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Jeremy, Certified Truck/Bus Technician
Category: Medium and Heavy Trucks
Satisfied Customers: 2721
Experience:  ASE Med/HD Truck Technician, NATEF Diesel Engines and Electrical/Electronics Technician, EPA 608/609 HVAC Certified, Steering/Suspension and ABS/Brakes Technician
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2003 Freightliner Century 12.7 VIN K74889. Had front end

Customer Question

2003 Freightliner Century 12.7 VIN K74889.
Had front end off, trs, srs, bull gear, and seals done because tone wheel chipped tooth. Not sure if that is related.
Left shop and cruise control worked like normal for 30 minutes.
Then setting cruise control, it accelerates to to maximum rpm (speed) in every gear.
I can turn cruise off by pressing the speed control switch "off" or pressing brakes. I usually just touch the clutch lightly, but clutch no longer disengages. If I push clutch in all the way, motor redlined.
With speed control switch left "on" it decided to set cruise to 63 after about 200 miles of driving. I didn't flip resume or set. When I tried to increase speed it tried to go to max rpm again.
I replaced the set/resume paddle switch on dash with no change.
I inspected ECM wiring and it looks clean since it was just taken off and away from anything it could rub or short on.
Submitted: 1 month ago.
Category: Medium and Heavy Trucks
Expert:  Jeremy replied 1 month ago.

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Customer: replied 1 month ago.
No shops open until Tuesday.
Expert:  Jeremy replied 1 month ago.

You only did the repairs on the front of the engine, correct? No other repairs were done and no wiring was touched otherwise? If that is the case, the issue shouldn't be related at all.

The issue will be somewhere in the cruise control system. Either the clutch pedal sensor is bad or there is an issue somewhere else in the circuit.

Now if your sensors were pushing oil, you could have oil in the plug for the ECM that would cause the shorts on the pins, but it is really odd to have the cruise engage and stay engaged. That is usually the clutch pedal switch that causes that. When the pins short near the ECM, the cruise just stops working.

Yours will be one of 2 setups. You will have the switch style with the rod attached to a spring that come out and it released from the pedal assembly or yours will have a sensor on the side that read the pedal position. They are both easy to change out, but if the shops are closed, it is hard to get a part to replace it. It is a pain, but you will have to turn the switch off since it isn't disengaging from the clutch switch.

NAPA sometimes carried them because they are generic if it is the little rod style. You will see the rod against the pedal if it is that style when you look up and under the dash. It is is the sensor style, there will be a pad of a square piece of plastic on the left side of the pedal.

Customer: replied 1 month ago.
Sensors were not pushing oil, threw code 41 so replaced sensors but it turned out to be chipped tooth on tonal wheel. The ECM software was updated, but like I said the cruise worked ok when I left Detroit diesel.I know the clutch switch you are talking about. Do you know if the switch makes the circuit open or shut during normal operation (clutch up, transmission engaged)? I can test this as a problem by removing switch and separating wires or wire nut together.
Expert:  Jeremy replied 1 month ago.

When the clutch is up, the circuit is closed. When you press the clutch, it will open the circuit. So what should happen is if you unplug it, the cruise should not work at all.

Customer: replied 1 month ago.
The problem is that cruise control won't stop building speed when activated.A symptom is the clutch pedal no longer disengages the cruise control when activated.Currently, cruise control works but won't set at a given speed-just continues to go faster.So if switch was open cruise wouldn't work at all. If it was closed cruise would engage (which it does) and would behave normally (which it doesn't)
Expert:  Jeremy replied 1 month ago.

Gotcha. That is in the smart switch then and you wrote you replace it. That leaves an issue in the circuit.

On the firewall you have that big plug on the driver side that comes out and another on the passenger. The driver has the cruise wires. See if there are any loose wires or if that round plug moves at all. There tends to be a lot of shorts there and rain water gets in it from the draining of the water in the cowl from the wipers.

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