23 replies to this topic

[Bob Lincoln]

Wow. You did some expensive shotgunning on this one. We're here to help, so it usually pays to consult here first. One or more of us will think of things that others don't, and you can work the list til you find the trouble.

First, always check fault codes. They are not absolute indicators of where the trouble lies, but it's free and easy to do so, and can lead to places to check.

Next, I would suggest that one common point of failure in these trucks is the power splice wrapped inside the harness between the battery and the relay box. It was known to corrode and fall apart, due to poor assembly at the factory. It caused a no-crank for me two years ago with no warning. So when you say you've checked continuity, that's not good enough. I measured 16K ohms between battery and the coil of the starter relay. That means the relay got less than 1 mA, not enough to close it. But there was continuity!

Another thing to check on these is the crankcase position sensor. If the computer doesn't sense the magnetic pickup for flywheel rotation, the fuel pump will shut down and not pump until it sees rotation again. The ASD relay shuts down the spark, and depending on whether the relays are cascaded as they are in my car, the ASD can shut down the fuel pump relay.

The computer rarely, if ever fails, so if you can take it back (unlikely at most stores), at least get some money back.

[AllanC]

Another thing to check on these is the crankcase position sensor. If the computer doesn't sense the magnetic pickup for flywheel rotation, the fuel pump will shut down and not pump until it sees rotation again.

There is no crankshaft position sensor on 1991 Dakota engines. That did not appear until the 1992 model year with the advent of the revised Magnum V6 and V8 engines.

The pickup unit in the distributor pulses with rotation of the distributor, camshaft and crankshaft. This pulsing is the signal sent to the engine controller so that it knows the engine crankshaft is turning and allows the ASD (automatic shut down) relay to engage. The ASD relay must be engaged and closed to provide electrical power to the fuel pump, injectors and spark coil.

With all those parts replacements it appears you have a wiring problem somewhere in the system. Trace the black / gray tracer wire from the negative terminal of the coil back to the wiring connector at the engine controller. Make sure this wire is intact, not broken and not shorted against metal. The positive side of the coil has a gray / black tracer wire that originates from a splice near the ASD relay. This is probably the splice Bob L mentioned.

I agree that engine controllers are reliable and typically this type of problem is NOT with engine controller failure.

[AllanC]

Yes your understanding is correct. If the ASD relay is closed, 12 volt battery power is distributed to the fuel pump, injectors and spark coil. Now it is possible that the fuel pump can have an electrical or internal pressure problem which prevents engine starting but you should still have spark at the coil and plugs. But if you are not getting any spark from the plugs it is possible no power is going through the ASD relay when its coil is energized or the gray/black tracer wire from the negative side of the coil to the engine controller is shorted or broken.

Take a multi-meter or 12 volt test lamp. Touch one lead to the positive side of the coil and the other lead to a good ground on the engine. Engage the starter. If no illumination then no power from the ASD relay. If the lamp illuminates you know that the ASD is functioning properly and providing 12 volt power when the crankshaft is rotating.

[Volunteer]

If still no success, try replacing the Hall-Effect pickup in the distributor.

[LH body New Yorker]

If I read right...... it has 255,000 miles on it?

Can I ask a silly question?

Have you had someone crank it without the distributor cap off to see if the distributor is turning?

[valiant67]

Good point. At that mileage the timing chain could be shot (even a second one could be bad with those miles).

[Seed]

Is it possible that with that many miles on it that the distributor is shot?

[Bob Lincoln]

But if the Distributor was bad, it wouldn't account for no gas right?

Yes, it could. No dist signal, the truck shuts off the fuel pump.

Repairing those wires is the first step and may solve the problem.

