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System Home |
Snap-on
Modis System Case Studies |
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Click
Images for a
Larger View |
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| 1999 Dodge
Ram Van |
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| Figure 1 |
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| Figure 2 |
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| 1992 Ford Ranger
3.0L. |
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| Figure 3 |
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| 1994 Dodge Ram
Van. |
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| Figure 4 |
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| Figure 5 |
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| 1990 Crown Victoria
5.0L |
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| Figure 6 |
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| 2002 GMC Sierra
4.8L |
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| Figure 7 |
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1999 Dodge Ram Van. Customer
complaint: Speedometer doesn't work until 20 MPH. |
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MODIS in lab scope mode.
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Signal: RABS speed sensor.
The speedometer signal is derived from the
Rear Anti-lock Brake Sensor (RABS) mounted in the differential. The sensor
generates an AC signal proportional to wheel speed. This signal is then
sent to the Controller Anti-Lock Brake Module (CAB). From there, the signal
is bussed to the instrument cluster to drive the speedometer. |
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Note the signal amplitude in
Figure
1. This image was taken at approximately 20 MPH when the speedometer
began working. With the MODIS Lab Scope set to 100mV per division, the amplitude
of the signal is approximately 200mV, which is too low for the given speed.
At low speeds, the instrument cluster never recognizes a signal and the
speedometer doesn’t operate. |
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Note the amplitude difference
in Figure
2 after replacing the RABS sensor. This image was taken at approximately
20 MPH. The fix: A new RABS sensor |
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1992 Ford
Ranger 3.0L. Customer complaint: Runs rough intermittently. |
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MODIS in graphing scanner
mode. |
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Signals:
TPS, MAF, O2S1 & O2S2. Figure
3 was taken while the symptoms occurred with black smoke coming out
the tailpipe. While comparing both O2 sensors, it’s quite obvious
there is a problem with O2S1 (bank 1 O2 sensor). Note as the throttle increases,
O2S2 voltage increases to approximately 900mV, but the O2S1 stays relatively
flat (40 to 60 mV) signaling a defective O2 sensor. The fix: A new O2S. |
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1994 Dodge Ram Van.
Customer complaint: cruise control cuts out intermittently.
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MODIS in scanner mode.
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Note the parameter LAST CUTOUT
___P/N and DISABLED___ P/N, Figure
4. These 2 pieces of information are the effect of the PCM commanding
cruise off due to a Park/Neutral input to the PCM while vehicle speed is
present. The cause for the intermittent cutout was a P/N switch input wire
chafing on a transmission bracket. |
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The fix: repair P/N wire at
transmission. See after repair image for comparison, Figure
5. |
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1990 Crown Victoria 5.0L |
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MODIS in lab scope mode.
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CH1 Primary current |
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CH2 Primary voltage |
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CH3 SPOUT (spark output) |
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CH4 PIP |
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Screen shows known good relationship,
Figure
6. |
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2002 GMC Sierra 4.8L |
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MODIS in graphing scanner
mode. |
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This image, Figure
7, was taken KOEO on a known good vehicle. The purpose of this test
is to verify the operation of the O2 sensor heaters. If the heater is inoperative,
the bias voltage applied to the O2S signal will NOT be pulled low. |
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At key on, full battery voltage
is applied to the O2 sensor heater. As the heater warms, the bias voltage
on the O2S signal is pulled low. This is normal. |
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Note: this test should only
be performed on an engine that has not run for at least 30 minutes. |
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Diagnostics