Tech Stuff

‘Been there, done that’ -- Advice from experienced service technicians


We had a 2004 Chevy Silverado 2500 in the shop, equipped with the 6.6L Duramax diesel engine. The MIL was on and the vehicle exhibited intermittent low power. Code P1093 was stored, which indicated low fuel rail pressure during power enrichment.

We first connected a vacuum gauge at the fuel service port. While cranking the engine, it should show three to four inches of vacuum. At wide open throttle (in park), you should see four to five inches. During a test drive under hard acceleration, you should see eight to 12 inches of vacuum. If vacuum is too low, you probably have air in the system. If vacuum is too high, you probably have a restriction, which could be a damaged/kinked fuel line or a clogged fuel filter.

If your vacuum checks meet specification, inspect the fuel return from the fuel pressure relief valve (located at the rear of the left side common rail), and check the fuel return rate of the injectors. In our case, we found high vacuum. Upon further inspection we found a mashed fuel line, the result of someone’s previous service. It looked like someone had used a pry bar that pushed against a section of the line.

Information courtesy of Teri McCoy, McCoy’s Tire and Auto Service.

Place your voltmeter across these two pins to check SCP voltage.
<p>Place your voltmeter across these two pins to check SCP voltage.</p>

Place the voltmeter or ohmmeter across these two pins to check the high speed CAN network.
<p>Place the voltmeter or ohmmeter across these two pins to check the high speed CAN network.</p>


One of the most frustrating things that can happen when a technician is trying to diagnose a concern is to find a lack of communication with the module or modules he is trying to diagnose. In most cases, there are some simple tests that can be done to quickly determine why a module is not communicating.

Let’s start with the most simple, yet very common issue: Check power to the data link connector. In many cases, Ford uses the same fuse for the data link connector and the cigarette lighter, so it is fairly common for that fuse to be blown. Power will always be on pin 16, the bottom, far right pin of the data link connector. Power loss will affect all of the modules, and if there is a lack of power to the data link connector, none of the modules will communicate.

If only one module has lost communication, test the communication circuits. There are many communication systems, but this tech tip will only cover the standard corporate protocol (SCP) and the controller area network (CAN). SCO is used in earlier-model Ford vehicles with OBD-II systems. CAN is used on most current model year Ford vehicles, and for this tech tip, we’ll only cover the high-speed CAN.

Tags: diesel 
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