I have a 2003 motorhome on a F53 chassis. The cruise control will not turn on. This model was not on the recall list so no work has ever been done to the brakes or speed control system. I do not have a shop manual but I have pieced together lots of test suggestions that I found on the web. Still have not found the issue. EVERYTHING works except cruise control. All brake lights work. Hydromax works, ABS works. No trouble lights at all on dash. For fun, I have replaced the brake pedal switch. I have replaced the SCDS that is on a brakeline junction below the master cylinder. The F53 does not have the cruise control self test feature. (I tried) I found and performed several servo tests. The servo gets 12v power with engine off. It gets power with engine on and no brakes applied. I can detect resistance when testing the set/accel button. There was a suggested test between servo and abs that I did not do because I'm not sure where the abs module is located, but since abs system works I did not figure that could be the problem. Before testing the servo I had the steering wheel apart. I have 12 volt power to the swithes. I have checked every fuse that had anything to do with brake or speed control according to owners manual. All good. One thing is troubling. On the master cylinder is a brake pressure differential switch. When I put my meter on the switch I have continuity. But when I put the meter on the wires to the swtich I cannot detect current either with the ignition off or in the on position. I tested the ground wire for continuity against the frame. That was good. I tested the positive side of the connector by stripping of some insulation on the positive wire and checking for continuity and it was good. Questions: When does this line get power? Does anyone know what fuse this line gets power from? I've traced the wire as far back as I could. No in-line fuses that I can see. Suggestions please!!! Thanks!

 

The SCDS on the master cylinder is powered by ALWAYS HOT fuse F8 in the CJB. The SCDS itself is a normally closed switch that is supposed to supply power to the servo until the brake pedal is forcibly pressed.

The servo is also powered by switched fuse F26.

You cannot measure current unless you open the circuit and insert an ammeter.

 

Thanks for the help. I checked the mentioned fuses. Those are good. I put my meter on the brake pressure differential valve warning switch. It reads 0 ohms when no brake is applied and 3-4 ohms when some pressure is applied to the brakes. Since that switch is a normally closed switch, this appears to be a good switch??? There are 2 wires that connect to the switch. One is purple with a green stripe (P/GN) and the other is tan with a green stripe (T/GN). I applied pressure to the brake pedal and then I took my meter and put the ground to the frame and put the positive probe into each side of the connector. With the ignition in off, I do not detect any current from either wire. With the ignition in the on position (engine not running) I can read about .2 volts from the purple/green stripe wire and nothing from the tan/green stripe wire. My questions are: 1. What should I read as voltage and ohms of the tests that I did? 2. Where does the opposite end of the connector go and what does it plug in to? I crawled under the dash and there are about a million wires and I couldn't find the wire pairing. 3. What's the purpose for each wire? 4. Any suggestions on further actions?

 

It reads 0 ohms when no brake is applied and 3-4 ohms when some pressure is applied to the brakes. Since that switch is a normally closed switch, this appears to be a good switch???


A normally closed switch will have continuity when not acuated. Do you mean the meter reads open or truly 0 ohms? All switches have some resistance when closed, true 0 ohms is difficult to achieve. If you mean open, the switch is acting backwards for a normally closed switch.

If you mean 0 ohms, when you press the pedal it should go open, no continuity.

You have to test the switch with the wires disconnected.