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How did you run an Emissions Diagnostic in the 80's?


Veteran Member
Oct 22, 2008
Kamloops, BC, Canada
In my last article I discussed AMC's CeC fuel feedback and emissions control system and specifically how to depot it for the purpose of reverse engineering. This consisted of one Intel MCS-48 microcontroller and a dozen inputs and outputs. This was pretty normal for the time and other automotive manufacturers were using similar designs, if not borrowing from one another. The problem with going this route however was that the system was closed-minded. There was no OBD2 and there was also no OBD1. Those were still decades away from being standardized. Testing and manual diagnostic procedures could verify what was going INTO the computer and validate the responses that came out again but there was no way to talk to the microcontroller directly and alter anything. Likewise this was a very time intensive process which for the customer could result in costly diagnostic bills from the mechanic's shop. AMC made this no better for anyone when the addition of new features to the CeC resulted in a overtaxed CPU and the sacrifice of the Malfunction Indicator Light and Single Fault Stored Code system (the "Check Engine" light) to get everything the Department of Transportation and the EPA mandated loaded in.
Thankfully however a few manufacturers built devices that expedited the diagnostic process. AMC was one such company who in conjunction with OTC (who still makes diagnostic scantools) brought the ET-501 CeC Fuel Feedback System Tester to the market. Here it is being mentioned in an issue of Popular Science.


At $200 in 1982 that's around $500USD in today's dollars and that's still not a cheap option, but it was still an option.
I spent about a year keeping an eye on ebay for one. The key here is that you NEED to purchase the entire kit. Not just the tester. Everything is special here and everything individually is unobtaniun for the most part. Eventually however I nailed a complete kit that would get to my door for $250 shipped.


This kit is everything you need to diagnose an 80's AMC and even a mid-80's Jeep. In the carrying case is the diagnostic tool, five separate wiring harnesses for testing several different cars and their respective MCU's standalone, the instruction manual and the two test chips for checking both 4 and 6 cylinder engines.


The test chips themselves are really 2732A EPROMs in a custom plastic enclosure, keyed to prevent reverse installation.



The manual for the most part is ten pages of how to use it, followed by 70 pages of flowcharts, then space for wiring schematics and any service bulletins released by AMC. The tester is limited to short messages and saves you lots of referring to the manual by listing the most commonly displayed abbreviations on the back of the unit.



The inside of the tester itself is two hand drawn PCB's, a single line VFD display and controller, a high voltage driver for the VFD, a quartz windowed Intel MCS-48, some discrete logic and a rather strange purple ceramic IC which piggybacks the test chips. It must be occupied for the tool to run at all but has no markings itself ti indicate what it does



There are indications however all over the place that this was not the perfect tool. Handwritten notes warn to connect and disconnect battery power first, a kludgy extra "adapter" is fitted to one side of the tester and contains nothing but thermistors and the battery lead itself has a black plastic hobby enclosure with a rather large capacitor glued inside it, presumably for noise supression.

So how does it work?
-Insert the test chip for your specific engine and apply power
-Answer a few simple yes/no and either/or questions about the car such as the transmission type, year and if the altitude jumper is used
-Select if you want to test the system in the car, or the MCU standalone. It then tells you what harness to use and to then plug it in

If you are testing the entire car it runs you through a few simple steps such as flooring the pedal, idling the car, going above a set RPM and unplugging the distributor. At the end of the test it either tells you what you need to look at, repair and retest or if the test has passed. The whole process takes less than a minute.

If you are wanting to test the MCU standalone it waits for you to unplug the computer from the car, use the recommended harness to connect it to the tool and then proceed. I can only guess what it's doing here because again, the microcontroller has no way to talk to an external device. It's possible the tester simulates the same test we ran above and compares the performance of the MCU to that of the calibration data on the test chip. If it passes you are good. If it fails you were forced to replace the computer. In my case I was able to verify if my depotted computer still worked. It does.


Now for the next big question: Do you REALLY need it?

AMC's mechanics manual dedicated some 60 pages to testing and troubleshooting. These days no shop is going to invest in a test for a car that is over 30 years old. In other cases I've found that the inability to monitor realtime data from the car is a limiting factor in locating intermittent or "blind" faults the tester cannot see. (even though it appears the early unit seen in the Popular Science article is doing exactly that) For example it concludes an excessive rich or lean condition from the output of the stepper motor. It cannot directly observe the O2 sensor. It just hopes everything else is working.
Unless you find one for significantly cheaper than I paid for my unit, you are better off manually testing sensors and outputs. You might also find more confidence in your CeC system as well.

If you are still determined to get a unit then allow me to help you a little.
A PDF copy of the instruction manual can be found here - https://www.dropbox.com/s/s693xy4v0789ges/AMC ET-501 Instruction Manual.pdf?dl=1
A zip file containing BIN dumps of the 4 and 6 cylinder test chip EPROMs can be found here - https://www.dropbox.com/s/jl6155riempathz/AMC ET-501 ROM Images.zip?dl=1