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FIX-IT, Technician
Category: Heavy Equipment
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Freightliner FLD120, Series 60 DDEC III with erratic Jakes

Customer Question

I have a 1997 Freightliner FLD120, DD Series 60 with erratic Jake Brakes. The problem seemed to start gradually. Sometimes when I wanted the Jakes to work (descending a hill, etc.) they would not work. To fix the problem I would stop, turn off the truck, restart, and the Jakes would engage flawlessly. Now the problem is becoming worse, and sometimes restarting the engine does not fix it. It seems like when restarting the vehicle something was getting reset. I thought I might start by checking/replacing the throttle position sensor (this is the switch on the foot throttle pedal, right?) Any advice on how to check my TPS or what else the problem could be? I'm not an experience heavy truck mechanic, but I am good at figuring things out. Thank you for any help you have. -Sam
Submitted: 6 years ago.
Category: Heavy Equipment
Expert:  FIX-IT replied 6 years ago.

Hi there.i can help you with this..the best way to diag the tps is with a shop computer with ddc can check the tps buy using a volt meter but,it won't tell you much with a jake problem,when the jakes don't work try tapping the throttle to see if they kick in if no luck it could be something else,like:

clutch switch,or spread pins at the ecm

or i've seen the ecm its self cause this.

now one way to see if the clutch switch is causing this is on level road,try to get the jake to act up,once it does,kick in your cruz control,if it does'nt kick in the clutch switch or wiring will become suspect,if the check engine light comes on let me know

get back to me with your results..

Customer: replied 6 years ago.
Thanks for your reply. I want to let you know that I've never been able to get the Jakes to work when in cruise control, and I didn't know if this is normal or not - should they kick in? When descending a hill in CC (foot off clutch and throttle) the vehicle speed will increase and the Jakes will not kick in. If I flip the cruise control off then the Jakes come on (all off this assuming Jakes are set to "Auto" not "Off".

There have been times when I was able to get the Jakes to work by tapping the throttle, or reaching down and trying to push the switch back - but usually this has no effect and the Jakes stay off.

Also, sometimes they come on intermittently - for only a second or two, then go off for a second or two, then back on...

One final note, sometimes my tach quites working and shows 0 rpm. This does not seem to correspond with the Jakes at all, and lately the tach has been working 100% of the time.

Thank you. I hope this info can help you better help me.
Customer: replied 6 years ago.
Check engine codes:

I recently had code 22 and 46. My alternator belt was loose and have not had these codes since tightening the belt. I have no active codes.
Expert:  FIX-IT replied 6 years ago.

ok 22 pertains to your tps or wiring....related to your problem...yes

46 pertains to you ecm battery voltage being low..would'nt say its related at this time

how are you with wiring?..we may have to do some checks

but go to your ecm at the rear of ecm at the bottom is a rectangular plug with many circuts going to it,remove with a 1/4 inch nut driver,inspect pins for corrosion and being spread...very common on your vintage of engine..let me know bro..

Customer: replied 6 years ago.
OK, I'll check that harness tomorrow. By spread pins do you mean pins that aren't making contact (open circuit)? I haven't had that plug apart so I don't know what the pins look like, and don't know how to fix them - maybe it will be obvious when I see it. I'll take it apart tomorrow, inspect, and apply some dielectric grease. I'll also look at my clutch switch - I assume that should just be a simple on/off switch.

I have a simple multimeter and can test things like voltage and resistance.

This is the first and only truck I have with Jake brakes. Should they come on when using cruise control? When reading my manual I thought they should, but they never have.

Thank you
Expert:  FIX-IT replied 6 years ago.

with the jakes coming on with cruz,thats a parameter thats set in the ecm and it looks like your engine isn't programmed for this,it can be though,just have to go to dealer and have turned on..

yes those pins at the ecm are very small and just compare with the other ones,they should all look the same.or take a good picture and send it to me on this site,,go to the clutch switch and you'll see the wires going to ring terminals and be sure the screws that fasten terminal are good and could have a loose screw there at the cluch switch..possible...keep me in the loop.

Customer: replied 6 years ago.
I took a look at my clutch switch and it seems to be working correctly. I then started the truck and was not able to get the Jakes to come on. I removed and inspected all the plugs going to the ECM (DDEC III) and reinstalled with dielectric grease. They looked good to me. I took some pictures, but don't know how to post them here or send them to you. When I started the truck the Jakes worked! However, after shutting down and restarting they didn't work again - I played with the throttle pedal and clutch - nothing worked. After a restart the Jakes once again worked, and kept working (I haven't driven the truck, just testing in the shop.) I'm not sure how the TPS works - it seems that the ECM assumes that whatever position it is in when the truck starts is the idle position. If I start the truck with the throttle pedal part way down the truck idles at that position, but letting off on the pedal creates a new idle position. I can get a new TPS for $80.08 from Freightliner, or I can take it in to Freightliner and have them diagnose it for $104.50. Since sometimes it works and sometimes it doesn't I wonder if they'd find anything wrong. I could also take it to a different truck shop, but it seems that the folks at Freightliner could diagnose it as well as anyone. What are your thoughts? Thanks
Expert:  FIX-IT replied 6 years ago.

yes,take to freightliner

if it does'nt fault in the shop they can go for a road test with you with the computer and you'll know for sure.keep me in the loop and let me know how u did..

FIX-IT and other Heavy Equipment Specialists are ready to help you
Customer: replied 6 years ago.
OK, if I do take it to Freightliner should I have anything else done while I'm there? A tune up? The truck has about 422,000 miles on it and I've never had any work done on it (I think I've put less than 50,000 miles on it since I got it.)
Expert:  FIX-IT replied 6 years ago.

if it isn't broke don't fix it

but if you unsure when the last tune up was done then do it!

every 100,000 miles,should be done..

Customer: replied 6 years ago.
I thank you for your help. I accepted an answer, and now hope to give you a little more information. I took the truck to Freightliner and the jake apparently wasn't acting up when they diagnosed it, but they did recommend a new TPS and clutch return spring. I got a new TPS and installed it, but skipped the spring as I new that wasn't the problem. The truck hasn't got much use since then, but the jake has been working perfectly since the new TPS. It seems like I now have more freeplay on the throttle, meaning there is no throttle response for the first bit of pedal movement. They also recommended I update the software on the ECM, but I didn't have the time. Is this something worth doing? They couldn't tell me specifically what my benefits would be.

I also just replaced the alternator - when idling the volts were not always staying up at 14 where they normally do. Took the alternator in to get tested and was found to have a short - haven't road tested the truck since the new alternator. I don't think this problem is related to my jake problem, since they worked fine once the new TPS was installed - just throwing this out there.
Expert:  FIX-IT replied 6 years ago.
thanks for keeping me in the loop,as for your ecm software up date i would leave it,it'll cost over 250.00 $ and i haven't seen any letters on it ,drive and let me know how she pulls and ill see you around..