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Speedometer quicks working and engine rev limits to 1400…

Customer Question
Speedometer quicks working and...

Speedometer quicks working and engine rev limits to 1400 rpm

Mechanic's Assistant: What's the make/model/year of your truck? Engine type?

2011 Mack CXU 613

Mechanic's Assistant: Are you fixing your truck yourself? What have you tried so far?

Yes I am, changed the vss at back of tranni

Mechanic's Assistant: Anything else you want the Truck Mechanic to know before I connect you?

Also getting a message about scr system fault but that may be a unrelated issue

Submitted: 2 years ago.Category: Medium and Heavy Trucks
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Answered in 2 minutes by:
3/26/2020
Technician: Mike Mcmillan, Shop Foreman/Triage Tech replied 2 years ago
Mike Mcmillan
Mike Mcmillan, Shop Foreman/Triage Tech
Category: Medium and Heavy Trucks
Satisfied Customers: 6,768
Experience: Volvo/Mack Master Tech, Certified with Cummins, Eaton, GM, Hino, Detroit Diesel
Verified
Hello and thank you for using Just Answer. My name is ***** ***** I’d like to try and help. Does this have an mdrive trans in it?
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Customer reply replied 2 years ago
Not exactly sure what mdrive is... its a 10 speed Eaton Fuller
Technician: Mike Mcmillan, Shop Foreman/Triage Tech replied 2 years ago
Okay. The mdrive is the Mack automatic trans used in these. Usually what we find with this issue is spread pins at the vss sensor or a bad spot in the wiring on the trans harness. Usually they get zipped tied to the metal brackets to tight on those brackets and the wires will break in the insulation. The vss signal then goes to the vecu in the dash. Take the main dash switch panel out and it is behind the abs module. Then check the connections and wiring around it. I hope this helps
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Customer reply replied 2 years ago
Ok will give that a try. Where is the abs module located?
Technician: Mike Mcmillan, Shop Foreman/Triage Tech replied 2 years ago
Right behind the main switch panel to the right of the cluster
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Customer reply replied 2 years ago
Do these trucks have a speedometer relay or solenoid somewhere? I'm wondering this because when I put the truck in reverse and back up the speedometer starts working again until I drive forward than it usually kicks back out fairly soon
Technician: Mike Mcmillan, Shop Foreman/Triage Tech replied 2 years ago
No there is no relay in this circuit. The vss wiring goes directly to the vecu in the dash then that module data links the signal to the engine, cluster, and abs modules
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Customer reply replied 2 years ago
What do you think could cause the reset when I back up?
Technician: Mike Mcmillan, Shop Foreman/Triage Tech replied 2 years ago
Make sure the output shaft is tight and the tone wheel is tight. All that sensor does is create an ac voltage signal this is the same signal whether it is moving forward or backward so the vecu does not know if it is going forward or backward. Those mechanical components are the only things that change between going forward or backward
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Customer reply replied 2 years ago
Sorry I'm not the most educated on the proper names! What is the output shaft and tone wheel you are referring to?
Technician: Mike Mcmillan, Shop Foreman/Triage Tech replied 2 years ago
That is the shaft that comes out of the back of the trans. The time wheel is on that. This is what moves under the vss
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Customer reply replied 2 years ago
That is all tight.... could the wheel speed sensors be causing this problem?
Technician: Mike Mcmillan, Shop Foreman/Triage Tech replied 2 years ago
No. This speedo signal comes right from that sensor
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Customer reply replied 2 years ago
I found out both abs sensors were broke off in the back so I replaced them and it seems to have fixed my problem
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Mike Mcmillan
Mike Mcmillan, Shop Foreman/Triage Tech
Category: Medium and Heavy Trucks
Satisfied Customers: 6,768
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Experience: Volvo/Mack Master Tech, Certified with Cummins, Eaton, GM, Hino, Detroit Diesel

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