1984 Ram Crank No Start

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matthon

matthon

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84 Ram, Mopar 5 pin ECU, crank/no start.

Everything new 9k miles ago, not that it matters.
Hit it with the pressure washer the other day, started it up after to dry it off, parked it in the sun for the day to dry.
Left the doors open to air it out, came out to start, dead battery.
I forgot the interior light only comes on when the pass door is open, thinking that caused it.
Jumped it, no go.
Cleaned up battery terminals, no go.
Tested for spark at the # plug wire, no spark, at the coil wire, no spark.

Put it on the battery charger, 12.87 volts, charged up enough to start, no go.
It seemed to fire a bit a few times, not really though, think it was wishful thinking.

ECU grounded to battery.
11.9 volts on ECU pin 1 on run.
Pulled the ECU to check the back, looks good, not burnt anyway.
Coil - voltage 1.2 in Run position.
Coil + voltage 6.75 in Run position.
Disconnected coil wire and tapped to ground, coil fires a mad spark.
Disconnected wire connectors, checked for good connections and corrosion, looks great.
Test light on coil in run position is bright, blinks in start.
Resistor is 11.3 volts on one side, the other 6.7 on run and 9.6 on crank.
Tested for spark at the coil wire, it's sparking.

Pulled the plugs, not flooded, but I broke a plug so off to the parts store.

Think it will fire when I return and put the plugs back?

Whether or not it does, since I know how to wire things well, but not very good diagnosing electrical, with all I did check, I still do not know what it tells me.
A bad resistor could be starting but not running, I think.
What would point to a bad coil or ECU?
 
Clean/gap plugs, crank no start.

Fail! And failed to learn anything.

I didn't want to just throw parts at it, at least I succeeded in that.

Something is making me think it's more than 1 thing, based on nothing though.

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Does it have a dual pick up distributor? As in, two plugs coming out of distributor?
 
Just the one 2 wire plug.
I did pull the cap/rotor for a visual inspection, cleaned them up, distributor looks brand new, not that it matters.
 
Want to be able to run a business card in between the distributor impeller and the pickup, think it's supposed to be like .008 ths. of a gap there.

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I don't think I checked that directly, just the resistor on both sides.

What would cause that?

My battery isn't fully charged yet, 80%, but that's enough to crank/start it.
 
George Jets said:
between the distributor impeller and the pickup, think it's supposed to be like .008 ths. of a gap
Click to expand...
I'll check tomorrow.
Ran fine, then nothing, that gap 'shouldn't' have changed, stuff happens.

Years ago I had a bad pickup, it was cracked, only when hot it wouldn't start.
Start cold, run hot fine, turn it off no start.
Let it cool, start fine.
Opposite of current issue, but got to check everything sometimes.
 
Just so you know;
>Both the coil and the ECU will run on battery voltage.
>With the Coil-wire yanked out of the cap and near-grounded; in a normal system, the ECU will send one spark, every time you shut the ignition switch off; this assumes the coil is dry on the business end.
>the pick-up should send a voltage spike of ~3.5vAC, every time a reluctor vane passes by the pole piece. and
> just for kicks, the rotor has to be turning, lol.
>in a pinch, you can run a 5pin ECU on just one ballast resistor, or even on a bypassed resistor.
> your engine with it's 5-pin ECU, will run on ANY ECU from any car any year that fits in the plug. All it is, is an amplifier for the pick-up signal, and a translator that then interrupts the power to the coil, in the very same manor that the points did.
 
I'm not understanding a few things:

I have a 5 pin ECU, the plug has 5 female pins but only 4 wires, why?
I have a single ballast resistor, factory appearing harness, is this correct?

The biggest hurdle for me is I see no cause and effect, or maybe the reverse, for everything I have checked.

For example, I disconnected the coil wire and tapped to ground, coil fires a mad spark, but I have no idea what that tells me.
I've seen it suggested multiple times in various threads, but it is not clear what it means, the coil is good, the ECU is good, etc.

