Why No Cel?

imp

imp

Mustang Master
Jul 13, 2017
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#1
Recently acquired (May 1) '94 GT 5.0 HO, 5-speed. The vapor canister is gone. The smog pump lies idle, belt bypassed. According to previous testimony here by accepted expert knowledge, there should be at least CEL for canister missing. No CEL.

My '04 Explorer, OBD-II, on the other hand, stranded me when the ABS Module "bought it". That threw

"P2106 FORD
Throttle Actuator Control System - Forced Limited Power", as well as flashing 4X4 HIGH, BRAKE, Wrench, and ABS icons. Reacting to the ABS, the great whatever-his-name-is Makurko or something, Ford Super Technician, video showed him going right to the rear differential housing, and pulling out a mangled ABS speed sensor. That fixed it. I never went that far, but rather bought into other opinions regarding the Drive by Wire problem. They unanimously recommended replacement of the Throttle Body. I did. You can guess the result. Problem unchanged.

Here's my point. The DTCs are intended as a guide, but are not Gospel. At any rate, the OBD-I earlier system is I think far more misgiving. But, nonetheless, how did the guy who threw away my canister convince the PCM to not throw a code?? Car runs perfectly, but eats too much fuel, IMO. No DTCs. No CEL.

Help!! imp
 
#3
OBD-1/EEC-4 is just a more primitive system. It's not going to operate like OBD2 simply because OBD2 didn't exist yet, so we can't backward-apply the same logic to it.

To sum it up, MOST of the trouble codes associated with OBD1 will not trip a CEL. IIRC, only the 13 most critical in terms to how the engine runs will display the light.

Charcoal canister is not one of them.
 
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#7
karthief said:
Are the 94-5 charcoal canister is the same place as the fox?
I thought they moved them inside the fender on the opposite side of that same frame rail.
Purge canister, excuse me, no code 85?
Click to expand...


It's a different code for Sn95's. I'm actually not too privy on EEC-4 in the Sn95 cars, but yank a canister in a fox and you get a code 85.
 
#8
Mustang5L5 said:
It's a different code for Sn95's. I'm actually not too privy on EEC-4 in the Sn95 cars, but yank a canister in a fox and you get a code 85.
Click to expand...
@Mustang5L5

Well, someone yanked out mine, and no code(s). Now the question is: how would PCM detect a "no canister" present?
All it knows is it sends an electrical signal to the purge valve, which admits vapor into the engine via a vacuum line. With programming trickery as in later OBD-II, it could detect no current flow to the valve, and throw a code. But whether or not actual vapor flow takes place, rather doubtful, IMO. imp
 
#10
I honestly think it's just detecting a change in resistance of the solenoid to verify the valve opens and closes. I doubt it's conforming flow.

Is the harness plugged into a purge valve left hanging?
 
#11
karthief said:
I think you're looking in the wrong place, it's behind the inner fender liner, look up into the cavity between the front wheel and the bumper cover.
Click to expand...
@karthief

Am pretty familiar with that hole, actually, since there is an opening in the inner fender apron, through which the guy had put a long, curved 3-1/2 inch tube, with a small K&N stuck inside the cavity. Could not get it out to service withoiut disassembling the entire inlet ducting. That's gone now. imp
 
#12
Mustang5L5 said:
I honestly think it's just detecting a change in resistance of the solenoid to verify the valve opens and closes. I doubt it's conforming flow.

Is the harness plugged into a purge valve left hanging?
Click to expand...
@Mustang5L5
Yes, that makes sense. Every one of those "self-checks" requires quirky added circuitry and programming within the PCM. Some are easy, like displacement (movement of a part), this is done in the DFPE which detects and measures movement of the EGR valve.

The harness has no connection left for the purge valve. I suspect it's been cut off and hidden within the harness from which the MAF extends. There is a round rubber grommet surrounding the MAF wiring, about 6" from the MAF plug. There must have been a bracket, which is gone, and right below, the TFI Module is screwed to the fender apron, kind of cock-eyed, not like Factory installed. imp
 
#13
The computer uses current sink technology; it does not supply power but it does supply ground.

The computer controlled ground circuit has protection via an op amp comparator that measures the voltage drop across a low value resistor of known resistance.

As current flow through the resistor increases, the voltage drop across the resistor increases: Ohms Law states E=IR. When the op amp comparator detects a voltage greater than its trip point, it turns off. That turns the power FET or transistor that supplies the ground off.

The turn off signal can either trip a D-Flop to shutdown the circuit until the computer power gets turned off, or it can run in PWM mode and provide a lower power level that won't damage the power FET or transistor that supplies the ground. Everything that drives a coil, relay or some other feature uses this principle.
 
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#14
jrichker said:
The computer uses current sink technology; it does not supply power but it does supply ground.

The computer controlled ground circuit has protection via an op amp comparator that measures the voltage drop across a low value resistor of known resistance.

As current flow through the resistor increases, the voltage drop across the resistor increases: Ohms Law states E=IR. When the op amp comparator detects a voltage greater than its trip point, it turns off. That turns the power FET or transistor that supplies the ground off.

The turn off signal can either trip a D-Flop to shutdown the circuit until the computer power gets turned off, or it can run in PWM mode and provide a lower power level that won't damage the power FET or transistor that supplies the ground. Everything that drives a coil, relay or some other feature uses this principle.
Click to expand...
@jrichker

Very informative. But, what is your point, Grandma? imp
 
#16
I don't know about you guys but that cleared things up for me, explains what happened that night my resistor needed draining and I d-flopped causing my circuit to shut down for 5 or 10 minutes. My resistor was ok after I had a cup of coffee. I'm just glad I didn't damage my op amp comarator!

:hide:
 
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#17
karthief said:
I don't know about you guys but that cleared things up for me, explains what happened that night my resistor needed draining and I d-flopped causing my circuit to shut down for 5 or 10 minutes. My resistor was ok after I had a cup of coffee. I'm just glad I didn't damage my op amp comarator!

:hide:
Click to expand...
@karthief

Any problem, real or just perceived, lends itself to the exclusive use of left-handed tools! imp
 
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