4 & 5 pin ECUs

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Richie

Richie

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Hi everybody:

Besides the number of pins what's the difference between a 5 & 4 pin ECU

s-l300.jpg


s-l1000.jpg
 
Date code.....5th pin was just another ground IIRC. saved ~8 cents per ECU without a 5th pin.
 
The fifth pin was another power supply which was eliminated in later designs. Some later five pins are really just 4 pins with one pin not connected. You can use a four in place of a five.
 
Mike69cuda said:
The fifth pin was another power supply which was eliminated in later designs. Some later five pins are really just 4 pins with one pin not connected. You can use a four in place of a five.
Click to expand...
Exactly correct. You cannot tell by looking if a 5 pin box is "really" a 5 pin box. You must check resistance from the "5th pin" to the other 4 to find out. I doubt ANYBODY sells 5 pin boxes, unless they are very very old stock
 
5 pin boxes also have the dual ballast resistors. The 5th pin is power not ground.
 
My ‘74 power wagon has the 5 pin with dual ballast resister. I don’t know what or why they did the things they did. I got that 5 pin you see from Halifaxhops.

EC921071-BDD0-4C22-878A-0216B1B2B1D8.jpeg
 
I do find them on occasion usually NORS from the 70's or early 80's.
 

67Dart273 said:
The only difference in the circuit / ECM with the dual ballast is that the second ballast resistance feed power into the 5th pin on the ECU. That is no longer used, and unless you find a really old original vehicle or really old NOS, most any new ECU WILL be 4 pin.

You can use a 4 pin box in a 5 pin harness, but not the other way around

Incidently, the "single" (or other half) of the ballast is the same exact circuity/ wiring as the old points setup. In fact in an emergency, if the thing quits, you can stuff in a breaker point dist. hook the wire to the coil, unplug the ECU and go

In today's Chineseoficianicated products, hard to say, but ECUs usually don't fail that often
Click to expand...
 
This is good info. Thanks for sharing i always wondered the same I knew the 4 pin used a 2 post ballast and the 5 used a 4, and you could run a 4 in place of a 5 but not the other way but I never knew why.
 
The resistor to the coil should be around 0.55 ohms
The resistor to limit the current to the ECU should be 5 ohms
1972 Imperial & Chrysler Ignition System Service Book (Session 292)

When the engine is running, if current is flowing through the 5 ohm resistor, there will be a measurable drop in voltage. How much will depend on the current draw of the ECU.

Here's an example from a member whose truck had aftermarket resistors and ECU.
He measured the resistance (revealing it is an incorrect replacement) and then the voltage while the engine was running.
He posted the measurements on this photo
lil-red-ballast-resistor-readings-jpg.jpg


The voltage drop across the 1.3 ohm resistor is the result of resistance to the flow.
The lack of voltage drop indicates no flow to the ECU through the 5 phm resistor. Most likely because the replacement ECU isn't using the 5th pin.
upload_2020-5-22_8-45-16-png.png
 
Mattax said:
The resistor to the coil should be around 0.55 ohms
The resistor to limit the current to the ECU should be 5 ohms
1972 Imperial & Chrysler Ignition System Service Book (Session 292)

When the engine is running, if current is flowing through the 5 ohm resistor, there will be a measurable drop in voltage. How much will depend on the current draw of the ECU.

Here's an example from a member whose truck had aftermarket resistors and ECU.
He measured the resistance (revealing it is an incorrect replacement) and then the voltage while the engine was running.
He posted the measurements on this photo
View attachment 1715757868

The voltage drop across the 1.3 ohm resistor is the result of resistance to the flow.
The lack of voltage drop indicates no flow to the ECU through the 5 phm resistor. Most likely because the replacement ECU isn't using the 5th pin.
View attachment 1715757869
Click to expand...
Pardon me if this is a dumb question, but the pictures imply that the resistor can be mounted upside down, and hence you can connect the resistor backwards - is that correct?
 
Maybe this:

The 4 pin resistor is not "a" resistor, it is TWO resistors "in one box". IF the factory wiring connectors are undamaged, they have locating pins which prevent incorrect connections. that is what the "U" cutout is at one end, not only an "index" for us not using factory connectors, but the factory connector fits into that "U"

So simply, there are TWO different resistor values, one for the coil and one for the "box."

The resistor and wiring connecting to the coil, and to the key, IS UNCHANGED from the old points system. IN FACT if you were to "go dead" on the road, you could slap in a breaker point dist, hook the dist wire to the coil NEG, and pull the connector out of "the box" and be on your way
 
Righty Tighty said:
Pardon me if this is a dumb question, but the pictures imply that the resistor can be mounted upside down, and hence you can connect the resistor backwards - is that correct?
Click to expand...

If you have the factory connectors with the plastic locating “nubs” intact, they fit into slots in the ceramic resistor so it cannot be hooked up backwards.

It you don’t have the factory connectors or the nubs are broke off, it can be connected backwards. Probably wouldn’t run well ( or maybe at all) with 5 ohms in place of 0.5 ohms.
 
Thanks gents. Just went out to confirm, and I do have the factory style connectors with the nubs. I noticed it's possible to switch left and right, but it can't go together upside down.

I'm going to just start my own thread about my issue so I can stop stealing other guys' threads - I've been lurking in the ignition forum for a few days trying to work it out on my own, but no luck.
 
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