Check Memo for Intermittent Electronic Problems

Three years ago we had a problem with one electronic control we produce that was mounted onto a new, off-road truck. At the heart of the system was a PLC (programmable logic controller).

The system kept failing intermittently. We sent a man over 1,000 miles in a truck (at far greater expense than the cost of the system), but he could find nothing wrong. We replaced the entire system anyhow, just to be sure.

It failed again and we sent our guy back. He checked everything, tried every possible cause, and could not make it fail. He checked power. It was a little low but well above our minimum. He assumed this was due to an undersized/over-utilized power feed (positive) wire. We see this all the time. Since he was there, he completely replaced the entire system again.

It failed again the night after he left.

We have over 400 of these systems in service and no other trucks failed. The end user, a major oil company, blamed our system for not being rugged enough to handle the truck environment. Over my protests they had the truck shipped to the manufacturer, at great expense to my company, so they could modify it to add an extra battery and an isolation system to protect our "fragile" electronics.

We were called out to the factory for a meeting with the customer to go over the truck. The top truck repair/rebuild mechanic was puzzled. He knew our system was rugged; he had worked with it for years. He could not make this one fail. When everyone went to lunch, he stayed and worked on the truck.

When everyone returned from lunch, he announced he had figured it out and demonstrated. First he turned on our system, then he turned on an area light, only used at night, and our system failed. He repeated it over and over. It failed every time. The customer asked how this could be and the mechanic put a voltage meter on and showed that when the light turned on, the voltage to our system dipped to less than 9V DC.

He said, "No electronics can run on less than 9 volts in my experience."

The culprit? He pointed that they had grounded (for the negative power connection) our system and the light through a stainless bracket, one with lower conductivity than steel. The solution? A copper negative wire to the vehicle power system.

He said, "In my repair shop we have been putting copper negative wires in for all electronics on every truck we rebuild for years."

His boss asked, "Well then how did this new truck get built this way?"

The old mechanic smiled and said, "I have no idea, I sent a memo to the factory on this three years ago and I guess no one read it."

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