TRANSFER CASE - TOD

 GENERAL INFORMATION AND OPERATION

TOD system means the full time 4WD system and the
registered trade mark of Borg Warner. TOD is an
abbreviation of Torque On Demand.

TOD control unit automatically tailors torque distribution
to road, offering full time four wheel drive. TOD
distributes electronically-controlled power into front and
rear wheels whose ratio change from 0 : 100 to 50 : 50
complying with wheel speed differences.

Also TOD control unit analyzes data from wheel speed
sensor and engine output then changes pressure of
electromagnetic clutch. This pressure controls front
propeller shaft and power to front wheel. Power to front
wheel depends on degree of pressure corresponding
clutch slip.

TOD is designed to distribute the power to front and
rear axle by operation of 4H/4L switch and shift motor.
Shifting 4WD high (4H) to 4WD low (4L) is performed
towards reducing high-low collar by means for
connection high-low shift fork with output shaft in order
to join with planetary gear. Torque transmits input shaft
then sun gear rotating front planetary gear. Front
planetary gear joins with output shaft and drives at
low position.

The TOD system has 2 selectable mode, 4H and 4L.
4H is the normal operating mode when drive of which
gear ratio is 1:1 and 4L mode distributes power to
front and rear wheels 50 : 50 of which gear ratio is
2.48:1.

TOD SYSTEM FUNCTION

4H Mode

The TOD system transfer case controls the clutch
mechanism to comply with rotation in front and rear
propeller shaft and if its difference exceeds the
permissible range, corresponding power is distributed
into front wheel through electromagnetic clutch (EMC).

Hall effect speed sensors are located front and rear
propeller shafts, send signals to TOD transfer case
control unit (TCCU). The EMC coil is activated by
variable current on exceeding difference of speed in
front and rear propeller shafts.

4L Mode

When select 4L mode, EMC is locked to apply
maximum torque into front and rear propeller shafts.
Shift motor rotates also 4L position by rotation of cam
thus propeller shaft torque changes from 1:1 to 2.48:1
by planetary gear set.

Transfer Case Control Unit (TCCU)

Transfer case control unit (TCCU) receives, front and
rear propeller shaft speed, shift motor  position and
4H/4L switch signals and controls electromagnetic
clutch  (EMC), shift motor. TCCU communicates with
scan tool with K-line for diagnosis. It located under
the front LH seat.

quando cambiare la frizione

• Avete notato che col passare del tempo i cambi di marcia sono diventati più ruvidi e meno fluidi? Molto probabilmente vi è l’olio del cambio da cambiare: molti sottovalutano l’importanza di avere un olio del cambio di buona qualità e soprattutto di effettuarne la sostituzione ogni 60-80 mila chilometri. Un buon olio nella scatola del cambio aiuta l’innesto delle marce, diminuisce gli attriti tra gli ingranaggi e ne prolunga quindi la vita
• La frizione strappa nelle partenza talvolta anche nei cambi marcia o quando si sfrutta appieno la potenza del motore nei rapporti bassi? Probabilmente si è danneggiato lo spingidisco o le molle. In questo caso la frizione va cambiata quanto prima per evitare che vi molli definitivamente e vi impedisca di utilizzare l’auto.
• Se la frizione slitta invece la colpa potrebbe essere del disco, o troppo consumato oppure sporco. Anche in questo caso la frizione è da cambiare
• Il pedale della frizione è diventato duro: Se la frizione è a comando meccanico è probabile che si sia danneggiato il cavo della frizione, bisogna quindi controllarlo e lubrificarlo.
• Lo slittamento della frizione può essere dovuto a varie cause.
• La frizione non stacca
  Se la frizione non stacca, la causa non è necessariamente la frizione stessa. Spesso la causa è imputabile al sistema di disinnesto della frizione, al cuscinetto pilota o a errori di montaggio.

La frizione è rumorosa >Rumori della frizione come fischi o colpi sordi non sono sempre indice di una frizione difettosa. Fischi possono essere causati ad esempio da un contatto fuori centro del cuscinetto reggispinta, da alberi d'ingresso del cambio non centrati o da cuscinetti pilota difettosi. Rumori sordi si possono verificare in caso di variazioni di carico, ad es. se sono installati dischi della frizione con pre-ammortizzatori. Questi non compromettono il funzionamento o la durata del disco della frizione. Rumori possono essere generati anche da un'installazione non corretta di componenti o da un disco della frizione installato in una posizione sbagliata.

OPV RELAY

Mercedes OVP – A Basic Primer and Test Process
I am dealing with my second failed OVP and reviewing the various links prompts me to try to consolidate some of the scattered information into this single format. I hope it is useful. See Figure 1. below for a picture of a typical OVP relay.

Over Voltage Protection [OVP] – devices are used to protect sensitive electric circuits from damaging high voltage. Two OVP protected circuits in the 124 series Mercedes are the Electronic Engine Control and the ABS [anti-lock brake] circuits. Hard starting/stalling are often symptoms of a failed EEC circuit, often in conjunction with an illuminated ABS light on the instrument panel. The OVP device supplying power to both these circuits is located under the hood, behind the battery, somewhat behind a removable plastic splash panel on the passenger side of the vehicle (North American configuration). It looks like Figure 1. The first thing to check is that the 10 amp fuses on the top of the device are not blown. Assuming the fuses are OK, replace them, and to further test the device; remove it from the car by wiggling and pulling it free from the plug base. Figure 2. shows what it looks like inside with the metal canister removed; but it is not necessary to dismantle the device to test its basic function. Figure 2. below is mainly intended to identify the circuit pins referred to in the next section.

The basic function of the OVP is this: Battery voltage is always present at pin 30 (nominally 12.5v – higher when the alternator is charging); and the device is connected to chassis ground through pin 31. With the ignition off (that is no 12v current delivered to pin 15) there should be battery voltage at pins 30a and 30b, but no voltage at the protected circuits 87E and 87L. If there is voltage at the 87E or 87L pins with the ignition off there is an internal short in the device. When the ignition key is turned on, battery voltage is applied to pin 15 (the skinny one) which pulls in a relay switch and battery voltage is then supplied to both the 87E and 87L protected supply circuits.

To test the basic functioning of the device out of the vehicle you only need a 12v power supply (a battery), a couple of (actually 3) jumper wires with alligator clips, and a DC voltmeter. First connect the 12v positive jumper (preferably through a 10 amp fuse) to pin 30, and the 12v negative jumper to pin 31. Use the voltmeter, with its ground lead attached to the negative terminal of the power supply (battery), to check for positive voltage at the other pins. There should be 12v at pins 30a and 30b, but no voltage at any of the other pins [15, 87E, or 87L]. Without disconnecting the jumpers to pins 30 and 31, now connect a third jumper wire from the positive side of your power supply to pin 15 (this is the same as turning on your ignition switch when the device is in the car). You may hear a slight click as the relay pulls in the internal switch, and now when you test the pins with your voltmeter, 87E and 87L should be reading 12v. If there is no voltage or reduced voltage at the protected circuits 87E and 87L this indicates a faulty OVP.
Note that passing this basic test may not be the whole story, as electric devices can fail intermittently, or partially, when in their real working environment. Further testing after heating (hair blower) or cooling (freezer compartment) may be useful if a temperature sensitive intermittent failure is suspected.

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