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STILL FLAKEY PROBLEM

Jim,

On pin 80 of the PCM, I am getting an ~ 5volt level. Now, when I first turn the car to the run position, this applies operating voltage to the module/s and, for a second or so, it is low. Normally, one might think that is the low or ground signal that you say should appear on pin 80. However, I really doubt that it is a valid signal SINCE, it is going from off/low/ground to off/low/ground.

If I were designing this module's protocol, I would ensure that pin 80 of the PTCM provide a high-level to the Variable Load Control Module and let the VLCM go from there. That way, if that line ever shorted to ground the fuel-pump would not be actuated (by pin 80, at least).

Yes, I can see the feed-back loop on the PTCM to pin 40 and after your description, can see that the diagnostics can have a chance at checking the fuel-pumps circuitry.

What I question is the EXACT level expected by the VLCM in order to ground the fuel-pump relay for its 'prime' purpose.

Naturally, by providing a low-level to the VLCM does not automatically ensure that the fuel-pump is grounded if that signal is present on pin 80 to pin 12 of the VLCM. A positive signal, could, if designed that way, provide a ground to the fuel-pump relay through a transistor or chip.

If I had a rock-solid, unambiguous answer on this pin 80 question, I could go from there. As it is, it starts low then goes to 5 volts and that is throwing me.

Thanks for your efforts on this.

Cass

So, as it appears to me, now, the fuel-pump circuit can have problems and > the diags won't catch it. I wonder if this is a correct assumption. > > Cass

"Fuel Pump Signal: Description and Operation

Purpose: The Fuel Pump Monitor (FPM) circuit is spliced into the Power-to-Pump circuit and is used by the PCM for diagnostic purposes.

Operation: The PCM sources a low current voltage down the FPM circuit. With the fuel pump Off, this voltage is pulled low by the path to ground supplied through the fuel pump. With the fuel pump Off and the FPM circuit low, the PCM can verify that the FPM circuit and the Power-to-Pump circuit are complete from the FPM splice through the fuel pump to ground.

This also confirms that the Power-to-Pump or FPM circuits are not shorted to power. With the fuel pump On, voltage is now being supplied from the Fuel Pump Relay to the Power-to-Pump and FPM circuits. With the fuel pump On and the FPM circuit high, the PCM can verify that the Power-to-Pump circuit from the Fuel Pump Relay to the FPM splice is complete. It can also verify that the fuel pump relay contacts are closed and there is a B+ supply to the Fuel Pump Relay."

sounds like they've got the entire circuit monitored for problems.. what are you seeing on pin 12 of the VLCM? What happens if you ground that circuit/ test with a test light?

Regards,

Jim

Reply to
Cass
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Put down the voltmeter..

I think it's better for the PCM to sink rather than source.. with your way, tell me what's happening to the PCM with 'that line shorted to ground'..

grab your LED testlight...

Testlight clip to battery B+, probe pin 80. If testlight lights for a second or two at key on, or lights up with engine cranking, your problem lies elsewhere.

You could even probably get away with using a normal testlight if it doesn't draw too much current.. the one I use draws about 0.04A. Best not to use a Hi-Z meter for stuff like this.. save it for measuring state-of-charge of the battery. Now, if your meter has a low-Z setting, that might work.

If pin 80 lights the testlight, probe pin 7 of the VLCM (for a positive voltage, but less than battery voltage), see if it lights. If not, check for B+ on pin 6 of the VLCM.

Pin 7 is power to fuel pump, low speed. Pin 6 is B+ for fuel pump. Pin 10 is high speed fuel pump relay output. (All on VLCM)

It should take all of 5 minutes to check to see if the problem lies with the VLCM or elsewhere.. this does not sound like a difficult problem to diagnose.

Regards,

Jim

Reply to
Jim

Jim,

I don't use a test light for several reasons. One is that depending on the light, the voltage could be marginal and give false indications. Some test lights can destroy some electronic circuitry; not only due to excessive current draw, but, in the case of filament lamps, inductive kick-back has killed more than one chip. Rare, to be sure but it happens.

