install? I would love to hear more technical details. Does your tach work well? The electromotive tach output will work the tach just fine, just not filtered through the "turbo" control unit. Did someone run a straight wire to the tach from the ignition? That's what I had to do. I also had to do a few "wiring" hacks to make the other functions of this control unit work. This is what I would suggest if we are talking about the same control unit. The biggest side effect is that the fuel pumps will run anytime the ignition is on which is not terrible. Many older 911/930's have theirs unplugged to run all the time anyway. I would be happy to share the info on bypassing this control unit if you use it at your own risk!!! I am still using the overboost for protection. If you connect the white wire going to pin 1? directly to the brown/yellow wire going to pin12? The fuel pump relay should run w/key on as long as overboost switch is grounded. You could try just that one bypass and see if it fixes your overboost problem. I seemed to have installed a relay in it's place for something else but I can't remember what it was for. I could go find out if your considering this and need more advice but it's cold in there so just let me know if you actually give a ***** first I just think the tach signal from your electromotive could be freaking out the control unit in question and possibly shutting your fuel pumps down... Could be way off here, FWIW I spent about 3 days staring at the factory manuals making this all work. We took off tec2 and put on a complete CIS system sourced from Andial and then put Electromotive HPX twin-plug 'cause my boss at the time was obssesed with twin-plugging tricked-up turbos. The car split a cylinder w/ the tec2 and none of us felt qualified to tune it after we built the motor (we didn't install tec2). In hindsight, the single plug setup was probably better but it wasn't my call. Good luck! hope this helps.
Old 02-01-2008, 07:16 PM
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