Dont underestimate the alternators twist on driveability COLUMN BY KEVIN MARKEL n the February issue (DC voltage and fuel control repair, page 6) we discussed the reality of problems caused by DC voltage on engine control. There- fore, it seems fitting to cover the AC side of voltage. The most overlooked driveability prob- lems are caused by excessive AC voltage or as it may be known, ripple. Common practices have technicians checking DC voltage and then taking for granted that the AC side of the charging system has to be OK. This could not be further from the real problem. What have we seen? One problem example that I will never forget, and iron- ically enough still run into, involves the owner of a Dodge truck with a Cummins turbo diesel and automatic transmission. The complaint the customer would typi- cally convey involved poor shifting char- acteristics of the transmission. More commonly than not the vehicle had already been checked for any possible internal transmission related problems or control problems of the transmission. By some twist of fate we would end up with the truck, listen to the complaint and advise the customer of our in-depth diag- nostic procedure. We would explain that diagnostic charges are billed out in a way that reflects the testing of everything because one system failure may cause a multitude of symptoms. If the added expla- nation was understood by the customer, we got a signed work order with the approval for the diagnostic time and charges, and we were on our way. Depending on the year of the truck, we may even be working with mechanical injection, so instantly someone reading this article is thinking to themselves “He only does driveability work” and you are right. The controller on this vintage is only there for limited body and transmis- sion control. After some very limited scan tool data acquisition and no trouble codes, we make our way through the rest of the beast only to find it has excessive, and I mean excessive, AC leakage from the alternator. After the balance of our diagnostics tests are complete, it is time to call the cus- tomer. Imagine informing the customer they need an alternator for their shifting problems. The first time I had this situa- tion I was accused by the customer of not being competent, if we want to be nice about it. His response to my diagnosis of the alternator being the problem involved the following remarks, “The truck runs, the battery warning light is not on, the gauge shows charging and the eye is still green on the battery. What do you mean it needs an alternator?” Inevitably when the customer could not understand, I would come back with, “If it does not fix the problem, you don’t have to pay for it.’ You guessed it. We would replace the alterna- tor, take the truck out for a drive and presto, problem solved. Now the cus- tomer’s turn for a drive, they return in complete disbelief and amazement. You may be wondering at this point how in the world the alternator caused this problem. Anything that has battery volt- age going to it can and, more often than not, is affected by the excessive AC volt- age. As you know there is always a small amount of AC coming out of the alterna- tor. In simplest terms, if there is a large amount of AC voltage, it will affect the transistors in modules and control units such as an ECM or PCM (electronic control module/power train control module). The sad truth is that I have seen several parts thrown at a vehicle and have had other occasions involving Dodge trucks with the Cummins, which had already seen a trans- mission replacement or overhaul. Ouch! This may have seemed hard to believe but this is only one example of alternator problems and engine control. Take that extra step to look at the AC side of things and more importantly make sure it is not excessive. Kevin Markel is owner on Pro Auto Tech, Ft. Meyers, Fla. Please e-mail technical questions to polly@erenewspaper.com to be addressed in future issues. ET RIE|