ECM tools

To properly work on a Fiero or any other GM car with a C3 or P4 family ECM requires a few tools. Without these tools you could change every sensor on the car and still never figure out what is going on. You might not even get the problem fixed. Believe me, more than a few forum members have done just that. If you move up to the OBD2 cars it becomes even more difficult to work without the tools. Fortunately, We only have to mess with the C3 and P4 computers, which are a heck of allot simpler to deal with.

Warning! Never do resistance tests on the ECM or sensors still connected to it. You could damage the ECM.

Definitions

ALDL: Assembly Line Diagnostic Link also called ALCL (Assembly Line Communication Link) in some literature. The terms are interchangeable. We'll use ALDL in all Fiero Cave documents.
DTC: Diagnostic Trouble Code. The industry standard term for error information stored in an ECM/PCM computer.
Code reader/reading: The process of retrieving ECM error codes through the dash lamp or a small tool with it's own light. A reader device has no intelligence and cannot do anything but read the DTCs from the ECM.
ECM Scanner/Scanning: Scanners are intelligent devices that can read DTCs and all the other information available via the ALDL. They can also initiate ECM resets on units that support software error clearing. (None of the Fiero ECM units support resetting via the scanner.) Scanners can be handheld units or full PC systems.

Code reading

You need nothing more than a small wire connected between the A and B terminals in the ALDL connector. The check/service engine light will flash out the codes for you. You don't need any of those 20-30$ readers you see all over the place.

If you are uncomfortable with a wire you can get a little stamped metal key at most auto parts stores for a couple dollars. I'm told AutoZone may even give you a free one if you ask the counter man.

Scanning

There are many tools available for this job. There are even homebrew tools for use with IBM PC compatible computers. The commercial products can read DTCs, sensor data, and other ECM parameters, often in real time. These tools allow the operator to see the sensor feeds as the ECM does and can often help determine if a sensor is working correctly.

The least expensive scanner I know of is the AutoXray GM OBD-I Scanner part # 10110. It lists for $199 but if you shop around you can probably find it for less. It covers 82-95 GM cars with the 12 pin ALDL such as Fiero. It even comes with the cable to hook it up.

There are other scanners available but nearly all are "universal" scanners. You have to buy the scanner and then buy additional modules or cables to hook them up and use them. These units are good if you have multiple cars to deal with but can be quite expensive.

I have the older version of the AutoXray scanner. I've used it extensively to gather data to write articles and to do numerous repairs. It's well over a year old now and still has the original set of batteries running it. It's one of the most important tools I have.

Since I own a scanner I haven't really fooled with using a PC. Doing this with a PC would only be practical with a laptop computer and I don't have one. There are numerous sites on the Internet where you can dig into freeware and commercial products to scan using a PC.

Digital Volt Ohm Meter

It's hard to work without a good one of these. You need to be able to measure resistance of sensors to test them other times you need to read the voltage output of them.

You can get inexpensive DVOMs at Wal-Mart and many other stores. What you want to watch out for is the meters internal resistance. You want to make sure the meter specifications list 20,000ohms per volt or higher. This information is usually printed on the meter and/or its packaging. Most DVOMs meet or exceed this specification but you want to try and make sure of the cheap ones. If the meter doesn't meet the spec it can put load on the circuit under test that will cause errors in your readings.

You can also test for some of the more annoying things. Like bad grounds. Doing a test for voltage drop across a ground strap is a much more reliable test than checking its resistance.

Static Ground Strap

A ground strap is a common item in the computer industry but most auto service people are unaware of them. These straps ground you to the car so that you don't build up static electricity and damage the ECM. If you are working directly on the ECM you should pick one of these up at any electronics supplier. They are fairly cheap and could save you blowing the ECM. Especially in dry weather where you can build up some really huge static charges as you move around in the car.

The static strap comes with a wristband, a wire with a resistor in it, and an alligator clip. You put on the wristband and clip the other end to any bare metal. The resistor in the wire is to prevent you from getting injured or killed if you grab hold of a live circuit. Never use just wire to make a ground strap. That resistor is critical to your safety.

Test Plugs

These are just used plugs in good condition cut from salvage yard cars. They make it easier to do resistance tests on the sensors. You can clip your test leads to the wires and plug into the sensor. This way the connections stay steady while you do other things. You want to try to have one of these for each resistive sensor on the car. (TPS ECT MAT)

They can sometimes be used for tapping the sensor harness for voltage checks but you have to Very careful setting up for that or you could blow the ECM.

Bed Of Nails clips

These are from the telephone industry. They are modified alligator clips with a bunch of tiny pins inside them. The pins will pierce the insulation of a wire so you can test it. The pins are fine enough so that the holes they leave close and resist entry of dirt and water. You can get these at electronics supply houses but you usually have to find places like Graybar that are big suppliers to the telephone and computer networking people.

Like test plugs make sure they won't cause a short that could toast the ECM.

"Noid" Lights

The injector test light, or Noid Light, is an important tool for almost all ECM and fuel system tests. This tool should be one of the first things you reach for whenever you suspect injector problems. Noid lights are cheap. $5-$10 is allot less than even one injector.

The Noid light plugs into the injector connector at the engine. There are Noid lights for most common injector systems. It has a small light that flashes anytime the ECM is trying to fire the injector in question.

If the light is flashing, as it should then you can be reasonably sure the ECM and wiring are ok. If the light is on steadily or not at all then the ECM or wiring have problems and you don't have to mess with the injector. (At least not right away.)

Do note, in bank fired injector sets you may have to unplug all the injectors in the bank under test. This also applies to multi injector TBI units that may fire 2 or more injectors at once.

IAC test light

This is similar to a Noid light and plugs into the IAC connector on the wiring harness. It has 2 LEDs in it. The lights will flash as the ECM changes the IAC position.

This tool is available for both the old and new style IAC. The old IAC screws into the throttle body and has 4 large Weather-Pack terminals. The new style IAC is held by 2 Torx screws and uses a Metri-Pack high-density connector. These testers cost about $10 and can be found in the tool department of larger parts stores.

You want to use this along with the ECM scanner. The ECM scanner can tell you which IAC motor coil, if any, is turned on. The LED for that coil should be on at the same time. If the ECM says a coil is on but you don't get a light on the tester then the ECM or the wiring has a problem.

IAC Motor Tester

This tool plugs into the IAC itself. It contains driver and controls that let you move the IAC motor a few steps or full stroke. This is the only true way to test an IAC beyond coil resistance.

Coil resistance will tell you the IAC coils are probably in decent shape but it does not let you see if the IAC is working mechanically. The coils can be perfect but if the IAC is binding or worn out mechanically it's just as dead as if the coils are gone.

This thing is rather expensive. The ones I've seen are a couple hundred dollars. I listed this just so people would know it was out there. Unless you have a shop you probably would not buy this item. (People with electronics background could probably build one easy enough. All you likely need is stepper motor control chip and the application notes. I can't imagine it's hard to build.)