You ask the question as to what would happen to 'that line' on the PCM if done my way. Well, if sourced current/voltage any competent engineer would put current-limiting resistors in the line. Even better would be to use opto-isolators with proper dampening for noise glitches falsely triggering the device. So, you could short that line all day long for years and as long as currrent limiting was designed in (simple) nothing would happen.

You say that it should be simple to trouble-shoot this circuit yet, the problem remains. Can you tell me EXACTLY what level this pin 80 should be in order to activate the fuel-pump, high-speed relay?

Guessing at this is going to take a chance of buying a module without knowing that it is defective.

One MUST know what level the VLCM expects on its pin 12 and going to pin 80 on the PTM.

I think it is safe to assume that 12 volts appears on pin 40 of the PTM from time to time when the computer says it is time to turn on the fuel pump for another shot of fuel. Pin 10 of the VLCM is expecting to be low or grounded in order to turn the fuel-pump on by applying full voltage to the pump. Pin

7 of the VLCM is probably pulse-width modulated signal applied when fuel-pressure drops and sends that signal to the PTM.

As I have mentioned, I have verified that the battery voltage on the VLCM, which is pin 6 for the fuel-pump but the other battery pins are okay, too.

Also, the state-of-charge of a lead-acid battery cannot be determined by nothing but a voltmeter. But, let's leave that discussion for later.

This should be a very, very simple thing to diagnose ONLY IF and ONLY IF, one knows for certain what level the VLCM is expecting from pin 80 of the PTM.

Thanks

Cass

Reply to
Cass

Yes, you have to be careful with a testlight and computer circuits (that's why I spec'd an LED testlight), but, speaking from experience, you'll burn yourself if all you use for automotive diangostic work is a Hi-Z meter.. especially in situations where current has to flow.

More parts equals more cost.. manufacturer's will never go for that.. while some drivers in the ECM use current protection, much of the stuff controlled by the computer (like injectors and ignition) are ground side controlled.

Pin 80 has nothing to do with the high speed relay.. it's the ground path for the low speed relay control side.

You're speaking in terms of logic circuits.. pin12/80 is the ground path for the low speed fuel pump relay (even though it may not technically be a mechanical relay). If you tie this circuit to ground and you don't get anything out on pin 7 of the VLCM then you've found your problem.

Nope.. wrong.

Pin 10 is the control line for the high speed fuel pump relay.. but high speed is only activated under heavy load conditions.

Pin 7 is the power output for the low speed fuel pump.. it goes from pin 7 through connector C122 through splice S254 through connector C212 and ends at splice S254, which is the power feed to the fuel pump. PWM?.. perhaps. Ford has done that on some other cars already.

Good.. always check power and grounds.

.. love to discuss this later.

It's a ground path and it has to sink current..

You are _WAY_ overthinking this circuit..

Regards,

Jim

Reply to
Jim

Okay, Jim,

Well, are we going to assume that the VLCM grounds the high-speed pump relay through the data bus?

Cass

Reply to
Cass

crossposted back to rec.autos.tech, where the problem first appeared..

ground path or command line?..

Ground path for the high speed relay is through pin 10 of the VLCM.. the only input I see on the pinout for the VLCM is for fuel pump input, so I suspect that all the rest (cooling fan motors, high speed fuel pump relay and AC clutch are controlled through the data bus.. which looks to be pin 21 (SCP+) and pin 23 (SCP-).

One of the tests a scan tool would allow you to do is command fuel pump on.. if you commanded fuel pump on and it didn't turn on you could probe fuel pump input and tell if the signal is lost inside the PCM, or if the VLCM is receiving a signal, but not activating the pump.

Regards,

Jim

Reply to
Jim

Hey, Jim.

This has become comical. Since we really aren't sure of what signal is supposed to be where and when.

