Test Point 9 - Throttle Switch
TEST POINT 9 - DME 944Throttle Switch
1. Idle Contact
A) Connect Bosch test leads on one fuel injector. Connect oscilloscope on test leads as specified - see testing requirements.
This image should be displayed at idle speed.
Accelerate engine to approx. 4000 rpm - the injection time section (ti) should be wider.
Return throttle to idle stop quickly. The injection time section (ti) should no longer be visible on the screen (coasting shutoff).
The idle speed image should again be visible from a speed of approx. 1200 rpm (approx. 1300 rpm at 20 °C) on.
If the oscilloscope does not display coasting shutoff, there is a defect either in the wire leading to the control unit, the control unit itself or the throttle switch.
B) Checking Control Unit
Pull off plug on throttle housing.
Start and run engine at idle speed.
Bridge terminals 18 and 2 on disconnected plug with a test lead.
Boost engine speed to approx. 1600 rpm.
Engine speed begins to surge (coasting shutoff).
If not, check power flow between plug and control unit plug.
Replace control unit, if necessary.
C) Checking Idle Contact in Throttle Switch
Connect ohmmeter on terminals 18 and 2 of throttle switch.
Display: approx. 0 - 10 ohm (switch closed)
Open throttle (approx. 1 - 20 throttle valve <).
Display: infinite ohm (switch open).
This test can also be made on control unit plug term. 2 and ground.
2. Full Load Contact (Testing at Idle Speed)
A) Checking Control Unit
Pull off plug on throttle switch. Bridge terminals 3 and 18 on plug with a test lead.
Ignition timing must change to approx. 0° TDC (USA: approx. 3.5° before TDC).
If not, check power flow in wire from throttle switch plug to control unit plug (term. 3 and ground).
B) Checking Full Load Contact
Connect ohmmeter on terminals 18 and 3 of throttle switch.
Display: infinite ohm (switch open).
Move throttle to full load position (wide open).
Display: 0 - 10 ohm (switch closed).
If not, replace throttle switch. (On control unit plug term. 3 and ground.)