D8t serial number KPZ01569. Brakes will randomly engage while in first gear. Only does it in first gear not second and only under 1400 rpm. I hooked a pressure gauge up to the brake pressure tap and have good pressure around 345 psi at low idle in neutral and while traveling in first gear. Most the time the pressure is good and no issues then randomly only at lower rpm the brakes will grab and my brake pressure will drop 100+ psi. What would be causing the sudden drop in brake pressure like that? And why only in first gear? Second gear doesn't do it. No codes come up. The transmission was pulled out and rebuilt. Clutches were worn out. It was doing this prior to transmission rebuild. I had the floor out and there were two wires on the same plugin that were bare going to one of the brake solenoids i believe it was for the secondary brake solenoid. So i fixed the wires and replaced all the solenoids on the brake control valve. It's like there's a current drop to the brake control valve but why only in first gear?
Is there any relation between the brakes grabbing and the operation of the service brake pedal.? When that pedal gets close to the bottom of its travel it sends a signal that will apply the park brake. Check switch adjustment maybe.?
The pedal isn't applied at all when it happens and I don't see any brake pedal engament signal when it happens. Wouldn't it do it in 2nd gear as well? I know the signal goes through the ecm. Would 1st gear have a separate brake signal? It definitely seems like it's electrical.
No 1st gear wouldn't have a separate brake signal from the pedal. I agree. The fact that there are no Diagnostic Codes indicates that something is behaving as it is programmed to behave in a certain set of circumstances, even though that behaviour causes the undesirable result of a parking brake application. The only two inputs that I know of that affect the application of the parking brake are the park brake switch and the service brake switch. Digressing for a moment. How are you accessing the Codes.? 1. The PB switch is a double-pole switch. If the primary pole circuit malfunctions it will set a CID 0070 Code, either 02, 03, or 04. If the backup pole circuit malfunctions it will set a CID 0618.02 Code. 2. If the service brake switch circuit to the brake dump solenoid malfunctions it will set a CID 0722 Code, either 03, 05, or 06. However if the switch has a some sort of problem (out of adjustment.?) that causes it to activate when it shouldn't then there will be no Code - but the dump solenoid will still operate and engage the PB. My suggesrion is that, because the service brake switch is the easiest one to access, temporarily disconnect the wire from the NO connection of the secondary brake switch. This may create a Code whenever the pedal is pressed but it ought to work for a test. Operate the machine and see if the random PB application continues or stops.
I'll have to try that out. I can see the codes on the dash display screen and I also have cat et I plug in.
Sorry for a bit of off topic, but this issue is scary. Can you imagine pushing over an edge up over 100 ft and having the brakes lock at the last moment and can't back up? What a very dangerous feature. If they lock in 1st gear can you immediately grab second gear and go or what? In all my life of working with any sort of machinery the most dangerous things I have come across is any sort of safety feature or device.
If I’m right then to get out of the situation will require to shift the transmission control to neutral, cycle the park brake switch to ON then back to OFF again, then re-engage a gear. I may be wrong.
Do me a favour. Hook up ET and download a pdf copy of the Product Status Report (PSR) then post it up here. There are a few settings I’d like to look at.
The pressure seems low or off to me, I don't have E.T. but often the second speed clutch is around 370 psi, 1st gear at 320 psi and P1 pressure around 425 psi, with this pressure supplying the brake control valve??? 1st gear oil being controlled by the modulation valve can suffer with the oil feed via the hub seals, I have found a wire loose going into the plug of a modulation valve, does this Tractor have a Nitrogen accumulator fitted? p.s. the pressures I have posted are what is common to find on P.W.M. controlled transmissions so just a general comment, Nige will have all the correct detail.
Tony, from this OP I get the impression he is talking about the pressure measured in the park brake release circuit. At least that's how I read it. That's supposed to be around 350psi IIRC. Only on the implement pilot circuit, not the transnmission.
I agree totally.
If this one has the accumulator I would check that first.