Troubleshooting a Backhoe Electrical System

This winter I cruised up to Centerville, Idaho, coffee in hand, to troubleshoot a backhoe with a customer complaint of the transmission neutralizer switch not working

After confirming the customer complaint. I began by inspecting the switch, I swapped the neutralizer switch with the transmission kickdown switch and it made no change in the switch’s operation. I referenced the schematic and noted the parking brake switch also commands the neutralizer relay. Upon inspection, I noticed that the parking brake position switch was misaligned, which should cause a problem with the neutralizer circuit.

 I bent the parking brake position switch back into shape but it also made no difference.

Figuring it was something a little more complicated, I next looked at the fuse box. I noted the transmission neutralizer relay wasn’t getting a signal from the neutralizer switch. I manually overrode the relay but it made no difference. I inspected the wiring behind the display panel so see if there were any obvious issues. I unplugged the main cab wiring harness connector and noticed a terminal wasn’t fully plugged in. I looked up its wiring number, it was for the headlights…. so not our neutralizer issue. Still on the hunt. 

I looked at the fuse box some more. I thought it was really odd that I could override the transmission neutralizer relay, but the machine would not shift into neutral. The transmission neutralizer relay wasn’t receiving any signal but also would not enable or disable the forward or reverse relays. I was very confused. I referenced the wiring diagram (below) and saw that the wiring to the forward and reverse relays goes through the transmission neutralizer relay. It all made zero sense.

It seemed to me if the neutralizer wiring or relay had failed then the machine shouldn’t move at all. I was confused, I figured the machine must have some type of bizarre short that was allowing the forward and reverse relays to still get power

I crawled down under the machine and gave the transmission wiring a look. I noticed a new valve body and some wiring repairs — my curiosity was peeked.

I investigated a little further and found a splice on the transmission neutralizer circuit.

The transmission neutralizer circuit had been bypassed and spliced directly to the forward and reverse circuit. The hardest issues to troubleshoot are human caused. The reason the transmission neutralizer didn't work was that someone had overridden it for some reason. 

I called the customer and they then informed me that it had “broken” at some point years ago and that someone had “fixed” it. I contacted JCB and a few different online parts suppliers to source a new transmission wiring harness because the existing one was full of “repairs.” This machine was manufactured in 1999 and no wiring harness was available. 

So this job ultimately had an uneventful end. Unable to find a replacement harness, the wiring diagram does not provide a connector pin out so I could not make a new harness. Though successfully diagnosed, I wasn’t able to perform a professional repair for this issue.