I'm trying to check my injector wiring for possible rough idle. I have read that you need a noid light or a test light. I don't have a noid light, but I tested the wiring while the engine was idleing. The test light lit up and stayed lit on all the connectors. I thought they were suppost to blink?

Thanks,
Matt

 

The circuit should always have power. The injectors are fired by raising a voltage, raising a current, or providing ground to a signal that is already there instead of an on off.

A noid light is only ever good to detect a non-pulse situation. In some cases it's not even good for that. Because there are two basic types of FI circuits (voltage aka 'saturated' trigger and current trigger) and the noid light has to be made for the correct one.

A DVOM can be used to check for injector resistance and for voltage to the injector. If you have a nice one you can check dwell time with the duty function.

The only tool for all situations of injector circuit testing is an oscilloscope. Then you can view the actual wave forms.(clicky)

It's no shocker that the light is always on, on your test light. I believe that both L98 and LTx use a ground supplied by the comp to close the injector circuit.

There is an even lower tech way of checking the injector's basic function. Use a 3' garden hose or a stethoscope and listen to each of them. If one sounds different then there's your problem! If one of your plugs is gas fouled Bingo!

 

The injector is primarily a coil, one lead goes to +12v, the other lead goes to the ECM which gets grounded for a time pulse determined by throttle position, a/f ratio etc. A low powered 12v lamp connected across the two contacts that go to the injector must pulse when the engine is cranked or is running. If the lamp is on continuously, then the ECM power transistor is defective or the wire going to the ECM is shorted to ground. A side marker lamp should be ok to use in this test.
If you connected your test light from one wire on the injector to ground, then yes, the light would have stayed on continuously if the injector wire chosen was the +12v one.

 

Great post bud, i learned something from ur post. Hope this helps thread starter.

 

sami85L98: There is a way to determine if an injector is not firing without some of the high tech stuff. Go to Sears and get a mechanics stetescope (spelling = ???). Put this in you ears and place the probe on each injector. You can actually hear that injector firing or the mechanical sound of it operating. Compare one injector to another. You can also use any metal object, but the Sears tool is very easy to use. If there is a difference between injectors, you will easily find a "bad boy".

 

A resistance check of each injector will reveal a bad coil; the sound check may reveal a faulty or dirty injector.

Since you are looking for a miss, hook a timing light in turn onto each spark pulg wire to see if the miss coincides with a missed flash.

 

I used the test light on both terminals on each injector connector. Both terminals stayed lit and no blinks at either (both showing 12v with the test light). I have not tried a garden hose to my ear yet, use a voltmeter on the terminals, or using a timing light on each plug wire.

Just wondering if either of the terminals for each injector should blink or stay lit? I might have to get a noid light anyway.

 

ttt

 

Think this through with me a minute:

If you clamp a test light to ground and then probe for voltage with it and find voltage the light is on right?

So consider the FI circuit. The DC signal is always there but lacks ground until the comp supplies the ground. But when you probe with your light you are providing a ground path for the DC to travle across your light and it lights up. So the light should be on as you describe but it really only tests the circuit to see if current is there. It does not test the comp's supply of a ground, the injector's opening or closing, the time the injector is open or how one injector compares to another.

 

Yes, my test light has a probe, light in the handle, single wire and a alligator clip (ground). The light stays on when I have the alligator clip on the negative post of the battery and am probing on each injector terminal on each connector. I understand what you are saying.

What if I reversed this. Touching the probe to the positive post on the battery and touching the injector connector terminals with the alligator clip. It seems that it would blink, if the ECM grounds the circuitry??? I have not tried it yet just thought of it after reading the above post.

 

Brother, I don't know. I would hate to say try and see when I have no idea of the actual risks. Worst case is you get hurt, next you hurt the PCM, next you fry an injector, etc...

I would say go for it if proven tests were difficult or expensive. But the other tests are cheap. Used garden hose = free. Stethoscope = $15(clicky), New noid light with adapters for most cars $40.(clicky) Autozone or Advance may even loan one to you for free.

Sorry I couldn't answer the reversal question. Maybe others will chime in.

