Nice to hear about your goals. I taught Engine Rebuilding, Electrical, Brakes, and Suspension and Alignment for nine years and have a lot of notes pages that I produced. Holler if you have other questions or would like copies.
The place to start with this vehicle is the system operation. Manufacturer's service manuals include these sections. They do a much better job than the aftermarket manuals.
When you turn on the ignition switch, the engine computer turns on the automatic shutdown (ASD) relay for one second. That is to insure fuel pressure is up and ready for starting in case the pressure dropped off overnight. You should be able to hear the pump run for that one-second burp. What is more important is what happens next. During engine rotation, cranking or running, pulses of voltage are generated by the camshaft position sensor and the crankshaft position sensor. When the engine computer sees those pulses, it turns the ASD relay on again. Voltage from the relay goes to the fuel pump, alternator field, ignition coil, injectors, and oxygen sensor heaters.
In the case of a crash that ruptures a fuel line, fuel will not spray from the injectors when there is no pressure so the engine stalls. When the engine stalls, there are no pulses from the cam and crank sensors so the computer turns the ASD relay off and that turns off the fuel pump to prevent dumping raw fuel onto the ground. The system is designed to prevent fires and is very effective and reliable.
There are really three circuits. The ignition coil circuit is the one you have been working in. The fuel pump is the second circuit. Both of them give very little trouble. The third circuit consists of those two sensors and causes the most trouble.
One advantage you have is Chrysler has a very user-friendly way of asking the engine computer to tell you what is wrong. Cycle the ignition switch from "off" to "run" to "off" to "run" to "off" to "run" three times within five seconds without cranking the engine. If the engine does crank, even for an instant, turn the switch off, wait a few seconds, then start over. After the third cycle, watch the check engine light. It will be on for a few seconds as a bulb check, then will go off, and after a few more seconds will start to flash out a series of two-digit fault codes. Most vehicles will start with code 12; that is one flash, a short pause, two flashes, then a longer pause before the next code starts. Code 12 just means the ignition switch was turned off recently. Write down the rest of the codes. The last one will be code 55 which just means "end of message". If you think you miscounted, cycle the ignition switch off and back on once and the sequence should repeat. On a few vehicles you will have to cycle the switch three times to view the codes again.
During the 1980's and 1990's Chrysler was very consistent with their codes. You can use almost any service manual for any Chrysler vehicle to find a description for each one. Let me know what you come up with for code numbers and I will look up the descriptions. If we are lucky, one will have to do with one of the two sensors I mentioned.
The camshaft position sensor is in the distributor. The crankshaft position sensor is on the passenger side rear of the engine. I believe it actually sits in the transmission housing. It is a magnetic sensor that reads gaps in a metal ring attached to the flex plate to determine which cylinder is on top dead center. I never replaced a crank sensor on a truck, but on the front-wheel-drive vehicles, the air gap of the sensor is critical. A new sensor from Chrysler's supplier will have a thin cardboard spacer glued on the end to set the gap. Push the new sensor all the way in and tighten the bolt. That spacer will slide off when the engine is started. Some aftermarket sensors have a plastic rib molded onto the end to set the gap. It will partially wear away during engine operation. If that style sensor is removed, the rib must be cut off and a new paper spacer stuck on to reset the gap. The dealer's parts department has the spacers. Again, I do not know if that applies to the trucks, but on the fwd vehicles, if you push the sensor all the way in without a spacer, it will be broken the first time the engine is cranked.
The single most important test is to determine if the ASD relay is being turned on during cranking. You most likely will not be able to hear the fuel pump running so the easiest way is to use an inexpensive digital voltmeter or a test light on any of the points that get their voltage from that relay. The ignition coil is usually fairly easy to reach, and the twelve volt feed wire is typically dark green with an orange stripe. That same wire feeds the field winding in the alternator. It is impossible to tell which terminal is which on the back of the alternator because the two wires disappear into a black plastic block. It does not matter though. When the ASD relay turns on, one of the two small nuts holding that block on will have full battery voltage and the other one will have less, but not zero volts. Both will have zero volts if the ASD relay did not turn on.
A more involved method of determining if the ASD relay is turning on is to remove its cover, then watch the movable contact while a helper cranks the engine. You should see it flip for that first one second, then it should flip again during cranking. If it does flip for that first second but not during cranking, suspect the cam or crank sensor. That is where the diagnostic fault codes will come in handy. Any stored codes will be erased from memory if the battery was disconnected or run dead so you might have to crank the engine for a while for the problem to be detected and the code recorded again.
Another way to read the codes is with a scanner. If you plan on working on cars professionally, I strongly recommend you buy a scanner of your own so you do not have to wait in line to use the shop's equipment. I am driving around right now with my Chrysler DRB2 connected to my minivan. It will do everything necessary while aftermarket scanners never do all the things the manufacturer's stuff will do. The DRB2 works on all Chrysler products from 1983 through 1995 but it requires various plug-in cartridges. I also have the newer DRB3 which works on 1998 through most 2004 or 2005 models. With a plug-in cartridge, it will work on 1994 to 1997 Chrysler's as well as any brand and model of car sold in the U.S. After 1996. A different cartridge lets it work back to the 1983 models.
If you want to invest in an aftermarket scanner, the best one, I have heard, for the money is the Genysis version 3.0. I never used it but I heard it can be updated over the internet. Snap-on has a number of scanners too but everything they sell including updates is expensive. You can find good used equipment from the guys who drive the tool trucks around to the shops. They will trade in stuff from people who are upgrading or leaving the industry. Other times they will know someone who has a scanner they are trying to sell so they can buy a newer one. The common vendors are Mac, Matco, Cornwell, and Snap-on. If you cannot find them in the phone book, stop in at any new car dealership and ask the mechanics when the guys show up. It is usually once a week or once every other week.
If you really want to have fun, I sell the DRB3 at old car show swap meets. You can buy them over the internet right from the manufacturer, (OTC), but the kit is very expensive, around $6,200.00. I get them for less through the parts department of the dealership I used to work for. You can find a good used DRB2 on eBay for less than $200.00 which would be fine for your truck but be sure it comes with the proper cartridge. The last one had a yellow sticker matching the yellow service manuals for 1994. It is an updated version of all the other cartridges that came before it. The cost is reasonable for someone starting out, but the vehicles it covers are disappearing fast.
Both the DRB 2 and DRB3 include an "actuator" test that allows you to run stuff that is controlled by the various computers. In the case of your engine computer, with the press of a button, you can fire the ignition coil to see if it is working, fire the injectors, turn on the radiator fan, tun on the ASD relay, and turn on the fuel pump relay if the vehicle uses one separate from the ASD relay. I have never seen an aftermarket scanner that can do actuator tests, but that could have changed within the past few years.
Monday, August 16th, 2010 AT 5:19 AM