I don't know what each thing I'm testing is eliminating as the issue, or what it should lead me to test next.
 
I beleive that by '84 Chrysler had changed to ECU's that no longer required the 5th pin and 5 ohm resistor to control power for running the ECU. The plug was never changed. it may have the 5th ire removed.

What wire did you touch to ground?
If its the controlling primary wire (connected to the - terminal), touching to ground completes the circuit (assuming the key is in run or start). Breaking that connection causes the coil to discharge high voltage to the distributor through the secondary or high voltage wire.
 
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Top of the pentagon connector is power for the ECU
When the distributor signals the ECU, the ECU connects its left pin (black and yellow wire in diagram above) to ground. That's all it does. Works same as points closing.

Give that battery a long slow charge.
it should be around 12.6 to 12.8 V fully charged with nothing turned on.
When you hit start, that Volts should drop when the starter engages but not below 9.5 V
You can check for spark while cranking the engine with a spark checker. Used to be about $5
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No spark, then you can ask whether its ECU or lack of power etc
Yes spark, OK, then its something else.
 
Like Mattax said.

Just to add.
The 5-pin ECU used to require it's own power supply with an External Ballast resistor.
The 4-pin ECU, has the power-supply inside the case, so it no longer requires it's own ballast .
The 4-pin ECU, will run just fine using 5-pin wiring. The extra power-supply is just dead-headed in the socket.
But your truck shouldn't have 5-pin wiring, so this information is just information.

If you even once saw a stream of sparks, no matter how you got it, then EVERYTHING at that moment was working, in whatever position your key was in.
If you see the one-spark discharge when the key is turned from run to off, then both the coil and the ECU are working and the only questionable piece is the Pick-up, unless the distributor driveshaft has quit turning, or the reluctor gap is waaaay off spec..
The Reluctor gap spec is .008>.011 IIRC, but in my experience, the pick-up will signal just fine anywhere from zero gap to .030.
If the timing chain has ripped all the teeth off the nylon upper sprocket, then distributor driveshaft will stop spinning. But the engine will crank funny with out compression at the right time, and usually the pistons will bend some valves.
If the timing chain has only jumped some teeth, the driveshaft will still turn, the ignition will still fire, but at the wrong time, and the engine will not start. The timing chain, when it jumps, it almost always happens whilst cranking a cold engine.
Put the engine at TDC/compression of #1 cylinder. as close as you can figure out, close is good enough. trace the #1 wire from the plug back to the cap. Mark on the distributor body, the position of the center of whatever tower that wire terminates at. Pop the cap off, and make sure the rotor is pointing to that mark you made, within a few degrees.


OOps , I was typing while Mattax was already posting,
Sorry man,
 
Mattax said:
by '84 Chrysler had changed to ECU's that no longer required the 5th pin
Click to expand...
It seems odd the harness has 4 wires, the plug 5 female connectors, and the ECU 5 pins.
Does that mean a 4 pin ECU will work as well?

Mattax said:
What wire did you touch to ground?
Click to expand...
I disconnected the 2 wire connector from the harness to the distributor, each end has a long pin on one side.
Key in run, I touched the long pin from the harness to ground and the coil wire would spark.
 
matthon said:
It seems odd the harness has 4 wires, the plug 5 female connectors, and the ECU 5 pins.
Does that mean a 4 pin ECU will work as well?
Click to expand...
It's not odd. Take a look a some other multi-terminal connectors - like the ones on the bulkhead. Unless the vehicle is fully optioned out, some positions have no wires on one side and some positions have no wires or terminals on both sides. Same part used for different purposes saves money, makes the older/newer parts interchangable, etc.
matthon said:
I disconnected the 2 wire connector from the harness to the distributor, each end has a long pin on one side.
Key in run, I touched the long pin from the harness to ground and the coil wire would spark.
Click to expand...
Why are you doing that?