 

Sleeper: A thought for you! Remove each spark plug and number the plug from the cylinder from whicih it was removed. If you have a bad injector, bad wiring to the injector, etc.......that plug will have a very different appearance from the other, good plugs/cylinders. The different appearance may be due to the injector not firing, bad spark plug, defective injector, etc. What if you find all plug look alike? This may indicate you do not have a problem. Just a thought for you to consider. Have you checked/set the base idle on the engine?

 

Can you give a little more detail about this way of checking for a miss?

 

89Bham: There are so many things that cause an engine to miss or idle rough. If a specific cylinder is the root cause of the miss, that cylinder's spark plug will have a total different appearance ....... if you compare the visual appearance of it to the other seven plugs. This does not indicate what is wrong with the cylinder, it just directs you where to look for the problem. If all plugs look alike in color and deposits, the miss is probably not the result of a single cylinder. I have experienced blown head gaskets and it is relatively easy to find the bad cylinder(s) by just examining the plugs. I hope these words answer your question.

 

The above statments are about the best for checking the injector wiring. The garden hose or long screw driver....one end to the injector while connected and the to your ear. You can actually hear the clicking and all 8 should sound the same.

The noid light is a handy tool to have. Just unplug the injector connector and plug the noid light in. It will blink as the ECM driver grounds the injector circuits.

By using the timing light on all 8 plug wires, this can rule out a miss from recieving spark. All 8 should blink the same. You can even check the spark with a spare plug in each plug wire while held close to the exhaust manifold. The spark should be bluish/orange and ~1/2" long.

By pulling the spark plugs you can actually see if any are carbon or gas fouled (oil gettiing by the rings or leaky injector or wiring), white (lean) or a tanish normal color. Any that are a good bit different in color or smell could indicate a weak or non existing spark or a "bad" leaky or closed injector.

As for the wiring to the injectors...the FSM states which wire is which. On my 86 the right bank injector wiring is [lt blue is ECM controlled (ground) and [COLOR="royalblue"]pink/black is 12v ignition]. The left bank [lt green is ECM controlled (ground) and pink/black is 12v ignition]. Not positive the insulation color is the same for all years though.

Not sure why both terminals on each injector connector shows up 12v steady other than the stated above information. Also not sure if reversing the test light will show anything either (it should work), but may have a negative affect on the circuitry so I will not suggest it unless a last resort.

You have been given good information so far. Ohm the injectors individualy and they should be 16-17 ohm or as long as all 8 are over 10 ohms and are very close. Pull back the loom on the wires and visualy check them as far as you can and you can even test them at the ECM.

Good luck.

 

What you need to do is connect the test light to BOTH terminals of the injector connector. 12 volts is ALWAYS there, the ground is turned on and off. If you GROUND your test light it will always be lit, you are supplying the GROUND. If you connect it to BOTH terminals of the connector you will see the computer supply the ground in pulses.

 

It's always nice to know the year/model of the vehicle we are talking about.

Batch fire:
Except for the ZR1, all C4s before the '94 had "batch fire" injectors -- not sure about "Cross-fire", etc.
The batch fire systems only have two injector drivers in the ECM -- one for each bank.
So, unless you have a wiring problem, if any injectors in one bank are being driven correctly, the others are wired in parallel and should have the same signal.
Usually, with a batch fire system, if one injector only is having problems, it is the injector and not the signal.

Sequential fire:
Each injector has a individual driver in the ECM -- they are independently wired and driven.
So, if one injector has a problem, it could be the injector, wiring, or the driver.

Tom Piper

 

Kalister had that dead on Incidently the FSM calls for a 9V test light..not 12. I bought a neat noid light set, has about 5-6 noid lights for testing not only GMs, but alot of others as well, think it was about $30. The theory Tom Piper stated is flawed....sorry Tom, but what I found with my noid light was a bad injector pigtail wire...so, even though they're batch fired you can have a problem with wiring to ONE injector.All the injector wires run to a common connection, it's the length of each individual injector wire that can and does cause problems.

 

I will re-quote what I posted above:
"So, unless you have a wiring problem, if any injectors in one bank are being driven correctly, the others are wired in parallel and should have the same signal. "

So I am assuming, since our above posts have the same time, you didn't see it completed.


Tom Piper