XtraSlow_XtraCab

Mr. Brown (and Yotatech),


First off let me say that your cheap tricks/repairs website for Toyota 4Runners is excellent! I've been using it for a few years now.


I switched from carburetion to fuel injection about a year ago and recently the 22RE has been acting... funny, so with the aid of both your website and the 1985 FSM I have been probing all EFI components (which led to a TPS adjustment along the way).


My AFM failed the static resistance test between E2 & VB terminals. The FSM shows that the only difference between the VB and VC terminals is the addition of a second resistor for the VB terminal (I tested VB to VC and found that THAT additional resistor works to some degree but as there is no specified value in the FSM... all I can say is that it has ~100 ohms of resistance).


I ordered an Oreilly's remanufactured AFM and tested it prior to installation... sure enough it fails the EXACT SAME TEST (E2-VB).


In short, I am hoping you are willing to provide some feedback based on the above information and the following two questions:


1) Could I be testing this wrong? The FSM lists the spec range in full ohms (as opposed to k ohms). I am not an EE so please excuse any incorrect use of terminology. I have been testing using "full ohms" (the 200 selection under the resistance section of my multimeter). Funny thing is that it does register resistance under k ohms, but the FSM does nto specify for k ohms. Am I testing correctly and received a defective reman unit? Or am I testing incorrectly?


2) I am quite familiar with electrical schematics, but by no means do I profess in this field. Basically, I have no idea where the VB terminal takes part in fuel control. Would you happen to understand where the VB terminal specifically controls fuel (whether it be in terms of engine speed, etc.)?


Thanks for any and all information you can provide. For the longest time I 've heard/read about others' horror stories with "cheap, reman Kragen/Autozone parts". Believe it or not, this is the first time where I question the functionality of a reman part from chain parts stores. I will also post this at Yotatech for others to see/advise in.

--


Dan

MudHippy

1) It's sounds like you're testing it right. But you don't understand what a k ohm is. So let me explain resistance terminology first.

1 ohm = 1Ω
1µ(micro) ohm = 0.0000001 ohms
1m(illi) ohm = 0.001 ohms
1k(ilo) ohm = 1000 ohms
1M(ega) ohm = 1,000,000 ohms

So now that you know what is what, you might want to recheck those resistance values(or not, see 3).

2) The VB terminal is the regulated battery voltage supply to the AFM from the ECU. The battery powers the AFM electronics via a voltage regulated circuit running from the battery + terminal through the EFI main relay to the ECU +B terminal/pin, then from the ECU +B terminal/pin through a diode then resistor in the ECU to the ECU VB terminal/pin(and from the ECU +B terminal through a seperate resistor in the ECU to the ECU THA terminal/pin), then from the ECU VB terminal/pin to the AFM VB terminal/pin(and from the ECU THA terminal/pin to the AFM THA terminal/pin), then from the AFM VB terminal/pin through another resistor to the AFM VC terminal/pin(and from the AFM THA terminal/pin through AFM THA-E2 circuit to the ECU E2 terminal/pin), then through the AFM VC-E2 circuit to the AFM E2 terminal/pin, then from the AFM E2 terminal/pin to the ECU E2 terminal/pin, then from the ECU E2 terminal/pin to the ECU E1 terminal/pin, then from the ECU E1 terminal/pin to ground(engine/body), and finally from ground(engine/body) to the battery - terminal.

The resistor between VB and VC is there to further regulate the voltage supplied to the VC-E2 circuit. In layman's terms the use of the 2 resistors(1 in the ECM + 1 between VB and VC in the AFM) lowers the battery voltage supplied to the AFM VC terminal/pin by ~2 volts. As such, without having any ohms specifications for the resistance between VB and VC an approximately correct amount of resistance can be inferred by checking for that relative voltage difference. Measured with the engine idling, what you should see is ~14V between the VB terminal/pin and the E2 terminal/pin. And ~12V between the VC terminal/pin and the E2 terminal/pin. More voltage means less resistance in the circuit, less voltage means more resistance in the circuit.(See here for the wiring diagram and more info on Toyota's first generation AFM design and function)

