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That pin 1 should be the white wire that plugs into your overboost switch in the back. Pictures would be good, zoom in on that circuit board. I found the control unit out of the '91. I'll open it up and compare. It has never run except under stock conditions. I was scared to try the ignition trigger thinking I could fry one of the electromotive coil packs. Is yours twinplugged?
Old 02-03-2008, 07:48 PM
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I am away from home till thursday, but I'll get some pictures then.

Yes, my car is twin plugged.

Bill
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Old 02-04-2008, 07:22 AM
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OK, pretty much have it nailed down to the Multifunction Control Unit failing. I never plugged my freinds part it, its pretty obvious by the sound that mine is bad. I am going to go ahead and order a new one.

Thanks for the help. What I need to find out is whether or not it failed on its own, or something caused it to fail.

Bill
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2003 996TT
1992 964 Turbo (sold) Holy crap are these expensive now!
1986 911 Cabriolet (sold)

I don't know why I do the hillbilly things I do.
Old 02-13-2008, 12:37 PM
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Wait!!

Don't buy one. I have one I'll gladly sell you if you want it. However, I hate to see you spend money if it doesn't fix your problem. I don't want it back if you plug it into your setup though, for obvious reasons. I'll help you troubleshoot yours a little more if you want or we can work something out on my control unit. Let me know what you think.
Old 02-13-2008, 07:19 PM
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DAMNIT, crap, although I did get it for substantially cheaper than what I had originally thought. I'd still like to spend some time troubleshooting with you though.

Pretty much, if it isn't clicking, I get good boost and power. If it is clicking, I get random fuel pump shut off, basically overboost kicking in, at all kinds of different power levels. It is clicking pretty much all of the time now.

What should we check?
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1992 964 Turbo (sold) Holy crap are these expensive now!
1986 911 Cabriolet (sold)

I don't know why I do the hillbilly things I do.
Old 02-13-2008, 07:40 PM
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I have a feeling that control unit may do the same thing. If not right away, perhaps later. Hopefully not. I just don't think the ignition signals are compatible. I also realized that another function of that unit must be the original Rev Limiter. This may explain why the fuel pumps shut down sometimes, if whatever IC in that unit is going haywire it could be shutting down thinking it's at redline. I need to spend more time on the wiring diagram and see what all that unit does.

One thing I'd like to know is if the fuel pumps still run all the time with the key on if you unplug that control unit. Also, if the tach still works with the unit unplugged, although that could be difficult to test if the fuel pumps don't run.

Try that and let me know so I can ponder some more. Good luck
Old 02-15-2008, 05:51 PM
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OK, the fuel pumps still run all the time, with it plugged in, and with it unplugged.

I can't tell whether the tach works or not, as it won't start with the unit unplugged.

I don't think its so much the rev limiter, as once you get off the boost it keeps running and revving freely. I honestly think it has something to do with the boost, but I may be wrong.

I'm definitely open to any help from you, as on the elctronic side, I am not the brightest bulb in the drawer.

Bill
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1992 964 Turbo (sold) Holy crap are these expensive now!
1986 911 Cabriolet (sold)

I don't know why I do the hillbilly things I do.
Old 02-16-2008, 07:36 PM
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Any more advice?
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2003 996TT
1992 964 Turbo (sold) Holy crap are these expensive now!
1986 911 Cabriolet (sold)

I don't know why I do the hillbilly things I do.
Old 02-22-2008, 11:26 AM
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Hey Bill, Sorry buddy, I missed your last 2 posts. I'm rolling up my sleeves and working on a response. Hopefully it won't be too comfrusing.

Don't sweat it too bad yet. I have a feeling this problem is man made and hopefully will be quite conspicuous with a little pokin' around.

Too be Continued...

Last edited by rsrmike; 02-23-2008 at 09:01 AM..
Old 02-23-2008, 08:54 AM
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Hey no problem at all. Trust me, I appreciate any advice I can get.

Thanks.
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2003 996TT
1992 964 Turbo (sold) Holy crap are these expensive now!
1986 911 Cabriolet (sold)

I don't know why I do the hillbilly things I do.
Old 02-23-2008, 11:52 AM
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OK, sorry buddy, I haven't forgot about you. I've managed to confuse myself after reading back through what we've said. Given that you said the fuel pumps run all the time with the key, the overboost switch still kills the fuel pumps, and the fuel pumps run even with the control unit in question unplugged could only mean one thing to me. I would bet someone bypassed your fuel pump relay circuit out of the loop of this control unit. Which doesn't make sense that it acts up when the control unit starts clicking.

Are you positive your losing the fuel pumps when the car acts up? When I removed this control unit from the 91 that started this crazy thread, I just connected the white wire from the over boost switch to the brown and yellow wire that grounded the trigger side of the fuel pump relay at the connector for this control unit. I had to do some other hacks as well to cover the other functions of the control unit. As I said before, I wasn't proud of it, I was just told to "make it work" and "don't F-up the coilpacks" so I did what I could with what I knew.

