'95 motronic - when does it run in open loop?
 

Does the '95 motronic go into open loop under WOT? OR above what % into the throttle? I know the '86 goes in when the WOT switch engages, but that doesn't have a throttle position sensor, just on / offs at WOT and idle.

Reason I ask, I have a much freer flowing exhaust system than factory and am concerned that the engine would run lean under WOT. I believe the maps are pre-programmed for open loop modes.

I had intended to have the engine dyno - tuned to optimize, but not until break-in is complete.

I guess with knock sensors, maybe I shouldn't be concerned?

As always, thank you,

Doug

I believe, but need to research more, that closed loop is only used during warm, part throttle operation. There is probably a time component as well.

Your in open loop only during the warm up phase. once the engine is warm your in closed loop your o2 senor will keep you from going lean/rich.

The WOT puts it into open loop. That's what the switch is for. Open loop has a different map for looking up timing and injection pulse width.

The Motronic on the 993 goes into open loop when the throttle potentiometer reaches an opening angle of 66 degrees, activating the full load maps. The throttle potentiometer on the throttle body is used to determine the angular position of the throttle shaft. The Motronic unit being self adaptive takes the smallest opening angle of the throttle as the idle position, and as soon as the opening angle of 1 degree is reached, it goes into part throttle mode.

Steve:

Is this.. in principle....the same as the Motronic functioning for 3.2's ??

- Wil

Functional operation on a 3.2 would be the same, except that the Motronics on the 3.2s and 964s do not use a potentiometer so is thus not able to determine angular position, but use a separate microswitch for both idle and full throttle to signal those postions to the DME.

Guys,
Thanks for your attention to this. Logically, what Steve described makes sense.

Here's the next question:
If you are breaking in a new engine (stock DME and mapping) and you want to load up the rings (my rings seem to be putting up a bit of a fight in their seating process), would you be concerned about running open loop (IE close to full throttle) with a sport style muffler (much less back pressure + no cats). I don't want to run it lean, but do want to run it hard. We're not talking hundreds of miles, but I know bad things can probably happen quickly.

If I had an adjustable fuel pressure regulator, I'd be inclined to crank that up for a little while until I have it broken in enough to get on a dyno and do the AFR work.

Doug

Also, is there a good way to determine that the engine is running in closed loop mode? I used to take a spike probe into the O2 signal wire and look with an o-scope at the Voltage vs. time. I don't have an O-scope.
Can I check timing at a certain rpm to ascertain if it is in a limp home mode vs. closed loop?

I have no check engine light or OBDI interface in the harness.

Doug - you'd probably be ok on your air fuel ratios. The 993s are programmed very conservatively, and run so rich in the upper rpm zones (11:1), they actually need fuel to be cut back to raise hp.

Steve,
Great info, as always!!

Thanks for sharing!

Is there a good way to ensure that I'm running in open loop mode (since I have no check engine light)?

Doug

A "hammer" (BOSCH KTS310) showing the actual values is really the only way to tell whether the DME is seeing the open loop mode. The check engine light does not come on when you meet the WOT criteria (>66% opening of the throttle flap) or the microswitch on earlier 3.6) so its of no use here.

Cheers,
Ingo

Ingo,
Thanks for the consideration.

Wouldn't there be a timing map that I could compare static timing at a given RPM and see if it corresponds to that of limp-home-mode?

It is my impression limp home mode would scale back timing?

Thank you,

Doug

Doug, yes, but again the timing can be displayed in the "actual values" submenu of the hammer. It would be difficult to use a timing light unless you have a wheel-dyno, where you can run in WOT and check timing at the same time.

What are you trying to find out?

Ingo,
I am trying to confirm if it is running in limp home mode or normal.

Doug

Doug,

I think you really need to hook up a hammer to get this information. I have made my own diagnotics plug and have gotten me a hammer to play with. Neat!!! It allows you to do all that and then some.

An alternative is to rig up a simple device that mimiks the CEL. All it takes is a bulb connected to 12V on one side and to pin8 of the T16 connector under the drivers' seat on the other side. T16 is the 14pin connector that comes out of the DME harness close to the DME box.

The CEL will flash fault codes if there are any. It saved my bacon before (I had a bad hall sender connector at the dizzy) There is a detailed description in the WSM that explains the sequence (press accelerator to the floor, turn ignition to ON, wait 4 seconds...) That's when the CEL starts flashing codes. If you keep the pedal depressed more than 10 seconds it clears the fault memory and resets the DME.

Let me know if that helps,
Ingo