1998 Chevrolet Express



March, 6, 2009 AT 10:20 AM

Electrical problem
1998 Chevy Express V8 Two Wheel Drive Automatic 187000 miles

Speedometer reads high at low speed and corrects at about 35 mph. Shifting is eratic then also corrects.
Is fine at high speed. Local chev dealer changed the Trans hall effect but did not clear problem.
Also the battery discharges if vehicle sits a couple of days. I measured the drain to be in excess of 500 MA, seems high to me. I think I may need anew ECM
as I beleive that supplies the speedometer and interacts with the Transmission. Looks like it is difficult to remove and replace.

Any help would be appreciated.
Regards Warren Reynolds


3 Answers


Dave H

March, 6, 2009 AT 8:44 PM

Hello .. thanks for the donation .. much appreciated

I would be looking more into the VSS rather than the ECM .. some models of the express are also fitted with a buffer for the VSS ..I would have to know the VIN # of your vehicle to check if your vehicle has one ?? let me know

Vehicle Speed Sensor (VSS)
VSS is a Permanent Magnet (PM) generator mounted in transmission or transfer case. The VSS sends a pulsing signal to the PCM/VCM or Digital Ratio Adapter Controller (DRAC), which passes the signal on to the PCM/VCM. The PCM/VCM then converts this signal into miles per hour by monitoring the time interval between pulses. PCM/VCM uses this sensor input in controlling Torque Converter Clutch (TCC) engagement, shift speed, etc.

Digital Ratio Adapter Controller (DRAC)
DRAC compensates for various axle and tire ratios by monitoring the Vehicle Speed Sensor (VSS) signal and modifying it before passing it on to the PCM/VCM and speedometer. On models equipped with a DRAC, VSS buffer is an internal part of DRAC.



March, 9, 2009 AT 10:58 AM

Hi Dave,
The VIN is 1GcGG25R7W10262. Don't think it has seperate buffer. I looked at the VSS output with an osclloscope at different speeds and it looks fine, no noise etc. The Vss was changed by the chev garage. Their computer said to replace speedometer cluster. This did not make sense to me or their mechanic as the cluster doesn't influence the transmissin, unless thePCM/VCM is in the cluster.
Any other Ideas?
Best Regards Warren Reynolds


Dave H

March, 14, 2009 AT 7:00 AM

You are right there is no buffer .. and as you say changeing the cluster would have no affect on transmission function !! you could check these out ..or as you say it could be the PCM but without it throwing out codes .. it's hit and miss to locatle the fault ! I would check the TPS first before replacing the PCM

Transmission Fluid Pressure (TFP) Switch (4L60-E & 4L80-E Transmission)
The TFP is actually 5 pressure switches combined into a single unit mounted on transmission valve body. The PCM/VCM supplies battery voltage on 3 separate wires to TFP. As switches are actuated in various combinations during transmission operation, PCM/VCM can detect what gear range transmission is in.
Throttle Position (TP) Sensor
TP sensor is a variable mechanical resistor connected directly to throttle shaft linkage. TP sensor has 3 wires connected to it. One is connected to a 5-volt reference voltage supply from PCM/VCM, another is connected to PCM/VCM ground and third is a signal return which is monitored by PCM/VCM. Voltage signal from TP sensor varies from closed throttle (0.5-1.0 volt) to wide open throttle (4.5-5 volts). PCM/VCM uses this signal to determine control of fuel, idle speed, spark timing and Torque Converter Clutch (TCC). A problem in TP sensor circuit may set a related diagnostic trouble code.
Transmission Fluid Temperature (TFT) Sensor (4L60-E, 4L80-E & 4T60-E Transmission)
TFT sensor is a thermistor (temperature sensitive resistor) mounted to the transmission valve body. The PCM/VCM supplies and monitors a 5-volt signal to TFT sensor. This monitored 5-volt signal is then modified by resistance of TFT sensor. When transmission fluid temperatures are low, TFT sensor resistance is high and PCM/VCM sees a high monitored voltage signal. When transmission fluid temperatures are high, TFT sensor resistance is low and PCM/VCM sees a low monitored voltage.
PCM/VCM uses TFT sensor input to control Torque Converter Clutch (TCC) application and shift quality. Sensor circuit problem should set a related diagnostic trouble code.

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