Speedometer jumping car bogs down at sixty mph

2001 CHEVROLET SUBURBAN
200,000 MILES • 5.7L • V8 • 4WD • AUTOMATIC
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LSTALEY
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After travelling on highway for about five minutes around sixty mph the Speedometer will drop to forty or fifty and jump drastically. Rpm go to about 3000 then drop and car bogs a little but will still go and shift.
Mar 13, 2018 at 4:46 PM
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ASEMASTER6371
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Good morning.

When the speedometer changes speed, does the trucks speed change or does it maintain speed?

Roy
Mar 14, 2018 at 5:14 AM
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LSTALEY
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It bogs just a little but will maintain speed and will accelerate even when speedometer is floating.
Mar 14, 2018 at 6:45 AM
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ASEMASTER6371
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Okay, there is a speed sensor at the transmission that sends a signal to the PCM and that signal is transferred to the dash.

You need a scan tool to monitor this signal to see if the signal is steady or if it is lost in transferring between modules. it can also cause the bogging as it controls transmission up shifts and downshifts as well.

Do you have a scan tool?

Roy
Mar 14, 2018 at 6:48 AM
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LSTALEY
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No I do not.
Mar 14, 2018 at 12:45 PM
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ASEMASTER6371
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If you cannot get one, see if a shop can check it for you to be sure the signal is getting to where it needs. If it does need the sensor, you can do that part of the job.

Roy
Mar 14, 2018 at 12:48 PM
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LSTALEY
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Thank you if the issue only appears when going high speed do you think the tester will show the sensor being bad?
Mar 14, 2018 at 12:51 PM
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ASEMASTER6371
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sure, a scan tool can be used while driving and can record the sensor reading while driving.

Roy
Mar 14, 2018 at 12:53 PM