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Learning the Digital Storage Oscilloscope

Wednesday, November 1, 2017 - 06:00
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Let’s start with an older vehicle with a stubborn intermittent stall issue. It’s a 1995 Chevrolet S10 4.3 “W” V6 with a distributor and poppet style fuel injection. The shop has replaced lots of parts including plugs, wires, cap, rotor, a different distributor, CKP sensor, a fuel pump and an injector assembly! This vehicle is said to run fine but intermittently stalls and reportedly sets no codes when it does so. I ask what has been tested and get the list of parts replaced! I ask if a scope has been tried to aid in the diagnosis and the tech tells me “it’s not misfiring and runs fines…. It just stalls.”

I decided to approach the stall in a logical progression. I start by scoping the CKP, CMP and primary current with a long time base to get an idea whether it is an ignition related issue. It started and ran fine, I happened to go to my vehicle to take a call and returned to find the vehicle had stalled. The scope has captured the stall! It is in the buffer. I systematically save the capture and then zoom in. The CMP and CKP sensors appear to be present and yet the primary current shows the coil quits firing. I now have gained some valuable diagnostic real estate, I now can narrow my focus to the ignition primary and its input and output. The IC trigger circuit is now added and it becomes obvious that the primary trigger from the PCM to the ICM drops out precisely when the IC stops firing. The CKP and CMP are rock solid. The feeds, grounds and control circuit of the PCM are inspected and found to be OK. A PCM is ordered and fixes the vehicle. Leveraging the power of the DSO and having a systematic plan of attack made this a fairly straight-forward diag.

A scope can perform voltage drop tests on circuits a DVOM cannot.

Voltage drop plus
Another great use for the DSO is to do voltage drop testing. The scope does an excellent job of catching dropouts caused by voltage drop or excessive resistance. The following is an example of this. The vehicle in question is a 2000 Grand Am that the blower motor intermittently drops out, the MIL on the dash comes on, the IPC illumination lights dim and sometimes the vehicle stalls. Occasionally, the vehicle will not start. This seems to me on the onset that it could be a ground or power distribution issue. Due to the high failure rates and pattern failures known, I suspect the ignition switch may have an intermittent excessive voltage drop affecting the blower motor and IPC circuit. A quick check of the power distribution schematic gives me direction and I design the experiment, or best way to hook up my scope, to prove out or disprove my theory.  My scope is hooked up to the following; DLC pin 16 (B+), PCM ACC fuse, IPC BFC ACC fuse and the Ignition SW Bat 1 fuse. 

I am basically doing a loaded voltage drop, testing the circuits in the environment in which they live. Turning the key from Off to On, I notice there is an excessive momentary voltage drop in the switched ignition circuits at the two fuses. I also notice in the capture the cranking voltage is low due to a weak battery. The battery is changed out but the voltage drop and electrical accessory issues remain. An ignition switch is ordered and replaced and the vehicle is remedied. 

Testing mechanical integrity
Perhaps one of the great uses for a modern DSO labscope is to check mechanical integrity. The relative compression with sync test has been detailed many times in the magazine so I will not include it again. Scoping the PCM inputs of the CKP and CMP sensors can give us an excellent indicator of engine base timing between the crank and the cams. CKP-CMP sync captures help verify engines that are out of time. Moreover, it can be a great aid diagnosing those pesky CKP-CMP correlation codes caused by stretched timing chains. The vehicle in question is a 2007 Equinox with a 3.4 VIN F V6 that the MIL is on. The engine was a freshly installed remanufactured unit that would turn the MIL on shortly after start up. Multiple CKP and CMP sensors, both aftermarket and OE, were installed. The reluctors were inspected for chips or nicks and a used and new PCM were installed. Several hotline phone sessions have not resolved the issue. A local mobile diagnostic company (mine) was contacted to try to get some closure.

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