DME Update Instructions
Sometime in 1987, Porsche upgraded the software used in the U.S. 84-86 911 Carreras, from a 2K to a 4K program. The primary differences were refinements for an improved idle and additional part throttle performance and response. All 84-86 911s can upgrade to the later 4K program by a simple jumper move and readjustment of the base idle speed. If your 911 is an '84, '85, or '86 model, there is a 50% chance your computer may be a 2K DME, in which case you will need to unsolder and move a single jumper to update it to a 4K DME. After you make this mod you can still run your old chip without changing the jumper back.
Your computer is a 2K unit if it looks exactly like figure 1. If it does not look exactly like figure 1, you already have a 4K unit, and do not need to do anything further. Figure 3 shows examples of computers that are already 4K DMEs that do not need any modification.
Converting a 2K 84-86 DME to use the later 4K chip:
Just unsolder (or clip off) the jumper that connects "B702" and "B1" and solder it (or a new one) between "B703" and "B704"
Note: this conversion will do absolutely nothing for the ROW/European spec 911s and only applies to North American and Japanese cars that come with a oxygen sensor and catalytic converter from the factory.
1 - Before
FIG.2 - After
*Photos courtesy of Chris Bennet
of circuit board after conversion. (Click image for a larger
3 - Already a 4K DMEs - no conversion necessary
Idle Speed Adjustment
After fitting the 4K chip in your DME, your idle speed will be a bit higher than normal and may need to be readjusted down if it ‘hunts’ on start up or is higher than 900 rpm at normal running temperature. This can also happen on an '87 Carrera if the original chip is #1267355236. This is because in mid ’87, Porsche revised the programmed idle specifications from 800 to 880 rpm, and advanced the idle ignition timing by 6 degrees. To reduce the base idle speed if necessary to 880 rpm, adjust the idle speed screw on the throttle body by two to three turns clockwise. It may be covered with a yellow plastic cap, which will need to be removed. For a more exact adjustment, bridge B and C in the rubber test socket on the left side of the engine compartment (located behind the black removable cover) with a piece of wire, and adjust the idle speed to 880 rpm at normal running temperature. Properly adjusted, there should be no difference in idle speed with the jumper or without. Remove jumper when finished.