[whitingbay57]

Hello all, I am new here and need help pls. I am experiencing the identical problem with my 1991 Dodge Dakota 3.9 V6 described in this topic. The only difference is, my truck won't start if the ambient temp is 40F or colder. Above 40F and it will start fine and run fine. If there was a break in the electrical or bad relay it wouldn't start at all. I'm in the dark on this one. Any ideas?
Thanks all
Peter

[B10alia]

So you have no spark and the fuel pump will not come on below 40, but everything works fine above 40? Sounds like the ASD relay. Keep in mind that relays are electromechanical, they have metal moving parts that expand and contract with temp variations, and if the relay is worn from 21 years of service, it could very well be out of tolerance when thermal factors come into play. You might be able to swap in one of the other relays in the ASD area to see if that fixes it, a lot of times, the relays for different applications (A/C relay and ASD, for example) are identical. If that swap fixes it, it's the ASD.

[Bob Lincoln]

Check fault codes: http://www.allpar.co...odes/index.html

[chuzz]

Your problem could also be related to something as simple as the plug wires. With time and age they break down and cold temperatures will definitely affect them. It's resistence related and won't let the plugs fire due to an increase in the resistence. At least that's how I THINK it goes. Bob Lincoln works with electronics, so I'm sure he can explain it better. But the bottom line is that if you have old wires on it, maybe a new set could get you up and running.

[valiant67]

Are you sure there's no spark when it won't start? If it won't start but there's still spark when it is colder it could be as simple as the temperature sensor for the computer (probably not the same one for the gauge). It tells the computer the engine is cold and needs a richer mixture to start.

Next time it won't start, I'd pull a plug and see if there's spark. If there is, I'd see what happens with a little starter fluid sprayed down the throttle body. What happens with those tests would direct the next course of action.

[whitingbay57]

Thank you for your prompt help! The truck has 155k miles and in very good condition, Previous owner tried replacing the following:
coil, cap, rotor, pick-up coil, plugs, plug wires. no luck. (he was so mad he GAVE me the truck)
The ASD relay makes sense. I will try replacing.
One thing I did notice, the ambient temp was warm and the truck started. I turned the truck off. I used the "KEY-ON, KEY-OFF" method to extract any codes in memory. The only codes were "12" (replaced battery) and "55" end of code cycle.
The next morning was very cold and thought I'd try starting the truck. Cranked over fine..... Well, no spark, no fuel = no start. OK, I'll try checking codes again. I noticed the "check engine light" wasn't showing at all when I turn the key on. So I left it alone most of the day until the ambient temp was warm. About 2:30pm I tried extracting codes again. This time the "check engine" light did it's thing for me with no new codes. and yes, the darn thing started right up! I tried this 3 days in a row. Cold morning=no start=no check engine light(with key on position). Warm afternoon=start right up=check engine light functioning(with key on position)
Again, I will try replacing the relays and hope this solves the problem!
Thanks!

[Bob Lincoln]

Try unplugging the connector to the computer (I think there's a screw in the center of the plug that retains it), and clean the contacts with contact cleaner from Radio Shack, or Gumout spray. Fill it up with dielectric grease and plug it back in. Could be a problem with thermal expansion and contraction of the contacts, in conjunction with oxidation of the contacts. Cycling the connector a couple of times can do it.

There is also a known bad problem with a factory splice in the harness near the windshield washer jug. In my case, it opened the circuit to the starter relay so that it would not crank. It could also fail in a way that the ASD relay does not get power.

[whitingbay57]

I opened the entire harness and checked ALL the splices and connectors. Everything was clean and no oxidation or corrosion. I was able to isolate the problem by means of leaving the truck in the shop over night. The next morning I arrived 6am. The truck wouldn't start as usual due to the cool ambient temp.
I hooked up a small electric heater and pointed to various areas under the hood (about 20 min each point). I also left the ignition key in the "ON" position so when the system "woke up" i would hear the relays kicking in. ECU was next in line for the heat test. Shure enough, about 5 min the relays started kicking in and out. I turned off the ignition switch and waited another 15 min. with the heater pointed at the ECU. The truck fired right up after 20 min of heat pointed at the ECU. Now, all connections have been inspected. I always thought when a computer gives up then thats it but not in this case. I proceeded to disassemble the ECU for inspection and possibly resolder or replace some radioshack componants. When I opened the cover I was surprised to see the entire board sealed with some kind of ruberized jello. So I guess the ECU can not be repaired? Looks like I'll have to buy another ECU!