the vehicle has been driven through a complete drive cycle and the readiness codes have reset. But assuming the system hasn’t
been altered by clearing the codes yet, the procedure for checking the Long Term Fuel Trim is simple: • Connect your scan tool to the diagnostic link connector.
• Set it for Global OBD-II (figure 1). • Check for any codes in memory; record any codes you find.
• Don’t clear the codes yet. • Start the engine and let it run until the engine reaches normal operat- ing temperature.
• Check and record the Long Term Fuel Trim at: 1. Idle (figure 2) 2. 1500 RPM in park (figure 3) 3. Normal cruising speed while driving (figure 4)
Don’t bother checking it at
wide-open throttle; under those conditions the system will drop out of closed loop and your readings will be meaningless. Examine your readings: Under
each condition they should be around 0 to +7. If they’re outside of those readings, it can affect transmission operation without setting any codes or affecting emissions. So consider look- ing more closely for a problem in the computer system, or at minimal, docu- menting your readings. If there were any codes in memory,
check them first, before going any- where else. If there were no codes, start by checking powers and grounds to the computer system, including the battery terminal ends. Then move on to the sensor cir-
cuits. In most cases, the problem you’ll be looking for will involve a load sen- sor, speed sensor, or temperature sen- sor. These are the sensors that have the greatest effect on transmission opera- tion. (Of course, other sensors will affect the Long Term Fuel Trim.) One thing to look for is whether
there are differences between the banks on V-type engines. For example, if the left bank’s Long Term Fuel Trim is within specs but the right bank’s is pulled negative, you know the problem is related only to the right bank, such as a fuel injector or primary oxygen sensor. There’s no reason to waste time
GEARS January/February 2012
gcor-just-ask-quarter.indd 1
Figure 4: The Fuel Trim reading look normal. The low reading at an idle is worth noting. You may not have any problems now but it’s good to note this in case the customer has problems later.
checking things like the fuel pressure or sensors such as the MAF, TPS, MAP, IAT, or other systems that will always affect both banks. When it comes to oxygen sensors,
remember that the only oxygen sensors that affect engine performance and fuel delivery are those in front of the cata- lytic converters. The ones behind the catalytic converters are only there to measure converter performance. They have no effect on fuel delivery. When it
comes to check- ing Long Term Fuel Trim, Robert offered this sug- gestion: “I’d rec- ommend check- ing the Long Term Fuel Trim on every car that comes into your shop, whether it has a transmis- sion problem or not. And record the results on the repair order. This gives you a chance to become familiar with how Long Term Fuel Trim should look on a wide variety of vehicles. And that record could be useful if the customer comes back later with a
new problem that ‘wasn’t there before you touched it.’” Long Term Fuel Trim won’t point
you directly toward any one problem, but it’s a terrific way to help answer one of the most important questions when it comes to transmission diagno- sis: Inside or outside? It’s not a pinpoint diagnostic tool, but there’s no question that Long Term Fuel Trim is a nod in the right direction.
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31 12/30/10 12:54 PM
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