Tuesday, November 30, 2010

Mechanic 12 Volt System Troubleshooting-Reverse Warning System Stuck On

Working on a fleet of buses has some amusing moments. Especially when 12 Volt systems and moisture come into play. Mechanics go to school to get the basics, then you need to use those basics and go much deeper. Experience indeed is the best teacher and mechanic information on symptoms and troubleshooting can be shared and recorded for the benefit of the entire shop and right here on this blog for future use.

Below is what I call a 'Mechanic's Special'

The driver complaint is the back up alarm and reverse lights come on all the time even in neutral. So the first thing to think about is what could possibly cause this to happen without touching the controls. With the key on we have full operation of the back up warning system. The HDX Thomas pusher we are working on has the VIM (vehicle interface module) mounted above the steering axle (Lots of snow and ice). This is the first place to check since the interface module receives the reverse signal from the Trans ECU after the driver selects reverse.   CLICK ON PICTURE TO ENLARGE.
The VIM has relays and fuses (BELOW) to control the circuits on the chassis side of the bus including the high idle, neutral start and reverse warning. Luckily once we pulled off the lid to the VIM we heard a buzzing noise. This is not normal and is a mechanic troubleshooting red flag, you want to look for abnormal mechanical situations.

After opening the VIM up the problem was obvious. The corrosion was making a circuit from battery to the reverse warning system. Moisture, oxygen and 12 Volt electrical circuits love each other and create double trouble after uniting for a given period of time.
Here is my 'KEEP IT SIMPLE' drawing (BELOW) of the circuit in question. The operator chooses reverse, the transmission shifts and the reverse signal is fed to the Vehicle Interface Module to the reverse relay and 12 Volt power feeds the reverse alarm and lights.

Once you know how a circuit works it makes the problem solving much easier and common sense will prevail. This problem can happen anywhere so be prepared. Checking electrical problem circuits and components closest to the environment pays off more times than not. I hope you got something out of this Mechanic Information.

Please leave a comment below and share any 12 volt electrical experiences you've had in the past or just let me know you got something out of this info.


The Rudstroms said...

I'm always amazed at how much of my time spent working as a "mechanic" is actually working on electrical problems. Corrosion (caused by water and salt) is the source of most of those problems.


J. E. Whelan said...

We just finished cleaning up some corrosion that caused an intermittent engine code.

Anonymous said...

The strangest electrical issue I've seen was from a 89 honda crx 1.5. Out of no where the car would not start, crank only. I started noticing it would happen on slight inclines once parked. Good thing the car was lite in weight and I was young enough to push, jump and pop the clutch. Then one day that was it, not starting. I traced the circuit down to a fuel relay. the relay was a ganged switch relay and to this day i'm still fully cornfused about the circuitry. I take it the internals of the relay were sticking when parked on an incline. Rr relay and shazam! problem solved!

J. E. Whelan said...

Strange Indeed!
I had a 350 engine in a pickup that went 35 mph tops at full throttle. After some trial and error it turned out to be the EGR throttle plate. It broke off of the EGR and made it's way down to the inlet of the muffler blocking it off.

itcetera said...

I know what you mean about fighting corrosion. It can be a full time job in it self. I especially learned how important corrosion prevention and protection is as an aircraft mechanics at Boeing Field in Seattle. The best stuff I have ever found to stop-prevent corrosion is Battery Corrosion Guard. It is only sold in the north west at this time but it is great stuff.There is one web site that carries it that I know of, http://Theelectricaldepot.com. Several log trucking companies in the area use it not only on battery post but on any bolts they remove and put back on. Worth checking into.