Diagnostic tips: Keeping it simple

Jan. 1, 2020
  This month, I’ve got a few simple, but maybe not so obvious, tips to share with you. After that, TST member Ed Hazzard offers a few lessons learned working in the bays.

This month, I’ve got a few simple, but maybe not so obvious, tips to share with you. After that, TST member Ed Hazzard offers a few lessons learned working in the bays.

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Changing out a battery is a routine task, but have you ever been bitten by a radio that locked out as soon as you disconnected the cables?

Let’s start with a couple of ways to keep that “Keep Alive Memory” going when we change out a battery, or have to disconnect one to perform a system test.

One way is to get another fully charged battery or a commercial 12-volt jump pack and connect it to the feed side (that’s the B+ post) of the alternator and a good engine ground. If the vehicle has posts under the hood for jump-starting, then you can connect to those instead.

Another way is to use a jump box with a diagnostic link connector (DLC). There also are DLC jump leads that have a power cord connection on the other end designed to plug into the power cord outlet on your existing jump box. Simply plug right into the DLC connection under the dash.

Using either method will make sure the control modules stay powered up when you disconnect the main battery. That means no reprogramming radio codes or relearning any engine/transmission management parameters. Just remember, though, that the positive battery cable is hot. Don’t let it arc to ground while you’re working. Use a shop rag to wrap the end of the cable, then tie wrap or tape the rag in place until you’re ready to reinstall it.

This box has a DLC connector that will keep all the modules going. The old days of a 9-volt battery plugged into the cigarette lighter are long gone.

Using an Amp Clamp
Amp clamps are great, because there’s no worrying about blowing fuses, it’s non-intrusive and it’s easy. There are two kinds of amp clamps: low current and high current.  They work exactly the same. To work properly, though, the amp clamp’s jaws need to be entirely closed and the 9-volt battery that powers it has to be good before you calibrate it.

You attach the amp clamp to your COM and V/mV ports on your meter/scope (those are the same connections you’d use to take a voltage measurement), put it around the wire you want to measure the current in and that’s it. Some scopes require a BNC connector, which allows you to plug into one channel on your scope.

But wait, the meter you’re using an amp clamp with is not set to display amps. That’s because the amp clamp converts the current it senses into a millivolt signal your meter or scope can understand. Take a look at the meters we have connected at the battery during a check of key off battery drain. The readings are almost exact. The meter to the left is installed in series between the negative battery cable and the battery and set to the DC A scale, reading 14.14 amps (or 14 amps and 140 milliamps) while the meter to the right is using an amp clamp and is set on DC volts, reading 1.449 volts (which, when corrected per the amp clamp makers directions equals 14 amps and 490 milliamps). Note the headlights are “on” and a circuit breaker is used in series with the amp meter (left meter) to protect it from an accidental current overdose.

1994 Chevy Lumina 3.8L Late-Shifting
By Ed Hazzard, TST Associate
The vehicle in question is a 1994 Chevrolet Lumina with the 3.8 V6 (Vin L) engine with 118,000 miles on the odometer. The customer was complaining that the transmission would shift late, the engine would idle erratically and the check engine light would come on.

Identical meters, one using an amp clamp on the millivolt scale and one reading directly using the DC amps scale. Both reading nearly identical.

The technician working on the car told me he had a code P0122 stored in the engine control module (ECM). That’s a code for Throttle Position Sensor-Low Voltage. A throttle position sensor (TPS) that gives the PCM an incorrect reading could allow the transmission to shift improperly, so the technician replaced the TPS and the vehicle ran fine. But the repair didn’t last, and the shop replaced it three more times without discovering why the car was eating sensors. Now I know what your’re going to ask next, and yes, the tech did install a quality part, even using an OEM part from the local dealer on the last swap.

Even after replacing the sensor with the OEM part, the vehicle returned a couple months later with the same concern. Now this becomes “The Challenge.” It makes you ask yourself, “What did I miss, what didn’t I see?”

I hooked up my scan tool to look at the data the sensor was displaying and the voltage was, indeed, low. The correct specification for this throttle position sensor Key On Engine Off (KOEO) is 0.33-0.46 volts. My reading was only 0.06 volts. Next, I disconnected the TPS harness connection and checked the 5-volt reference from the PCM at the A terminal of the connector. The reference voltage was correct.

Checking the sensor’s ground at terminal B showed no problems with excessive voltage drop either. Sure is looking like a failed sensor, isn’t it? But the repeat failures is telling me there is a reason all of the replacements experienced a short life span.

To keep KAM alive, you can connect a jump box or free-standing battery to the alternator B+ post and a good engine ground.

So, what am I missing?  Based on my interpretation of how the system works it seemed like the sensor has failed again. After all, my reference voltage and my ground were good. I removed the sensor from the side of the throttle body to do a bench test (which on this vehicle is no picnic) and you won’t believe what I saw. The backside of the throttle position sensor was covered in a white powdery substance. I’d seen this kind of stain before: antifreeze.

The antifreeze was seeping out of the throttle body (which has a couple of coolant hoses going into it) and into the sensor. On this car, it was almost impossible to see unless you were looking for something that wasn’t obvious.

Upon removal of the throttle position sensor you could see traces of dried coolant residue where the sensor was mounted, but the coolant loss wasn’t enough to alert the technician that a coolant leak was present. Unless he was being really observant, of course.  Lesson learned? Use your eyes, your ears, your nose,  your hands, and especially your brain; the tools God gave you. Those are the tools you really need to diagnose the car.

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