Hi my name is Paul. I have been reading the forum for quite some time and have finally registered so I I I can interact. I'm having trouble with my 972h With the machine in reverse the backup alarm is quietly buzzing. I ordered a new alarm to find it did the exact same. Now I have checked voltage and it's showing 2.3 volts while trying to alarm but if I unplug the alarm I show 20 volts on the hot wire. I don't understand why I'm getting this big drop while connected. Any ideas?
First guess would be a bad connection where two harnesses plug together, broken wire somewhere along the way especially where pins or sockets crimp on, bad switch if it's a switched setup, or ECU output or relay if it's computer controlled. Voltage drop due to resistance. I'm sure one of the CAT wrenches will be along shortly that's familiar with this model.
Yep. Lantraxco's on the money. You've probably got high resistance between whatever powers the backup alarm, and the backup alarm. The reason you are seeing circa 24v with your DMM (digital multimeter) is because the DMM needs very little current (microamps?) to make it work. The backup alarm probably gets its feed from the transmission ECM. You'll need to tell us your serial number for more accurate information.
As above, need a Serial Number (or at the very least a S/N prefix) to check the wiring schematic. IIRC the backup alarm signal comes from the power train ECM.
It's a A7G00128
The 2 wires to the backup alarm are 321 & P990 (both Brown). 321 Brown (backup alarm power) comes from Pin 12 of Connector J1 of the Power Train ECM. P990 Brown (return) returns back to Pin 8 on Connector J2 of the Power Train ECM. I think the fly in the oitment might be the P990 wire because it is listed as a return for both the backup alarm and the start relay. In the machine chassis wiring harness there are no less than 3 x P990 Brown wires that all meet at a common splice somewhere in the harness before it reaches the cab firewall. The other 2 come from the Start Relay and the Start Relay Arc Suppressor. See below. I'd be disconnecting the two connectors that I've circled in red first and see if that makes any difference. If it puts the backup alarm right then reconnect them one at a time to find the fault. I'd bet money the Arc Suppressor (diode) has failed.
Two kinds of diodes, those that fail open and those that fail dead short. Gotta love it.
Might want to start with the arc suppressor diode. Also the 990 might be all joined at a ground cluster on a bolt. I have grounded out some of those bolts that hold clusters on cabs right down to machine frames to fix unfathomable problems. Later Bob
Thanks guys I'm on the chase
Any idea where this diode will be located? How I should test it? What is it going to look like? I'm fair at chasing electrical problems but this is a new one for me.
It's in the main power relay box. Top right according to the parts breakdown. It will look like a two pin plug on the harness which has been terminated with a blank socket. A diode is essentially an electrical one-way valve. If you check it with your continuity tester it will allow DC one way but not the other, but basically just unplug it and see if your problem goes away. You won't harm the machine running it for a short while without.
Bob, according to the electrical schematic the location where the 3 x 990 wires join is a splice inside the harness. Unfortunately the schematic gives no idea as to the location. Sometimes based on the location of components you can kinda figure it out but in my case if there is a loss of continuity I prefer to run a new wire outside the harness. Also 990 is NOT a machine ground, it's a dedicated ground back to the powertrain ECM just for the Start Relay & backup alarm. Also forgot to mention. Because it's the start relay involved I imagine you'll have to start the engine before disconnecting the relay and the arc suppressor to start troubleshooting. I was thinking start the engine first then unplug both the relay and the arc suppressor.
I'm hearing you. I suppose what I was getting at was that to operate the start relay once or twice without the suppressor connected wouldn't do any harm. But then again, I don't think you even need the engine running in order for the ECM to power the backup alarm.
You may be right about not needing the engine running.
Well I found the diode. Didn't make any difference in or out including the machine being able to start both ways. You can tell the alarm gets hit with a slug of voltage and makes one light beep then goes to a steady quiet buzz once voltage drops.
Did you also try disconnecting the start relay at the same time as you had the diode removed ..?
Well the cat field service was coming out to diagnose a issue on our 8R so I had him take a look at the loader first. He was initially thinking ecm. He went to check in coming power to the ecm and found the bolts on the wiring connectors were loose. Pulled the harnesses off voltage was good put back together tightened up the connecting bolts and the alarm is working. Big thanks to everyone for the help along the way. Hopefully this can help someone else some day.
"I'm pickin' up good vibrations.... "
More like bad vibrations shaking things loose
Strange thing that the allen bolts holding the ECM harness connector came loose. That doesn't happen often, hardly ever in my experience.
Time will tell but she's sounding strong.
Agree, maybe last person there did not have the plug fully seated when tightening the allens. Or simply forgot to tighten them.