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Speedometer swap not working properly

Tiny
TGOODMAN11
  • MEMBER
  • 2007 PONTIAC G6
  • 3.6L
  • V6
  • 2WD
  • AUTOMATIC
  • 199,500 MILES
I have the car listed above GTP. I replaced the transmission with one from a Monte Carlo. My speedometer won't read over 25 MPH. It shifts fine. Took to a transmission place and said everything is reading normal for shifting. Said it could be a bad cluster so I replaced it and same issue. The transmission guy said speed showed 42 MPH when speedometer read 17 MPH. The speedometer was reading normal before the swap. There are no DTR codes. Cruise control won't work until speedometer reaches 25 MPH and then works fine. I am doing approx 60 -65 MPH at that time. The OBD scanner cannot communicate with the cluster so not sure if that's the issue.
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Friday, February 12th, 2021 AT 6:40 AM

10 Replies

Tiny
DANNY L
  • MECHANIC
  • 5,648 POSTS
Hello, I'm Danny.

You didn't mention what year Monte Carlo you swapped transmissions from to put into your Pontiac. I did notice one certain thing was that both the VSS- vehicle speed sensors are different GM part numbers. This leads me to believe what is causing the speedometer signal being off. Let me know what year Monte Carlo the transmission came from, Get back to us and we'll go from there. Hope this helps and thanks for using 2CarPros.
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Saturday, February 13th, 2021 AT 12:36 AM
Tiny
TGOODMAN11
  • MEMBER
  • 6 POSTS
I believe it was a 2009 or 2010 Monte Carlo. The numbers are the tranny were the same. Is there a fix? I still have the old transmission. Where would the sensors be located? There is nothing external on the transmission. There is just a cover on the side where the fluid goes and the harness plugs in. There is very little info about this that I can find. I have spent numerous hours on Google and YouTube until I found you guys.
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Saturday, February 13th, 2021 AT 5:46 AM
Tiny
DANNY L
  • MECHANIC
  • 5,648 POSTS
Hello again.

I've attached picture steps below for VSS-vehicle speed sensor replacement and location for your Pontiac including location on your automatic transmission. I will look to see what will be needed to convert to a Monte Carlo transmission and will update you with what I find regarding your speedometer issue. Hope this helps and thanks again for using 2CarPros.

Danny-
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Sunday, February 14th, 2021 AT 10:49 PM
Tiny
TGOODMAN11
  • MEMBER
  • 6 POSTS
Sorry I didn't see this email. I thought I got a new alert but, found it. That is not what I have. I have attached pics of the one I took out and tried to get numbers as well. There are no plugs on this one other than the main harness. Maybe these will help. I appreciate it.
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Monday, February 15th, 2021 AT 3:01 PM
Tiny
TGOODMAN11
  • MEMBER
  • 6 POSTS
Here are more.
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Monday, February 15th, 2021 AT 3:03 PM
Tiny
TGOODMAN11
  • MEMBER
  • 6 POSTS
Did the 7 pics help any to try and identify the problem?
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Tuesday, February 16th, 2021 AT 5:36 AM
Tiny
DANNY L
  • MECHANIC
  • 5,648 POSTS
Hello again.

I did a bunch of research and found no real easy answer. Since you replaced the unit with the Monte Carlo transmission there are differences between the two and even though they both have vss- vehicle speed sensors those transmissions have different gear ratios which is what is throwing things off and not making your speedometer not work right. You might want to find a transmission shop or person that can manipulate or modify the vss to make the Monte Carlo vss compatible with your Pontiac transmission controls. Hope this helps and let me know if you have any other questions on this issue. Thanks again for using 2CarPros.

Danny-
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Friday, February 19th, 2021 AT 1:39 AM
Tiny
TGOODMAN11
  • MEMBER
  • 6 POSTS
I am sorry it was a 2008 Malibu not a Monte Carlo if that makes a difference. My Pontiac had a 7PDK gen 1 6t70 transmission and the Malibu had a 8PDK and I think is the 6t70 gen 2 transmission.
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Saturday, March 6th, 2021 AT 6:34 AM
Tiny
DANNY L
  • MECHANIC
  • 5,648 POSTS
Hello again.

Okay, I'll do more research and get back to you shortly. Thanks again for using 2CarPros.

Danny-
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Saturday, March 6th, 2021 AT 4:19 PM
Tiny
DANNY L
  • MECHANIC
  • 5,648 POSTS
Hello again.

Yes, we are going to be having the same problem with this transmission as well. The gear ratios are still different from your Pontiacs original transmission. Even though the Malibu transmission has an external bolt on VSS- vehicle speed sensor they are different ratios. You might want to look online to see what companies can make an in-line gear ratio correction to make the Malibu transmission compatible with your Pontiacs transmission electronics. Those kind of places are out there but none come to mind off the top of my head. I've attached pictures below showing the VSS-vehicle speed sensor replacement and location on the Malibu transmission. The last two pictures show the part numbers for both your Pontiac and the Malibu VSS-vehicle speed sensors. If you find a transmission shop that can make an adapter or somehow calibrate maybe the part numbers can help with gear ratio information. Hope this helps and thanks again for using 2CarPros.

Danny-
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Thursday, March 11th, 2021 AT 7:01 PM

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