First time poster, I’m a diesel tech who’s been out working on a cat 279d. The customer had another person install a new coupling between the motor and hyd pump, after this was done the machine wouldn’t start by the key. I was then called, and let it be known I’m not a CAT fan. I tested every safety switch, they’re all functioning. Tested power to the relay, good. Swapped relays as i know they’re a problem with a non oem relay, no change. I’ve ohmed the green and white wire from the ecm to the relay and they’re good. I hooked my meter to the green signal wire coming from the ecm to the relay, with key on and all safety switches down I’m only getting .14v, now i understand cat uses the relay signal and ground to communicate with the ecm that safety’s are down and it’s good to start. What i don’t know is what that voltage is supposed to read, I’m assuming my ecm is fried on this leg, any insight would be helpful
Since it was after major work, I'd be looking at ground wires. One was forgotten?
main frame ground was good, engine ground was originally loose but i tightened it down, and cab grounds were never touched
Arm bar switches alone are usually a 3 wire switch with common earth (to an ecm pin, not chassis), and two ecm inputs (one NO and one NC for redundancy). Has to be something silly like an earth. There's a lot of them. Did you just pull the pump to do the coupling or did the engine come out too? Does it have the monitor screen in the cab? If so any diagnostic codes?
All those tested good, and the machine will run, move and work just fine if i jump the starter. Also i am not the individual that removed the motor and replaced the coupling, I’m following behind the man who did.
The problem's not the interlock system then. Post the s/n and we can look at schematics.
See where this procedure takes you. One thing I noted was that you never mentioned having tested continuity on the 304 WH wire between the relay and the starter solenoid.
continuity between 304 from relay to starter is good, I’ll go through this procedure and see what i come up with
Also, s/n is: CAT0279cpmbt01019
Also, my green 306 is at pin 7 on the ecm, and white 808 is at pin 8
That's why it always pays to give a Serial Number up front. I should have mentioned the procedure I posted above was somewhat generic and ECM Pin Numbers might be different. The schematic for machines up to MBT1497 confirms what you found, as does the 444-5 troubleshooting procedure for the S/N.
Awesome, I’m going to pin out and test resistance between the two, jumped wire is in place. Get back with y’all on what i find. Thank you so much!
Zero continuity on green wire, believe the problem is found. Thank you sir
Hold on a mo' - from your OP......... I'm somewhat confused.
Me too.
Yes i had, but not from the relay pin, i used a tie in adapter at the wire with my meter. I had continuity, but once i went to the spade i had lost it. The problem was in the plug
Needless to say, don’t always trust new tools
You should invest in a pair of spoons for back-probing terminations in harness plugs/sockets if you don't already have them. https://www.amazon.com/Electronic-Specialties-616-Spoon-Probes/dp/B07Q2MZSYH