I checked the pin 80 and it starts out low/grounded and stays that way.

You see, we don't know what conditions have to be met for the VLCM to get the signals it requires at the time they are required. The fuel-pump sends back its pressure signal to the PCM and we don't know how the software is handling that info in order to tell the rest of the associated hardware what to do.

In my way of thinking, it seems to be the most logical approach to get the fuel-pump relay going. Since we are guessing in thinking that the relay is controlled via the data buss, and we DO KNOW that the relay is controlled by an mosfet, we can only guess at the rest.

Yes, this is being over-thought but, since there is a dearth of information, I/we are left at trying to hone out a logical approach to solving the problem. To me, it is fun. It is also perplexing.

Digressing to the pin 80/12 problem. As I said, we don't know what conditions have to be met for the low-speed fuel-pump supply to turn on. I think we know that pin 40 of the PCM supplys 12 volts to the f.p.

The driver gets in, turns on the ignition which should turn on the f.p. relay and that is just hot happening. There has to be some information out there somewhere that tells more about this fault.

Thanks

Cass

I am running an experiment re. cross-posting and I didn't recall where your post orginated

Reply to
C_a_s_s_no_DAMN_spam______

Maybe my information is better or there are other reasons, but this looks to me to be a fairly simple circuit..

This differs from when you said..

"On pin 80 of the PCM, I am getting an ~ 5volt level. Now, when I first turn the car to the run position, this applies operating voltage to the module/s and, for a second or so, it is low. Normally, one might think that is the low or ground signal that you say should appear on pin 80. However, I really doubt that it is a valid signal SINCE, it is going from off/low/ground to off/low/ground."

I'm not sure what you mean by it not being a valid signal though..

As mentioned, the low speed fuel pump relay is activated for a moment when the key is turned to the 'on' position, during cranking, and during engine run.

Huh?.. where is the pressure sensor?.. what circuit(s) does this all happen on?.. what pins are used at the PCM?

How many fuel pump relays are there?.. how are they controlled and when are they activated?.. It seems there might be a misunderstanding about the fuel pump relay circuits.

Here's what Ford says to troubleshoot this problem..

*Check for VPWR to VLCM Pin 16 *Check fuel pump circuit continuity *Check FP circuit for short to ground *Check VLCM FP circuit *Check PCM for short to GND *Check fuel pump (FP) circuit for short to ground
2 second prime when key turned on, on when engine cranking, on when engine running. Or, use a good scantool and command fuel pump on.

No. Here's what pin 40 is for.. (from one of my responses in rec.autos.tech, where this thread started)

"Fuel Pump Signal: Description and Operation

Purpose: The Fuel Pump Monitor (FPM) circuit is spliced into the Power-to-Pump circuit and is used by the PCM for diagnostic purposes.

Operation: The PCM sources a low current voltage down the FPM circuit. With the fuel pump Off, this voltage is pulled low by the path to ground supplied through the fuel pump. With the fuel pump Off and the FPM circuit low, the PCM can verify that the FPM circuit and the Power-to-Pump circuit are complete from the FPM splice through the fuel pump to ground.

This also confirms that the Power-to-Pump or FPM circuits are not shorted to power. With the fuel pump On, voltage is now being supplied from the Fuel Pump Relay to the Power-to-Pump and FPM circuits. With the fuel pump On and the FPM circuit high, the PCM can verify that the Power-to-Pump circuit from the Fuel Pump Relay to the FPM splice is complete. It can also verify that the fuel pump relay contacts are closed and there is a B+ supply to the Fuel Pump Relay."

Which FP relay?..

Key off.. PCM disconnected.. VLCM connected.. jump pin 12 of VLCM to ground.. key on.. DOES FUEL PUMP RUN? (this assumes that power and grounds have been checked)

If fuel pump runs, suspect PCM. If fuel pump does not run, suspect VLCM.

Try that and let us know what happened..

Regards,

Jim

Reply to
Jim

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