Eractic Speed Odometer

04-22-2012, 09:40 AM
2001 Buick Century: My speed odometer needle fluctuates up & down which causes the transmission to up shift or down shift. Is this a speed odometer problem or a trans problem?

04-22-2012, 09:51 AM
Another detail to my problem: the speed odometer is sometimes stablbe. When it is the transmission does fine.

04-23-2012, 09:14 AM
Welcome to AF!!

The speedometer is electronic and works off the speed sensor in the transmission. The computer relies on the speed sensor to shift the transmission and if it is eratic the trans will not shift normally. Replace the speed sensor and the problem should go away.

Tech II
04-23-2012, 11:59 AM
I agree with GTP Dad, but just for ha-ha's, check the harness to it, and the integrity of the connectors at the harness connector....

04-23-2012, 04:46 PM
Thank you so much! As you can see, I am a newbie to the forum and it is worth every minute of it.

I am an amatuer mechanic but very mechanically incline. I suspected the VSS but wasn't truly sure how it, the trans and speedodometer all tied together...which hand shook which and in what order. I was told by a GM mechanic that he suspected the cluster panel but now I will head for the sensor.

Thanks again and I'll let you know how it turns out.

04-26-2012, 05:27 AM
i'll put my money on speed sensor replace sensor and you should be fine..but how longs this been doing this?

good luck..

04-26-2012, 10:06 AM
We just inherited the car from my elderly inlaw. Not sure how long it's been like this. I did replace the VSS from a parts car that didn't have this problem. When I drove it up and down the street it seemed to still be doing it a little...I guess I need to get it out on the highway to know for sure. I did notice hard shifting when I test drove it but on the test drive I didn't get over 30 mph. I was going to flush the tranny, new filter and fluid to see if this helps. The fluid is brown and had tiny bubbles in it.

How can I check the wiring going to the sensor? Or can it be checked? Maybe the signal is not getting back to the PCM.

I did do a visual inspection of the wiring harness at the sensor and it appeared fine.

04-27-2012, 08:56 AM
Changing the fluid and filter is a good idea especially if the fluid is brown. When you change the fluid add a can of Seafoam transmission additive to it. This will remove any gum or deposits an help the transmission to shift better. If you still have the problem after that you may want to take it to a transmission shop and have them diagnose the transmission issue for you. It is generally free to diagnose a problem.

04-27-2012, 10:48 AM
Thanks GTP Dad! I'm performing the fluid/filter change today. I'll try the Seafoam and see where that takes me.

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