Mechanic Talk - Peculiar Problems In The Field

Mechanics Hub - June 8, 2014
The Bad Ground Syndrome

Have you ever experienced a problem with a light on a vehicle that shines very dim and seems to be illuminating another totally unassociated light? For example if you have the signal lights on and there are other lights flashing on and off as well you may have a bad ground. I always check the lights closest to the environment which would be the rear lights that get all the abuse from the weather.

If the signal light can not complete ground back to the battery the resistance will cause it to go dim. The battery feed to the signal lights will find the least resistance back to the battery ground. If another light has a good ground then the battery voltage will back feed through that light and it will flash just like the signal light since the battery supply from the signal switch is going through the signal flasher.

So if you can imagine the flow of voltage from the battery being like a flowing river and then it comes up to a dam that's half closed.....that is resistance to flow. Much of the same principle applies to electrical circuits with bad grounds, open circuits and short circuits. If a wire is hanging by a strand then the resistance is extreme. All of those tiny electrons can not get through to supply amperage to run the load.

A Simple Tool To Test An Electrical Circuit

In my shop we have gotten into the habit of checking a circuit with a load. If you have an accessory that does not work but you have a 12 volt reading you may be fooled into thinking that the circuit is good. Grab an incandescent bulb or headlight and hook the suspect wire to it and run a ground for the test light.

If it lights up normally you have a good battery circuit but if it goes dim there is a problem with that wire. This is a quick check we use all the time and saves grief when troubleshooting electrical problems. The other option is to find both ends of the wire and use a multi-meter to test the resistance.

Exhaust Restriction Causes A Peculiar Engine Power Problem

If you've ever seen movies where someone stuffs a banana in the exhaust tail pipe you'll notice that the vehicle dies and isn't going anywhere. Engines hate exhaust restriction. The back pressure is murder on proper combustion. Even if there is a percentage of restriction the symptoms will be no power and slow speeds. There won't be any engine miss or clouds of smoke just a lack of power even at full throttle.

Two problems I went through was an EGR valve blocking the muffler on a gas engine and the exhaust brake sticking closed on a diesel engine. We have a water manometer in our shop and there should be very little movement on a healthy engine exhaust system. However if you have a high exhaust blockage the reading will go through the roof.

The manometer is old school these days and now diagnostic software will do the same check. Just plug it in and set your eyes on the engine readings. You can also read turbo boost and fuel pressure as well when you need to. It's part of the job to find problems and repair them. That's what I like about this trade is the constant problems that pop up out of the blue. But that's what keeps it interesting.
Mechanic Life