I know that this is kind of off-topic but.... My father-in-law owns this '93 FLD and is having a problem with the cruise control. The cruise control is inoperative at anything under about 1200 RPM. The fast idle function works fine, the brake switches are working, and I have checked the speed sensor on the back of the trans with an AC voltmeter and it is producing voltage when the output shaft is spinning. I'm thinking that it is a module/computer issue but if I had a wiring diagram or some suggestions from someone who works on these things for a living, it would be very helpful. Does anyone know whats wrong? or maybe where I can find a wiring diagram online?
The truck has a 3406E CAT engine if that makes a difference.

Thanks in advance!!!

 

i believe 1200 is below the operating specs of a 3406. cruise thinks engine is being lugged below min. rpm and disengages or will not set.
Clark

 

When we have the drive axles off the ground, we can get the cruise to engage up to, but not above, 1200 RPM. Now this could very well be just the fast idle function working to that point, but I'm not sure. Anything above 1200 and the cruise will not engage.

 

Are there any active fault codes? Everything else working correctly, like the speedo, brake lamps. The cruise control and throttle advance both work through the same part of the ecu...so if the throttle advance is working I would start by checking the dash switches and all related connections. Becareful not to zap the ecu.....use a meter no testlights allowed. BTW Freightliner wiring sucks....... Access to a pro-link or a laptop with dealer software would be very helpful.

 

Not all, but most trucks that I have worked on with cruise, will not engage cruise control below 1500 rpm, or 60 kph ( 37 mph) Unless custom purpose set by the engine / truck dealer.

Anything above that, you are good to go.

 

I have no idea if there are any fault codes. I am an ASE Master Auto Tech but this big truck stuff is kind of out of my territory of expertise. Yes, the speedo and brake lamps are working. I suppose that I could backprobe the speed sensor input wires into the ECU, just to make sure that the signal is making it that far. Any idea where the ECU is?

Even thought I wasn't sure, I figured that was how it worked, since there is no cable/linkage to the injection pump.

This particular model uses the cruise control switches for the throttle advance and they both work, so I assume that those components are okay.

The cruise control worked fine on this truck until a couple of months ago. Since I don't have a way access any data from the ECU, and I fear that may be where the problem is, it looks like it may have to go to the dealer. With some Freightliner service information I may be able to check a few more things, if I can find any.

Thanks for the input guys! Keep it coming!

 

The ECU is located on the drivers side, mounted to the engine just above frame level, very easy to access. Sounds like your going to need to boot into the ECM to actually see what is going on. Have you done any recent battery or alternator installs? I have seen some options return to default from temporary voltage loss or voltage spikes. It might be as easy as re-enabling it.

Good luck, Tim

 

Thanks Tim. I'm not sure about any recent electrical service but I will find out. I don't suppose the service program is available online, is it? I don't have a cable to join my laptop to the truck's DLC, but I could probably rig something up if I knew which pins to use. If heavy truck manufacturers are anything like light duty though, the software will be proprietary to dealers only.