D-Jetronic chapter 6: Throttle switch

6.1 Function of throttle switch (TS)

Throttle switch (TS) forwards information of up to three states to engine control unit (ECU):

1. Gas pedal in idle position, a position contact
2. Gas pedal is pushed down (acceleration) via 2 sliding contacts (not available in VW till 7/69)
3. In later years additionally Driver demands full power, a position contact

TS forwards no other information about throttle, also not how wide it is opened, it is no posiition potentiometer. You can only adjust idle-run contact by turning whole throttle switch against throttle axle that it is mounted on. Its axle moves with throttle axle while its housing is fixed. Housing is made of a metal bottom to which a PCB board is connected. This PCB board consists of lines and pins for sliding contacts to which a 2, 4 or 5 pin connector is mounted. It is covered by a black plastic cap that you can easily remove. It is either srewed on or later with clips, but be careful with the clips!

Pin 12 or 47 connects throttle switch directly or via ECU to ground. The other contacts forward this ground to ECU. Pin 17 is the idle-run contact. It signals idle position of gas pedal to ECU and needs to be adjusted properly. You have to turn the throttle switch so far that it safely closes in idle position and opens when the trottle is opened by 1° . Adjustment is described here.

All other contacts cannot be adjusted. The two sliding contacts pin 9 and 20 slide over a comb of contacts facing each other. While sliding over these combs pin 9 and pin 20 signal 0V and open connection in 10 times alternation. If pin 9 is 0V, pin 20 is open and vice versa. These 0 - 1 signals acceleration to ECU while pushing down gas pedal which in turn enriches fuel mixture, meaning enlarging injection pulses. In order no to have the same effect when releasing gas pedal, a drawing contact disconnects pin 9 and 20 in that case. When holding or releasing gas pedal there are no signals to ECU.

In late version of D-Jetronic full-load enrichment was taken out of MAP sensor and moved into a full-load contact in throttle switch signalling full-load via pin 2 or 14 to ECU when throttle is fully open. It cannot be adjusted.

6.2 Three types, many forms

There are three basic types of throttle switches, either with 2 connectors on early VW without accerleration contacts, or 4 connectors or later with full-load contact and 5 connectors. MAP sensors with black caps always demand a 5 pin throttle switch. But there are many more throttle switches than just these 2 types, nearly always one per engine type. Why is that so? Throttle switch must fit to turn direction and turn angles of throttle flap and also must fit to cable connector. All these details differ a lot in:

• Left or right turn
• Angle of idle position
• Angle of full-load position
• Angle of cable leads towards throttle switch (straight or in angle)

Therefore the throttle switch must fit to engine and ECU / MAP sensor. Opel, Volkswagen and others for example changed from 4 pin to 5 pin on their engines in a certain year. A throttle switch of another engine or manufacturer mostly will not fit. Exception are early throttle switches with same turning direction without full-load contact. They were used on several cars in early years and you might even find replacement PCB boards for them.

6.3 Possible failure

Throttle switches are not water proof, so they can get wet or rusty from oil or spray water. Afterwards contacts or PCB can rust or even connectors can fail. Additionally sliding contacts can create a wear on PCB, especially on acceleration contacts. Thus they will not send proper or false pulses to ECU. That will cause bad acceleration or stuttering when cruising with your engine. Of course also full-load and iddle-run contact can fail. You can open your TS while mounted and clean it with break cleaner. But it is a better choice to remove and clean it. If necessary you can try to repair it with conductive silver. After mounting it again idle-run contact needs to be adjusted. If it is rusty or if combs are worn, you need to replace it. For some TS you can buy replacement PCBs. But it must be exactly fit your TS. If you can, find a good used TS as replacement and continue to drive happily another 40 years. If you can't find one, learn your TS PCB board no. and then shop a new PCB board accordingly.

6.4 Testing your TS

Always test throttle switch contacts measuring at ECU connector to surely detect wire harness problems. Take a multimeter and measure pin 12 - 17 (or ground - pin 17) for idle-run switch. It must show 0 Ω in idle position and open immediately when pushing down gas pedal. Don't adjust it too narrowly, half a degree af lash is important so that it always closes safely at all temperatures. Now push down gas pedal fully and measure pin 12 - pin 2 or pin 12 to pin 4 for full-load contact (if you have one). At last connect your multimeter to pin 9 - pin 12 and pin 20 - pin 12. While SLOWLY pushing down gas pedal your multimeter must show on both contacts 10 times 0 Ω and in between infinite resistance. You notice the comb on the image with sliding pins.

Your Dr-DJet (Volker)