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rsrmike rsrmike is offline
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Join Date: Jan 2008
Location: Birmingham, AL
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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.

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, 12:14 PM
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