Testing Diodes; Measuring Engine Rpm - Bosch FIX 7677 Mode D'emploi

Testing Diodes

A diode is an electrical component that
allows current to only flow in one direc-
tion. When a positive voltage, generally
greater than 0.7V, is applied to the an-
ode of a diode, the diode will turn on and
allow current to flow. If this same volt-
age is applied to the cathode, the diode
would remain off and no current would
flow. Therefore, in order to test a diode,
you must check it in both directions (i.e.
anode-to-cathode, and cathode-to-
anode). Diodes are typically found in
alternators on automobiles.
Performing Diode Test (see Fig. 13):
Fig. 13
1. Insert BLACK test lead into COM
test lead jack.
2. I n s e r t R E D t e s t l e a d i n t o
3. Turn multimeter rotary switch to
function.
4. Touch RED and BLACK test leads
together to test continuity.
Check display – should reset to 0.
00.
5. Disconnect one end of diode from
circuit.
Diode must be totally isolated from
circuit in order to test its functionality.
6. Connect RED and BLACK test
leads across diode and view dis-
play.
Display will show one of three things:
• A typical voltage drop of around 0.
7V.
• A voltage drop of 0 volts.
• A "1" will appear indicating the mul-
timeter is overranged.
Cathode
Anode
Red
Black
test lead jack.
7. Switch RED and BLACK test leads
and repeat Step 6.
8. Test Results
If the display showed:
• A voltage drop of 0 volts in both
directions, then the diode is shorted
and needs to be replaced.
• A "1" appears in both directions,
then the diode is an open circuit
and needs to be replaced.
• The diode is good if the display
reads around 0.5V–0.7V in one di-
rection and a "1" appears in the
other direction indicating the multi-
meter is overranged.

Measuring Engine RPM

RPM refers to revolutions per minute.
When using this function you must multi-
ply the display reading by 10 to get actual
RPM. If display reads 200 and the multim-
eter is set to 6 cylinder RPM, the actual
engine RPM is 10 times 200 or 2000 RPM.
To measure Engine RPM (see Fig. 14):
Fig. 14
Typical
Ignition
Coil
Ground
1. Insert BLACK test lead into COM
test lead jack.
2. I n s e r t R E D t e s t l e a d i n t o
3. Connect RED test lead to TACH
(RPM) signal wire.
• If vehicle is DIS (Distributorless
Ignition System), then connect RED
test lead to the TACH signal wire
going from the DIS module to the
vehicle engine computer. (refer to
vehicle service manual for location
of this wire)
• For all vehicles with distributors,
11
Red
Black
test lead jack.