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Check Engine Light Won't Come On with Ignition

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Old 05-12-2010, 03:41 PM   #1
jamieson
San Francisco, CA
1994 XLT 150k miles
 
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Check Engine Light Won't Come On with Ignition

First of all --

Hello Everyone!

This is my first post, and it is a question rather than a solution. Hopefully someday soon here I'll have more of the latter, but until then, I could really use some advice.

I inherited a gift 1994 Explorer from my folks and drove it from Oregon to California last week. A couple times during the drive the Check Engine Light came on, but seemed like no biggee. Therefore I know the CEL bulb works.

Now, I'm ready to get it smogged to California standards -- I have changed out plugs, wires, PCV, air filter, fuel filter, oil, etc...........
My first attempt at smog test was that the CEL will not illuminate during ignition test.

Luckily, they were very nice and didn't fail/charge me.

So, yesterday I checked all fuses with voltmeter (incl powerbox), swapped EEC relay with fuel pump relay, and pulled codes (only bad code was user-error 587). Still no CEL.

However, when I was driving to the store last night, the CEL magically worked! Hoorrayyy! Except that this morning, it was not working again.

This morning I checked/cleaned about 4 or 5 grounds I could see in the engine compartment, and checked the power supply (for lack of better name for it) to the EEC. Only half of the EEC gets power during ignition, and I have no way of knowing if the other half is supposed to only get power after start-up (as the truck wont start with EEC unplugged).

Any thoughts would be greatly appreciated!

I really never thought I would WANT the Check Engine light to illuiminate!

Thanks a Bunch,

Jamie
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Old 05-12-2010, 05:00 PM   #2
TedJ
Fair Lawn, NJ
'94 Sport
 
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Just to make sure we are on the same page as to what should be correct CEL operation, it should be on under the following conditions:

1. Key on in run position, engine off.
2. Cranking engine until computer detects crank rotation (about a sec or so)
3. Codes are set

So it would seem you should have codes if it is going on while you are driving around. Check the codes again....
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Old 05-12-2010, 05:07 PM   #3
jamieson
San Francisco, CA
1994 XLT 150k miles
 
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I'm guessing the codes disappeared when I disconnected the battery for several hours to do all the other work (like the fuel filter, etc).
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Old 05-12-2010, 05:29 PM   #4
TedJ
Fair Lawn, NJ
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Yep. Drive it around until it comes on again then check the codes.
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Old 05-12-2010, 05:31 PM   #5
MrShorty
Spanish Fork, UT
92 XLT and '87 Bronco II
 
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Quote:
This morning I checked/cleaned about 4 or 5 grounds I could see in the engine compartment, and checked the power supply (for lack of better name for it) to the EEC. Only half of the EEC gets power during ignition, and I have no way of knowing if the other half is supposed to only get power after start-up (as the truck wont start with EEC unplugged).
There are four wires that feed power to the PCM (I don't have a wiring diagram in front of me, so I don't remember pin assignments -- should be able to get that from a wiring diagram): a) one that is "hot at all times" (yellow I think) that feeds power for the PCM's memory when the key is off and b) three others (should be red) that should come from the EEC relay and should have power whenever the key is on.

When you say only half has power, it makes me wonder which of those four wires doesn't have power with the key on.




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Old 05-13-2010, 04:18 AM   #6
jamieson
San Francisco, CA
1994 XLT 150k miles
 
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Okay, so when I got back to the car today after work (and after the post), the CEL came on again.....

It was 7pm, so I raced, as fast as a 94 Exploder can, through Oakland, CA (actually, from El Cerrito, CA all the way through West Oakland) looking for any smog shop that might be open, to no avail.

Fingers crossed that tomorrow morning the test CEL shines through and still illuminates.....

I'm beginning to wonder if the EEC is controlling whether the CEL does not appear on test (ignition only) after the battery has been disconnected and yet before much driving has occurred on a freshly wiped EEC/PCM? (Wiped meaning that the battery had been disconnected for quite some time)....

The drama will continue....Updates tomorrow.........
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Old 05-13-2010, 09:28 AM   #7
MrShorty
Spanish Fork, UT
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I'm beginning to wonder if the EEC is controlling whether the CEL does not appear on test (ignition only) after the battery has been disconnected and yet before much driving has occurred on a freshly wiped EEC/PCM?
I'm not entirely sure what you are wondering. Simply put, anytime the key is on with the engine off (KOEO), the CEL should be on.




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Old 05-13-2010, 09:42 AM   #8
Turdle
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I am betting on a bad connection to the instrument cluster. Probabably the plug to the left side.
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Old 05-14-2010, 12:15 AM   #9
nexis
Cedar Rapids, IA
93 Limited
 
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any other issues with the instrument cluster? Other gauges acting up, other lights not coming on? If so, i would look for a bad ground coming off of the cluster, If everything else but the CEL light is fine, i would suggest you pull the cluster out and get a good look at the bulb holder for the CEL. Remember this truck is almost old enough to vote, and plastic is getting brittle and breaks. The holders for the bulbs are plastic and do fail. The contacts get dirty, and the magic smoke escapes. If you get it all opened up and find a broken contact, you can move a bulb holder from another light over so you can pass smog. I had a similar issue with my 93 limited, part ended up costing me 1.94 at the dealership and came with the bulb installed.
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Old 05-15-2010, 02:57 AM   #10
jamieson
San Francisco, CA
1994 XLT 150k miles
 
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Great suggestions from everyone! Yes, certainly the truck is old (but looks and feels like brand new) and the light could be just about anything!

Okay, so....I noticed that the CEL light test does NOT come on in the morning, but DOES come on in the evening for some reason (moisture content or heat maybe?), so I took it to get smogged yesterday evening when it was working.

The engine and visual passed with flying colors, but I failed EVAP with Gross leak. (The tech said it was a HUGE problem, so should be easy to identify)

So, now to look at the Fuel Fill area and a couple of other typical trouble spots I've seen in other posts.

The best news? While I was following a post about CELs not coming on, there was a suggesting to check the EEC power through the TPS terminal. I did just that, and once I plugged the TPS back in after cleaning it up, my acceleration hesitation went away!

Weee, I love this truck so much already!

It's going to be frustrating yet fun times, and I like that!

I'll re-visit the CEL light issue after I pass EVAP and keep this thread updated to the resolution...................
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Old 05-15-2010, 12:46 PM   #11
huntman58
Fremont calif.
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if the light dose not come on in the am I would change the light first thing so that when you take it in will not matter easy to do kind of and a very cheap fix most likely moister or filament almost broken in the bulb and when cold or damp no go warm dry lights on had a few do that to me tell I changed all of mine out. By the way I was in the Fremont area when I did that.

As for the evap start at the gas cap check the fill hose really good any leak f any kind will kill yaw then also use your nose to sniff as it can lead you to it also at times.
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