Check Engine light

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No its not stuck on, its stuck off. It does not illuminate briefly when you start the truck and thus will not pass a smog check in CA.
I'll be fixing this 1988 Ford Ranger P/U this weekend and having done some
homework already, I still need a tip. As background this truck did have a problem where it just stalled out and wouldn't start again. That problem was traced to a wire harness that came loose and got burned on the manefold causing (at least) one of the wires to go open. Evidently the one needed to keep things running. That was fixed through visual inspection with a little help from a Ford explorer 1993 shop manual.
Now I suspect the check engine light problem is the same cause (open wire in harness) only I don't know which wire to trace. I checked several repair guides and none show the check engine light and what wire it is associated with it. If I could just determine the color and connectors it runs through this would be easy.
I'm not usually one to suspect conspiricy but really. WRT the ECC and associated sensors and idiot lights; Chiltons and Haynes only show a picture of some parts and say its too complicated to troubleshoot. OK expected. The FORD shop manuals for my 93 Explorer have a single page saying check the third manual (you don't have). The EVTM (electrical/vacuum troubleshooting manual) for the 84 Ranger (by ford again) shows most of the wires to the ECC but not the idiot light (check engine).
If anyone has the ECC repair manual for just about any ford truck (ranger, bronco II, Explorer in the 1983 to 1993 range you can probably determine the wire color for me.
Yes, I will check the bulb first. So far I have only visually traced the wire harness back to the ECC so it seems likely there is another open wire in there.
My second option is to trace the wire from the lamp back to the open using a signal tracer (triplett fox and hound). That will work but at the cost of more effort.
Does anyone have any insight as to why this indicator lamp would be excluded from most service manuals.
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On Tue, 13 May 2008 13:00:20 -0700, "pipedown"

Nope other than Haynes and Chiltons are not the first place I'd go for schematics.
Check out www.eautorepair.net. It's the online version of Mitchell On Demand and has the *actual* schematics. You can buy one week, one month or one year subscriptions. One week is $10, I believe. I just got a one month subscription for one of my vehicles for $15.00. and you can print whatever you need to, although I'm not sure how you'd go about downloading anything.
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wrote:

Does it come on if you force a code, like say unplug the engine coolant sensor once tghe engine is at operating temp.
Whitelightning
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wrote:

I may try that but I suspect it will not. Seems like an easy test though.
Its either the bulb (maybe a fuse), wire or controller and I will check them in that order.
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wrote:

I would not expect the Check light to be on its own fuse. It should get ground to turn on from the computer -- the pink wire -- If this is true, the pink wire should show 12v if the light works.
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wrote:

That's exactly what I expect. I think the ECC gets power directly from the ECC relay which goes through a fusible link (from memory the EVTM book is at home right now) which if open would do a lot more than extinguish the lamp. But an other poster did indicate fuse position 10 so its easy enough to check, that's an open item since I have not found the diagram yet.
My plan, remove dash, test bulb with DMM, test ground side connection with DMM, check for 12V on wire despite bulb. Connect FOX near bulb, trace wire with Hound to open (ideally) or back to ECC, verify ECC output with voltmeter. I see some ECC on eBay less than $50 incl shipping that should work for this vehicle and then there's pick and pull.
Indecently I did a diagnosis like this about 10 years ago on a GMC blazer. In that case the ECC had a burned out pin on the control IC chip (verified through electrical bench testing) so it had to be replaced. Instead the bum of an owner for that car rigged a switch and manually faked the operation during the smog test. (A bum for more than just that lame act)
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wrote:

If the circuit works as I suspect, the bulb will get its power from a buss that powers other lamps in the same area, but will get its ground from the computer. If this is accurate, you should be able to feel 12v sitting on the pink wire all of the time, and the light should come on if you pull the pink wire to ground. You don't have to take the dash apart to test the bulb. You do have to take it apart to replace the bulb, but if there is 12v sitting on the pink wire, the bulb is good.
____________ 12v | | lamp | | / | ------------ GND
In this really bad diagram, the computer is the position of the / and this represents the switching device that goes low to turn the lamp on.
There should be 12v above this point all of the time. When the / turns into | (the switching device closes) then there will be 12v on one side of the lamp and GND on the other, and the lamp will come on. The switching device is not really a switch at all, but it does the same job, it provides ground to a lamp that has 12v sitting on it keeping it ready to be turned on. If there is not 12v sitting on the pink wire, the bulb is bad or the wire is broken between the lamp and the connector. I'd consider the lamp first, but you said that the wire bundel has already had a repair because it fell across the manifold or something like that.
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wrote:

I understand but that 12V source will have at least one protection device and maybe a relay between it and the battery. The switch in your diagram would be the ECC pin.
Here are the wiring diagrams. http://www.autozone.com/addVehicleId,1071901/initialAction,repairGuide/shopping/repairGuide.htm?pageId 00c152800781b2
I have looked at every one and I can find no check engine light. Pin 20 is always black and grounded, fuse 10 is for general exterior lamps (if used, diagrams disagree) and I can find no pink wire on any diagram. I am not so confident in finding a pink wire anymore. These diagrams are very similar to the ones in the EVTM
In any of the cases, it should be obvious as soon as I start probing the lamp. I have a clear plan of action when I start. I'll try to post results. Haven't looked at the truck in a few days.
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wrote:

http://www.autozone.com/addVehicleId,1071901/initialAction,repairGuide/shopping/repairGuide.htm?pageId 00c152800781b2
but only on that one, not sure which diagrams apply to this engine. I'll figure it out later.
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On Wed, 14 May 2008 21:06:11 -0700, pipedown rearranged some electrons to say:

It's in the very first one that comes up.
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http://www.autozone.com/addVehicleId,1071901/initialAction,repairGuide/shopping/repairGuide.htm?pageId 00c152800781b2
I found the Check Light on the diagram without any problem at all.
http://www.autozone.com/shopping/repairGuide.htm?pageId 00c152800781b2
You are interested in Fuse 10, and Connector Pin 20 on the ECM.
This is on the left side of the diagram, near the top.
I don't recall the motor you have, so I selected the larger of the two 4 cyl options.
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I looked this morning. Fuse 10 spot is unoccupied and the sticker next to the fuse panel says #10 is for the rear brake towing option. I suspect that diagram only applies to the 83 model. I have the EVTM published by ford and nothing like the first diagram is included, the remaining diagrams match the EVTM very well. With no captions on the diagrams its partly guesswork. The engine is EFI and the controller is marked ECC IV.
Can't hurt to try a fuse in #10 but I have little hope it will lead to a 1 minute fix.
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My mistake, I thought you had an 83 Ranger.
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So now I am perplexed. I pulled the dash and checked the bulb, it was good and powered from the high side same as a few other lights that do work like the check oil lamp. Then I identified the wire (pink/lt green) and traced it back into the engine compartment to the first connector. I get good continuity to that connector. The wire color and lamp match the EVTM for the 1993 Explorer up to that point
Now the crazy part, the mate to that connector has no wire in that pin location, no pin contact and the outside is plugged up so it was never there, not just broken. Furthermore, there is no PK/LG wire at the ECC or the self test connector as it would be if it continued to agree with the 93 diagram.
Pin 17 of the ECC which should have that wire has a tan/red wire that leads to the self test connector same pin but I don't know if simply connecting those nodes will result in the check engine lamp working or just burning out the pin.
I'll advise the owner to go back to the smog check place and try to prove there is no lamp. I bet that flees like a lead balloon. Maybe she can call and verify first. Given that I can't find any verified diagram showing the lamp, it might not actually be implemented. Can anyone conform or deny that? Anyone own a 1988 ford truck?
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Why don't you just tie it in to another lamp, say the oil pressure, and call it good?
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should come on in run, stay on in start for a moment and then turn off.
Its close but a good tech might spot the patch and fail the vehicle. Right now there is a free retest for this one thing. It must have passed for years like this.
Im going to look into just wiring the lamp to where it should have been all along and see how that goes. Pin 17 of the ECC http://www.auto-diagnostics.info/ford_eec_iv I'll try that tomorrow.
Did I mention this is not the original engine but I wouldn't expect this kind of difference from an engine swap.
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On Mon, 19 May 2008 21:58:46 -0700, pipedown rearranged some electrons to say:

If someone tried to take a computer from a different year and plug it into your 1988 wiring harness, yes, that would explain a lot.
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Yes it would ...
For one thing, it would explain why there was not a wire on both halves of a connector ...
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That prompts me to check the VIN. That should tell me what the original motor was. It has EFI now. The swap was done so long ago its impossible to determine how much of the wiring was altered.
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