Those two wires just carry a weak signal that is amplified. There's a couple threads here that go into depth of how it works. I've not followed them in detail. I'd be afaraid you'll burn out the circuit and I don't know what shorting it proves or disproves.
Basic operations of the ECU here: 1972 Imperial & Chrysler Ignition System Service Book (Session 292)
Basic ignition operation and concepts here: Ignition System Analysis (Session 259) from the Master Technician's Service Conference
 
Battery charged overnight, now at 13.18 volts.
Cranking it goes to 10.7.

It gave a 'I'm gonna start' for a few seconds then nothing.
Spark at the coil wire, spark at the plug wire.
The Reluctor gap spec is .008>.011, literally, .007 is too loose, .008 fits, .009/.010 fit nicely, .011 is too tight.
Distributor is spinning, aligned 100% with tdc.
I even tried a different coil wire, because my slant 6 car had spark but would not start, turned out it had what I describe as a leaky coil wire, could only see it leaking spark in the shade.

It has a 1/2 tank of gas, carb is full and squirting gas.

It last ran for 10 minutes, shut it off, washed it, started it, moved it into the sun, shut it off, all during a 70+ degree day.
 
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By this time, I wonder if the oil has all been washed off the cylinder walls , and the compression is really weak. If you have been pumping the gas pedal, guess where the gas has been going.
Ima thinking it would be a good idea to put some oil back into the cylinders.
 
I have not been pumping the gas pedal at all.
It takes 1 pump and it starts right up, since day 1, any day, any weather/temperature.

I gave it 1 pump this morning and that was it, even after multiple attempts to start. Came back in the afternoon, I gave a small pump at the carb to make sure it was getting fuel.

I even pulled all the plugs to make sure it wasn't flooded in any way, that was when I broke 1 plug the first day.

Today, when checking for tdc, I stuffed a rag in #1 to get it close with the key and it shot it out with a blast.

I checked the oil today as well, gave the dipstick a sniff, no gas.

Completely baffled.
Beginning to think it is sparking intermittently, everytime I check it sparks, then it isn't when I try to start it, and it's effing with me.

I don't beat this truck, drive it occasionally, limited really, engine is rebuilt 9k ago, reliable, runs strong.

I've got gas, spark, air, compression, timing, voltage, distributor spins, etc.

No disrespect to anyone making suggestions as I pull my hair out over here.

I'm going to get a cap and rotor tomorrow, kiss principal.

If it's not something simple, it's intermittent, either way completely unrelated to me spraying it with water.
Prior owner put dielectric grease on everything.

Late night rant to myself over.
 
Same thing happened to me back in 2020, just up and died out of nowhere, '76 D100.

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Replaced with a new Ignition Control Module from the local parts store, never failed again.

This one was a Standard LX100 5 pin.

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Happy Trucking


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You're assuming it's the ignition. I have 73' Dart sport 340. I did a bottom end rebuild with a new thumper cam and left the Heads alone, Big mistake! Car started right up and ran to break Cam in the took it for a drive. The next time I tried to start nothing, just cranked. Ignition and fuel checked out. It Turns out the INNER SPRING ON THE #4 EXHAUST WAS BROKEN! Replaced all the springs and valve guide seals and It started right up and runs great!!!!!!!
 
If you install a new ECU, scrape the paint off the backside of one mounting bolts, and the firewall, and put a Star washer between the unit, and the firewall. This will ground it good. Or you can run a separate ground wire also.

Dave
 
barracudadave67 said:
Or you can run a separate ground wire also.
Click to expand...
I've done that on every car I've owned, this truck is new to me a few months ago.
I'm doing some of the wiring over, installed both a ground and positive hub today, as well as a bigger ground cable, separate ground wires for ECU, starter relay, a ground wire all the way to the rear taillights, the headlights, etc.
This all began because I was going to install trailer breaks/light harness, which I did most of it today.
Picked up a new ECU as well.
Going to finish up the re-wire/trailer wiring I had planned anyway, then try to start, then replace the ECU, then with any luck this will be over.

Still do not understand what steps I took diagnosed/ruled out what issue/part.
 
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