I would suggest some testing first to determine for sure what is shutting down. And for sure if the fuel pumps do in fact work like we think they do on your car. I guess you're aware there is a valve that buzzes in the engine that sounds kinda like a fuel pump to some, just in case. I would try to hook a test light of some sort either directly to one of the fuel pumps, or to the output side of the fuel pump relay to see if they are indeed losing power when the car runs bad. You could also use a fuel pressure gauge, but that's fairly involved in your case.

Given the odd chance someone wired yours like I did, or if the fuel pumps are still run through the control unit, another possibility would be if the wires for the overboost switch and the gauge sending unit were crossed. The gauge sending unit can sometimes provide enough of a ground off boost to ground some relays and then lose continuity as the boost builds. Is the white wire connected to your overboost switch? I did this on an '87 w/ a Kokeln one time in the early years... I don't think the gauge goes through this control unit though so who knows.

I hope I'm not just confusing you more. I would learn more about what's been rewired and how and start there if I were you. Good luck, let me know what you think.
Old 02-29-2008, 08:45 PM
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No worries on taking long Mike, you know way more than me. It is in the high 60's here today, so I buckled the kids in (wife is out of town) and took the car out for a pretty lengthy drive.

In each gear, at about 4900 rpm, it starts to faul on its face. Meaning the tach bounces around for a second, and you can feel definite hesitation, and the motor cuts out. I brought the rev's up very slowly a couple of times, without using any boost, and the result was the same in 1st - 3rd. I couldn't really check out 4th or 5th, as I would be reaching ludicrous speed, smile.

Now, when I get on the gas, and the boost builds super fast, it doesn't make it to 4,900 rpm.

I will go check the wires to the overboost and to the boost pressure sending unit. I can tell you that when I got the car, the boost guage read .7 bar all the time. After we had the motor out, reconnected the overboost switch, and did a couple fo various other things, the boost gauge worked just fine.

On a somewhat related note, is it possible for the crank fire sendor to go bad? Maybe it would work at the lower rpm's and then somehow crap out at the higher end??? I am just thinking outside of the box.

Let me go look and see what wire is connected to the overboost switch.

Bill
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2003 996TT
1992 964 Turbo (sold) Holy crap are these expensive now!
1986 911 Cabriolet (sold)

I don't know why I do the hillbilly things I do.
Old 03-01-2008, 01:04 PM
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OK, the white wire is connected to the overboost switch, and the green and brown wire is connected to one side of the boost sending unit. The other side of boost sending unit is empty. I am going to look at one of the wiring digrams I have to make sure that is correct.
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1992 964 Turbo (sold) Holy crap are these expensive now!
1986 911 Cabriolet (sold)

I don't know why I do the hillbilly things I do.
Old 03-01-2008, 01:17 PM
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That is correct, oh well, would have been an easy fix. You need to find out if/why the fuel pumps run w/ key on, and what was done to achieve this. Are you comfortable with a test light and a volt/ohm meter?

A quick but poor fix for no fuel pump relay trigger would have been to remove the relay and place a jumper from a switched power source to the output terminal of the relay.

Take a look at relay #61 in the fuse box...
Old 03-01-2008, 01:58 PM
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The relay is slot 61 is in there. I can see where the relays are in the engine compartment as well, and they look like they are all there as well. There is a connector in there that has a Merecedes Benz logo on it though.

I am comfortable with test lights, etc.

I also started the car, and after it warmed up, with a friend with me, revved the engine up over 5,000 rpm, and didn't experience any problems. It ran silky smooth. The MCU wasn't clicking except when I let off the gas.
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2003 996TT
1992 964 Turbo (sold) Holy crap are these expensive now!
1986 911 Cabriolet (sold)

I don't know why I do the hillbilly things I do.
Old 03-01-2008, 03:50 PM
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Quote:
Originally Posted by BSiple View Post
No worries on taking long Mike, you know way more than me. It is in the high 60's here today, so I buckled the kids in (wife is out of town) and took the car out for a pretty lengthy drive.

In each gear, at about 4900 rpm, it starts to faul on its face. Meaning the tach bounces around for a second, and you can feel definite hesitation, and the motor cuts out. I brought the rev's up very slowly a couple of times, without using any boost, and the result was the same in 1st - 3rd. I couldn't really check out 4th or 5th, as I would be reaching ludicrous speed, smile.

Now, when I get on the gas, and the boost builds super fast, it doesn't make it to 4,900 rpm.

I will go check the wires to the overboost and to the boost pressure sending unit. I can tell you that when I got the car, the boost guage read .7 bar all the time. After we had the motor out, reconnected the overboost switch, and did a couple fo various other things, the boost gauge worked just fine.

On a somewhat related note, is it possible for the crank fire sendor to go bad? Maybe it would work at the lower rpm's and then somehow crap out at the higher end??? I am just thinking outside of the box.

Let me go look and see what wire is connected to the overboost switch.

Bill
crap, dude, I totally missed this whole post somehow?? my bad...

