Mechanics

FUEL SYSTEM

1998 Dodge Ram • 78,000 miles

Theory of operation.
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David d
March 30, 2012.




There's at least three different books put out by Chrysler that cover that. I don't have enough time left in my lifetime to retype all that. Can you be a little more specific? What's the symptoms. Any observations or clues? Which engine do you have? Is the Check Engine light on? If so, have you checked the diagnostic fault codes?


Caradiodoc
Mar 30, 2012.
I just wanted to know the path that the currant follows from the time that the key is turned until the truck starts and runs. And what might might be in series that could take that flow out if not working properly I have more diagrams now for reference.


Tiny
David d
Apr 8, 2012.
Actually the ignition switch doesn't do much in that regard. It simply sends 12 volts to the Engine Computer to wake it up. A different constant 12 volt supply keeps the fuel trims and fault code memory alive when the ignition switch is off.

As soon as you turn on the ignition switch, the computer will ground the coil for the automatic shutdown (ASD) relay for one second. You might hear the fuel pump run for that one second. Next, during engine rotation, (cranking or running), pulses are received from the crankshaft position sensor below the rear of the right cylinder head and the camshaft position sensor inside the distributor. When those two signals appear at the computer, it turns the ASD relay on constantly.

Voltage from the ASD relay goes to all of the injectors, the ignition coil, alternator field, oxygen sensor heaters, and fuel pump or pump relay. Safety is the reason for doing that. Unlike the troublesome inertia switches that frustrate a lot of Ford owners, with Chrysler's system, if you're in a crash that ruptures a fuel line, there will be no fuel pressure so the engine will stall. With no pulses from the sensors, the computer turns the ASD relay off. That turns off the fuel pump so no raw fuel is dumped onto the ground where it would be a fire hazard.

Different flex plates were used with different engines and in different years so you have to get the correct one when you need to replace it. There are three or four sets of square holes with different spacings to let the computer determine which piston is coming up on top dead center. With the newer coil-on-plug ignition systems, that's how it knows which coil to fire. That's left up to the distributor on older trucks. Those notches are advanced quite a bit before top dead center; 30 to 40 degrees, as I recall. Lets use 30 degrees. The computer is programmed to wait that 30 degrees to fire the coil. That puts the spark right at TDC. It advances ignition timing by shortening the delay. If it only waits 20 degrees, the spark timing would be 10 degrees before TDC. They do it that way because the computer can't get a pulse at top dead center, then fire the coil ten degrees sooner.

The camshaft position sensor's signal is used to synchronize the injectors. Turning the distributor has no affect on ignition timing like it did years ago. Only the injector timing will change and you will never notice that.

Before around the early 2000s signals were needed from both sensors to keep the ASD relay turned on and the engine running. If one sensor failed while driving, the engine would stall and not restart. Often it would restart after allowing that sensor and engine to cool down for about a half hour. Those sensors often fail intermittently by becoming heat-sensitive. On newer models, by 2004, the engine would continue running if one sensor failed but a diagnostic fault code would be set and the Check Engine light would turn on. As a backup mode, the injectors would fire along with the coils. I don't know if just the cam sensor could fail or if it would run if the crank sensor failed.

You can trick the computer into running the engine with a failed cam sensor on the older models by bypassing the ASD relay. The computer will still fire the injectors and coil but it won't turn the relay on by itself due to the missing cam sensor signal. It seems to me the crank sensors fail more often, but if you're faced with a failed cam sensor, bypassing the relay might keep you from having to walk home. I included my sad drawing showing the ASD relay on the left, and terminals 30 and 87 to bypass it.

Also be aware that the crankshaft position sensor's air gap is critical. When you replace it or reinstall it, there is a thick paper spacer that must be stuck on the end before you stick it in. That spacer sets the gap, then it will just slide off when you start the engine. Many aftermarket replacements have a thin plastic rib molded onto the end. That rib will partially wear away so to reuse those you're supposed to cut the rib off and use a new paper spacer.

To read the fault codes without a scanner or code reader, Chrysler makes that easier than any other manufacturer. Turn the ignition switch from off to run three times within five seconds without ever cranking the engine. After a few seconds the three-digit code numbers will display in the odometer readout. On '95 and older models, two-digit codes were displayed by flashing the Check Engine light. You can go to this page to get the description of the codes:

http://www.2carpros.com/trouble_codes/obd2/P0100

It's important to understand that fault codes never say to replace parts but that's the first thing many people do. In fact, a sensor will actually be the cause of the code about 50 percent of the time, but you also have to consider corroded splices and electrical terminals, stretched terminals, cut wires, and things like that. Some codes set in response to a defect in an electrical circuit. Some codes set when the computer isn't happy with something, typically lean or rich exhaust, and it can't correct the condition. Chryslers are very good about having a code in memory once the Check Engine light turns on even if the problem goes away while you're driving. If the problem never occurs again, the code will erase automatically after 50 engine starts. Be sure to read and record any codes before disconnecting the battery or letting it run dead because that will erase codes too, then that valuable information will be lost.

As for injectors, Chrysler has probably the least trouble with theirs of all the manufacturers. GM, for example, grabs a handful out of a big bin and throws them into an engine without ever comparing them. That often leads to a misfire on high-mileage engines from one injector not flowing as much fuel as the rest. Chrysler buys their injectors from Bosch in flow-matched sets. Electrical and mechanical problems are extremely rare. There are some companies that rebuild injectors, then sell them in flow-matched sets. GM products are their biggest sellers, and many customers say their engine never ran so smoothly as they do with the new injectors.

Your truck will set a fault code if there's a misfire, and the code will even tell you which cylinder is causing the problem. It does that by seeing where the crankshaft is when the momentary slowdown in rotational speed occurs. It's that slowdown that causes us to feel a misfire.
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Caradiodoc
Apr 8, 2012.

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