But it's really not necessary to do so. Because the ECU is already programmed to detect any abnormalities in the VC-E2 circuit's voltage through feedback data from the VS circuit(voltage signal to the ECU representing air flow). If it does detect that the VS circuit's signal voltage/feedback data is too high or too low(caused by an abnormality in the VB-VC circuit's voltage and/or the VC-E2 circuit's voltage) it will throw a diagnostic trouble code. It determines that such a condition exists by comparing the feedback data from the O2 sensor circuit to the AFM VS circuit feedback data. If it decides that the O2 sensor's signal voltage/feedback data values are too lean in comparison to what the AFM VS circuit signal voltage/feedback data suggests it should be it throws a code 25(A/F ratio too lean). Conversely, if it decides that the O2 sensor's signal voltage/feedback data values are too rich in comparison to what the AFM VS circuit signal voltage/feedback data suggests it throws a code 26(A/F ratio too rich). It uses the same premise to determine if the O2 sensor is functioning correctly. Which is why codes 25 & 26 can be caused by a faulty O2 sensor or a faulty AFM.

It can also detect whether there's a short circuit between VB and VC, VC and E2, or VC and VS. And if there's an open circuit between VC and E2. If it detects any of those conditions it will throw a code 31(Air Flow Meter Circuit).

3) What codes are you getting that suggest there's any problems with the AFM?

XtraSlow_XtraCab

Mud Hippy- many thanks for the swift and thorough reply!

It is actually NOT throwing a code at all, but I don't know whether to be suprised by this or not. The 22RE fuel management system/computer is primitive compared to OBD2.

However, the truck has a severe lag at a certain RPM with the accelerator at a certain position (half way down roughly). I originally considered it a timing advance issue (hence the TPS adjustment) because runnability would severely improve at WOT. I know that the TPS jumps different circuits from idle, to a variable resistor that measures throttle plate position, to WOT. Anyway... it idled better but did not correct the initial problem.

At this point I printed out FSM testing procedures and made my rounds; eventually landing at the AFM. BUT, between your reply and Roger's (via email), it turns out there is a purpose for the numbers on the available ohms selections on my multimeter! My multimeter has '200' on it, which I took as full ohms. What I didn't understand is that this multimeter will only read up to 200ohms on the "full ohms" selection.

After switching to k-ohms, it read .280- which sounds like 280ohms! A healthy AFM! So, an operator error on my part... the specified resistance range exceeded the selection I made on my multimeter. Now I'll know better for next time

So, since you are definetly competent in this sort of thing, can you think of any scenarios in which an EFI component could be failing WITHOUT throwing the Check Engine Light on? Meanwhile, I'm going to test the Oxygen Sensor. I replaced it a few months ago and I felt, or thought I felt, the running condition become a bit rough. It ran great with the old o2 sensor, but it threw a CEL... so I just replaced it and left it at that. Maybe there is a discrepancy with it that doesn't trigger the CEL.

THanks!

MudHippy

Oh yes, they are plenty of opportunities for there to be a problem that wouldn't trigger the CEL or throw a DTC. But let's not get confused on another set of terms here. A CEL(Check Engine Light) is not equivalent to a DTC(Diagnostic Trouble Code). But rather a CEL refers you to a DTC. You don't have a CEL without a DTC, nor do you have a DTC without a CEL(at some point, though it doesn't have to be a constant CEL, because most DTCs are saved by the ECU). Meaning, if you have a CEL you definitely have a DTC. But just because there is no CEL doesn't mean you don't have a DTC. And you could have neither one and still have an EFI system electrical problem.

But that's all from me on the subject. I could sit here all night trying to explain the whys and whats of troubleshooting CELs and/or DTCs, and/or the lack there of were an actual electrical problem with the EFI system still does exist. So I will refer you to a previously written article on the matter.

Here's that: http://autoshop101.com/forms/h27.pdf

With lots more where that came from here: http://autoshop101.com/autoshop15.html

Enjoy!