ABS ECU Diagnostics

[Rubiclone]

Some background,

I have a converted 1993 FZJ80 with a 1991 1HDT, I did the mechanical conversion and a shop (Radd Cruisers) did the electrical work on the truck about 6 years ago now.

Long story short I need to have a functioning ABS light, the 1991 dash that I am using does not have a resource for this light as that truck did not have ABS.

The truck has the ABS installed and is working, the original ECU and everything is still in place. Upon removal of the dash I found a wire that was labelled ( ABS Warning Light) but I did not measure voltage or a ground connection through this wire (left over from the conversion work)

Where I am right now.

I found a wiring diagram for a 1993 FZJ80.

Now when I read this it appears that the ECU is ground switching the Warning light circuit. I back probed the ECU with the multi meter connected to chassis ground.

Point B - very little resistance to confirm computer ground connection is good
Ignition just turned on - 5Mohm
After a few seconds with ignition on - Open circuit.

This is as expected as the timer would allow the warning light to come on when the ignition is first turned on but then turns off after a few seconds for the bulb test.

The resistance is very large though and I was expecting a much smaller reading. In the manual it states that I need 0V when light is on (connection to ground) and 12V when light is off (no longer a circuit to ground so no current flow).

Obviously I dont have a bulb setup so I wont be reading any voltage at the computer. I had a test light which I connected to battery + and probed the "W" at point A, the light did not turn on at all.

Does anyone have any ideas or input on where to go next? Should the resistance of the computer be that high for low current flow?

Am I missing something?

Rob

 

[Rubiclone]

anybody?

 

[george_tlc]

Connect an actual bulb for testing.

Not sure what you expect from resistance measurements - they are essentially meaningless at the 'output' of the ECU.

Typical bulb drive for an ECU etc is to drive/switch the bulb 'output' to low (near ground level voltage), the other side of the bulb is at 12V so the bulb will then light. Once the ECU wants to turn off the bulb it will just stop driving the 'output' which will then 'float'. Since the bulb is essentially a resistor the one side of the bulb is at 12V and the other side will also go to 12V since the 'output' side is now floating. The bulb has 12V on one side and 12V on the other side and is not lit.

As an aside: This is one MAJOR problem doing LED retrofits since a LED does NOT act like a resistor. On vehicles with more modern ECU systems etc that expect a real bulb (incandescent) to be connected a LED can cause error messages or weird behaviour. Not a huge issue with the 80 - but do not folk have had problems with the cruise control turning off on its own when you turn the left/right indicators on on 80's that have had LED brake light retrofits done - this specifically due to the LED brake lights not acting like an incandescent (resistor-like) bulb.

cheers,
george.

 

[Rubiclone]

I did try it with a test light and it would not light the test light which made me think that the path to ground was not sufficient through the ECU. I realized after the fact I should have done my same test on the other bulbs which are actually working. I will go buy a proper bulb and wire it up, I was just hoping to get an idea of whether or not the ECU is doing what its supposed to and I was expecting to see a low resistance through the ECU when the bulb is supposed to be on and then an open circuit to turn the bulb off.

I think the logic works fine because I see high resistance (bulb test) then open circuit and when I remove the main ABS fuse the circuit never goes open. I assume this is because the AST output on the ECU is grounded and not opened by the solenoid.

Is the W the output of the ECU? I was seeing it as the Input and then the ECU would internally connect this to ground.

Rob

 

[george_tlc]

You can't measure resistance of an active output signal.

Meters inject a voltage (on the resistance range) but then an output signal is also outputting a voltage.

Meter resistance ranges are used for passively measuring a resistor or sensor etc - when NO other voltage is applied.

As I wrote, a typical ECU 'bulb output' signal will actively drive the output low (typically something like an npn transistor etc) and the bulb get's +12V on the other end from IGN hot, or from the rheostat dimmer signal. When the npn transistor is not 'on' the output just floats.

I'd test with a real bulb wired in - one of the correct wattage.

The ECU W signal appears to be an output (it will go to Ground/0V to light the ABS bulb). It can also be pulled to ground via the ABS relay (diode and W signal in the EWD). It's won't be driven high, it will just 'float' high through the bulb resistance.

cheers,
george.

 

[Rubiclone]

Alright I'm following, In the diagram the two arrows that say "To data Link Connector 1" I don't think are actually connected together am I right?

The way I read that diagram was that both of those go to the data connector and in the data connector you can jumper those wires together to trigger the ABS light to come on. I assume the diode is there so that when a connection tool is connected it wont feed current the wrong way into the data connector.

I will try to pull an existing bulb out of the dash in the next few days and wire it up into the W connector and post back with my findings.

Thanks for the feedback so far George

 

[george_tlc]

You're mostly correct, except the data link connector MUST HAVE THE LINK INSTALLED for normal ABS system operation.

i.e. there must normally be an electrical path in place from the W connection of the ECU/Warning light to the Anode end of the Diode.

You remove the link as part of the steps to read out ABS error codes (so that only the ECU can drive the warning light versus the relay cause the ABS light to remain stuck on).

Some hints on the operation of the toyota ABS system ->
http://www.testroete.com/car/Toyota.../Technical Training/05 - Brake Systems/18.pdf

cheers,
george.

 

[Rubiclone]

Where did you find that? That is a great resource.

You're saying I should have a plug in my data link connector? I am assuming its the one under the hood that looks like a small OBD2 plug, it has a cap on it that flips up but when it does I only see female connections inside, are you suggesting there should be something underneath this cap in the female receiving sockets?

 

[Rubiclone]

So I dont know what is going on with the ABS ECU, the "W" wire does not do what its supposed to, all the other inputs for the bulbs switch as required and I can get a 40kohm reading when it is "on" and an open circuit when the bulbs turn off. I can't get this from the W port so I dont know what is going on there.

My workaround, the main power wire going to the ABS motor (big red wire on the connector), this wire gets power a few seconds after the ignition system gets power. I tapped into this wire as the coil for a normally closed relay that goes through an LED from an ignition switched source. This way, when the ignition is first turned on the light illuminates for a few seconds and then turns off. When I remove the ABS fuse there is no signal to the normally closed relay so the light stays on.

I got an LED light that has an ABS symbol on it from ebay but it is delayed in shipping so I just mounted a red LED and wrote (ABS) beside it.

Booked the followup inspection for friday so fingers crossed they are happy with it now.

 

[vrlndr]

You can't measure resistance of an active output signal.

Meters inject a voltage (on the resistance range) but then an output signal is also outputting a voltage.

Hello

A light bulb on plus 12 also injects a voltage to the output signal, no systematic difference other than different voltage and resistance.

A multimeter on the diode setting should usually work OK to troubleshoot open collector (bipolar transistor) or open drain (MOSFET) outputs. But check first with a second multimeter that the red lead has positive voltage (my multimeters I tried have 2.5 V and 3V).

On the diode setting, an active (on) bipolar transistor should show about 0.6 V. An active (on) MOSFET should show just a few millivolts, mostly depending on the leads.

On the Ohm setting, provided that the red lead is positive, a bipolar transistor may show somewhat random kilo-Ohms, depending on the open-circuit voltage of the meter relative to the characteristic curves on the transistor.

On the Ohm setting, on a active (on) MOSFET output the multimeter should give an accurate resistance, usually in the low milli-Ohm range (MOSFETS are awesome).
I'm just writing this for educational purposes. I know the thread is old.

 

[Malleus]

There is no time limit on good to know details!