The check engine light on my '81 don't seem to work.
I wanted to check codes but I have no light.
It does not come on when I turn on the ignition.
Where is the light located?
The only light that comes on with ignition is the seatbelt light.
I'm not sure where to start checking.
Any advice for this noob?
Thanks
LF

 

Bump

 

Make sure the computer is plugged in, first. It's by the battery. The light is right below the seat belt light. Basically the bottom left light.

 

Well,if you live in Calif it is a blessing in disquise as it is only a functional test here....otherwords,if it don't come on, they don't check it..if it does,you fail the "functional test"...I passed everything till that light came on in my DD...turned out to be the non-vented gas cap not sealing....and the codes were not any help finding the problem,either....

 

If your check engine light doesn't come on it means one of three things.

1) the bulb is bad.
2) wiring problem
3) ECM is bad

The light is turned on when the ECM grounds the wire. It gets 12V+ from the center console and then a brown/white wire runs to the diagnostic port and to the ECM. A quick test is to ground the wire in the diagnostic port under your ash tray. With the ignition on, ground the 2nd pin (from the passenger side). The pin closet to the driver is a ground, but any ground will do. If the light turns on then you know the bulb and some of the wiring is good. If you don't have one, get a GM shop manual for an '81 vette, the wiring diagrams are priceless.

 

Thanks for the replies.
I'll check it out

 

Also it could just be loose.

 

My check engine light won't come on in my '82 I did the test you suggested and go no light, but if I connect a voltage test to the light itself I get current. I believe my ECM is good, I think this started when I had the inst. panel apart to fix the temp gauge. Everything works but the check engine light.
Help please.

 

HI, I think that in N.Y. that the light must cycle on and if it doesnt you will fail,,, I guess that too many people have tried to pass emissions by disabling the bulb so now it is required to come on at initial start-up and then go off. Clear as mud? Peace,,, Moosie

 

You are absolutely right! In my case mine didn't come on during inspection so I failed. I's got to be something related to when I had the instument panel apart. I checked and there is current at the bulb but it doesn't come on, try new bulb, same result

 

Also my test light comes on on each side of the bulb holder contacts, on other bulbs it only comes on on one side. (positive & negative right?)

 

Bubba took the light out
Now I have a light and can see codes 21 & 55
Let the fun begin.

 

 

Calling 81 ECM Jedi Masters - young padawan needs help with non-existent "Check Engine Light".

A little background info first:
Late last year I rescued an all original '81 with 76k miles from her rotting death here in Central Florida (outside on blocks for 10 years under beating sun and rain). Lots of surface rust inside cabin (seals are gone so water was leaking in. pulled rotten carpet out in chunks with bare hands). I went through a pretty thorough checklist of repairs before starting her up for the first time. I started driving her last month and trying to work through the problems. She idles pretty good but hesitates bad when accelerating and surges at highway speed. I wanted the ECM to help me pinpoint the problems after reading about it in the AIM CCC diagnosis and troubleshooting section. Problem is my check engine light doesn't work at all so I can't get a reading from it.

My "Fasten Seatbelt" light comes on so I pulled the "Check Engine" bulb and put it in my "Fasten Seatbelt" connector to see if the bulb was working. "Fasten Seatbelt" didn't light so I'm thinking - great - simple bulb change. I put the working "Fasten Seatbelt" bulb in the "Check Engine" socket - still nothing! Tried grounding the ALDL jumper - nothing!

Today I started working through the diagnostics section 6E1-15 of AIM ("Check Engine" light inoperative.)
1. Fuse checks out ok
2. Voltage from ECM "G" to ground with ignition "on" is 11.5V
3. Voltage from ECM "U" to ground is 14.8mv
4a. Voltage from ECM "C" to ground with ignition "on" is 16.5mv
4b. Voltage from ECM "C" to ground with ignition "start" is 12.16V

For step 4 I am not sure which reading to use. I would assume (from context) that I should use the ignition "on" reading but the manual is not clear on this. The flowchart branches at step 4 based on voltage > or < 10V.

Proceeding with reading 4a calls for repairing an open circuit in ECM terminal "C" (any advice on how to make these checks?)

Proceeding with reading 4b asks for voltage from ECM terminal "21".
5a. Voltage from ECM "21" to ground with ignition "on" ranges 30-70mv
5b. Voltage from ECM "21" to ground with ignition "start" ranges tight in a sequence of (~30,~100, ~250, ~400, OL)

In either of these conditions (anything under 4V) AIM says "Check for ground in circuits 19 and 21 (includes shorted or grounded TPS, MAP, BARO, Vacuum or Vehicle Speed Sensor)

Not sure where to go from here. Helps much appreciated! Also let me know if it is better to post these types of questions at the end of similar threads or if I should start a new thread.

Thanks!
Wade Bee

 

O.K., it's been a looong time but I'll try an remember as much as I can when I had ECM issues. If you search the C3 section (which you may have done already) you will find ECM related test procedures. BUT from your description of the condition of the car and it being out in the weather for so long I believe you would save yourself a lot of time by just replacing the box. I paid $79 for mine a few years back and it came with a lifetime guarantee. They are cheap because there isn't much to them. Considering the amount of water that got inside and the fact that it was outside in heat & rain for years, I would replace the entire wiring harness, alternator, EGR valve, speed sensor, MAP sensor, ECM, etc. You will spend a few bucks BUT you will save yourself a lot of hassle and time.

 

Thanks for the reply. I have reached out to a couple of forum guys who have replacement parts for sale. In the meantime I'd like to continue troubleshooting. Any ideas on Whether step4 above was supposed to be done with key in 'on' or 'run'?