The crank sensors go bad all the friggin time on turbos, especially mounted with a bracket on the dist. hold down stud. I put that system on 5 turbos over the years, only one was a newer turbo, and they all need sensors about every 2-3 yrs. But, when they quit, they quit for good in my experience. It's usually that they run fine, then one day they won't crank. This leads me to believe that the heatsoak on cool down is what kills them, which means you best cool your turbo down as much as possible before shutdown, not just for the turbos sake but the crank sensor as well.

The sensor really needs to be mounted on the flywheel, but that can prove to be a challenge.

When the tach bounces, that is usually always an ignition problem. I wonder if the airgap on your sensor is too large, I've never tested that scenario to see what happens but it seems like it just wouldn't crank. I can't see how it would affect the control unit. We've got to find out how intrusive the twinplug install was.

OK, here's what I would do, unplug the fuel pump relay from 61 and look at the pins on the relay. You should see 4 pins labeled 30, 87, 85 & 86 I believe.

30 is the one from the battery and should be hot all the time.

87 goes to the fuel pump and according to the wiring diagram splits into two wires somewhere awayfrom the relay

85 and 86 are what close the relay contacts when energized.

85 or 86 should get hot with the ignition switch and the other is typically grounded by the unit we keep talking about. You can't trust which one is which, you usually have to test each application.

Focus on 85 and 86 and see which one doesn't get power + w/ the key.
This should be #2, or 85 but like I said, sometimes they are reversed. it doesn't matter, it's just closing a relay coil.

I'll assume it's 85 from here on out. 85 should connect to the control unit under the seat at terminal 12, a brown and yellow wire.

See if that wire is grounded all the time at the relay plug with the control unit unplugged and plugged in both.

You can use a small low wattage test light but it's always best to use an LED test light so you don't pull too much current through the control unit and fry something. Hook to light to a positive source to check for ground

Most LED test lights hook to power and ground and give a green or red light for polarity.

If it loses ground when you unplug the control unit then the fuel pumps are still running through the control unit and this is most likely your problem

If it doesn't lose ground then it must be grounded somewhere else and if done poorly, could be breaking up. That wouldn't be RPM related though

If it is still grounded through the test light you could try removing the white wire and the brown and yellow wire and connecting themtogether away from the control unit. This would make the overboost switch the ground for the fuel pump relay and take thecontrol unit out of the loop.

You can pop the cover off the multiplug and remove wires one at a time IF you leave the plug connected to the control unit. This helps to keep all the wires from falling out of the plug and creating a mess. If you do this it's best to disconnect the battery for obvious reasons until you have all your wires secure.



The pumps would surely run all the time with the key on but you would still have overboost protection.

If you needed to work on the car with the key on, engine off, you would prolly want to disconnect the overboost switch while you sat there so the pumps won't kill the battery

In the long run, I think you'll probably wind up removing the control unit all together and replacing it with a couple relays. There's one function I don't understand according to the wiring diagram though, Somehow it ties into the other control units through a wire relating to the frequency valve. It's labeled 50% and I'm not sure what it does... Can anyone still reading this shed some light???
Old 03-02-2008, 01:14 PM
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OK Mike, I have to travel to San Diego on tuesday, so I will try this tomorrow night.

I really appreciate it.

Bill
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1992 964 Turbo (sold) Holy crap are these expensive now!
1986 911 Cabriolet (sold)

I don't know why I do the hillbilly things I do.
Old 03-02-2008, 02:48 PM
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Good deal, maybe you should give me a shout on the phone when you do if you want some real-time tech support. I have a wife and a six-month old to consider so timing could be an issue but it sure could help you make some headway

I wish some of the experts would chime in, surely someone out there has run aftermarket ignition in a C-2 w/CIS... Maybe not, I agree it's not the smartest move. The stock single plug setup is pretty nice really. I would not have twinplugged the 91 that started this thread but I was not the boss at the time...

I'll PM my cell if you want...
Old 03-02-2008, 03:40 PM
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Thanks Mike, I have 2 kids as well, and have to watch them tonight, so it might not be until later that i get to work on the car. We'll see how it goes. I might take you up on the help for later on in the week.

I agree about the ignition. The twin plugged issues has caused a couple of headaches for me.

Bill
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2003 996TT
1992 964 Turbo (sold) Holy crap are these expensive now!
1986 911 Cabriolet (sold)

I don't know why I do the hillbilly things I do.
Old 03-03-2008, 08:48 AM
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Mike I haven't forgotten about this thread. I actually pulled all the fuses in the engine compartment, and checked the wiring. Pulled off the overboost switch wire, celaned everything up and put it back together.

I still have the clicking sound coming from the MCU ( I haven't installed the new one yet), but the car has been running full boost all teh way up to redline, and hasn't been cutting out since I messed with additional fuses, and relays in the engine compartment.

Bill
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2003 996TT
1992 964 Turbo (sold) Holy crap are these expensive now!
1986 911 Cabriolet (sold)

I don't know why I do the hillbilly things I do.
Old 03-19-2008, 01:36 PM
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