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H2odiesel's Avatar
 
Join Date: Jun 2013
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Mechanical advance on a motronic 3.2

I started up my 1985 3.2 today after a rebuild down to the crank. Putting it back together I was surprised to see a centrifugal advance in the distributor. I thought all the timing was taken care of in the motronic. I hooked up my timing light at start up and at 2000 rpm I had 32 degrees of advance. Seems a bit much especially with the turbo I am running. I am running an MSD that retards on boost. I'd appreciate any comments as well as some timing target points at different rpm and boost levels. My waste gate has a .5 bar spring.
Old 07-05-2014, 06:03 PM
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It's not advancing, rather aligning the rotor to the terminal of the cap better.
Old 07-05-2014, 06:35 PM
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Tip, I'm not sure if you are pulling my leg or I didn't explain it well. I've got a digital timing light that lets me measure timing advance. At Idle I measure about 5 degrees BTDC, at 2000rpm I measure 32 degrees BTDC. I can see the weights and the springs in the base of the distributor and I'm sure it is contributing to the total advance. I was surprised to see a mechanical advance mechanism when I was under the impression the Motronic handled the timing duties. It leaves me a bit concerned about how much advance I might have at 5000 rpm and .5bar boost. I can't get back there with the light and measure that.
Old 07-06-2014, 07:31 AM
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Not pulling your leg. Without the advancing mechanism, your rotor would not be in optimum alignment when the Motronic determined the timing event.
Old 07-06-2014, 07:33 AM
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Quote:
Originally Posted by H2odiesel View Post
Tip, I'm not sure if you are pulling my leg or I didn't explain it well. I've got a digital timing light that lets me measure timing advance. At Idle I measure about 5 degrees BTDC, at 2000rpm I measure 32 degrees BTDC. I can see the weights and the springs in the base of the distributor and I'm sure it is contributing to the total advance. I was surprised to see a mechanical advance mechanism when I was under the impression the Motronic handled the timing duties. It leaves me a bit concerned about how much advance I might have at 5000 rpm and .5bar boost. I can't get back there with the light and measure that.
The distributor has nothing to do with triggering the Motronic ECU, that's done by the flywheel.
Maybe your car has a different distributor in it than it came with. Either way it has no effect on your year of Motronic fuel injection and ignition timing, it is only directing the high energy voltage pulses from the ignition coil to different spark plugs while rotating.
Old 07-06-2014, 08:15 AM
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Duh, OK got it. It has been three marathon days in a row to complete this rebuild, I'm afraid the old noggin is moving a bit slow. Also I have all the butter flies from initial start up, every noise or smell is an indication of immanent catastrophic failure. Just got back from the first short test drive. It seemed to run very well, but when I got back and popped the lid to look over the engine at idle I swear I can hear something, like maybe the alternator making a scraping noise. Thanks guys for setting me straight.
Old 07-06-2014, 08:37 AM
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I think I am going to be sick. All through this rebuild I have kept the ports and openings taped up to guard against a part dropping inside the motor. Yesterday I spent hours chasing a no start situation that ended up being the ignition timing being 180 out because that is how the cap was wired when I bought the car. I pulled all my turbo piping and when I finally got it running around 5pm I was too beat to put it back together. I came in this AM and hooked everything back up and went for a little ride. When I came back I heard a scraping noise that I attributed to the alternator that was giving me fits. Everything comes apart again, alternator out bench test and it looks good. Back together she goes and on start up my alternator light is out and everything is charging. Time to put the piping back on and just to be safe I get out the flash light and look down into the silicone intake elbow and sitting in there is the outlet side clamp I couldn't find this AM. That wasn't the Alt making the scraping sound, and yes there are a some chips in the blades. FML! to top it off my daily driver pulls a no start and throws a crank position sensor code.
Old 07-06-2014, 10:31 AM
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All of the '80's Motronic distributors have, shockingly enough, a mechanical advance mechanism. The actual timing is provided to the ECU via the sensors and tooth arrangement on the flywheel. These are used to both measure the actual timing, and set the actual timing. The mechanical advance does it's thing, and the Motronic does it's thing to come up with the desired ignition timing.

In the event that the electronics go tilt, the engine is assured of getting the mechanical advance to keep from melting the catalyst.
Old 07-06-2014, 05:41 PM
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I really don't think the advance mechanism has anything to do with actual timing, it's simply to position the rotor to a better alignment.
Old 07-06-2014, 07:11 PM
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Quote:
Originally Posted by Speedy Squirrel View Post
All of the '80's Motronic distributors have, shockingly enough, a mechanical advance mechanism. The actual timing is provided to the ECU via the sensors and tooth arrangement on the flywheel. These are used to both measure the actual timing, and set the actual timing. The mechanical advance does it's thing, and the Motronic does it's thing to come up with the desired ignition timing.

In the event that the electronics go tilt, the engine is assured of getting the mechanical advance to keep from melting the catalyst.


You are 100% wrong about the advance weights purpose and their capabilities.

Tippy is correct, the sole purpose of the weights is to better align the rotor to the terminal when the motronic fires the plug.

With exactly zero low tension wires going to the distributor with zero electronics inside, how do you think it affects the spark timing seen by the engine in any way?
Old 07-07-2014, 07:18 AM
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