ABS light is on, no fault codes, 1997 LX450

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Just checking to see if anyone has encountered this: 1997 Lexus LX450, ABS light is on, range selector in HI, brakes work fine; OBDII scan results in no fault codes, absolutely clean. Any thoughts? ABS relay, wiring harness short, ABS ECU perhaps?
ABS light is on in either LO or HI range. Again, no fault codes.
 
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Mine was a harness short after off-roading. My wiring was broken close to the sensor at the front knuckle.
Have you done any axle work such as knuckle or brake service?
No axle or knuckle work, nor brake service. Again, no fault codes indicating a bad sensor. In fact, no fault codes at all. It's clean.
 
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Mine was a harness short after off-roading. My wiring was broken close to the sensor at the front knuckle.
Have you done any axle work such as knuckle or brake service?
^x2, same here. Fortunately, you don't have a pre-'95. Those ABS sensors are made of unobtanium.

I'd check the continuity at the connectors, at all four wheels. Chances are that's where the fault is. If not, you'll have to work your way back up the harness until you find the open. AFAIK, the only way you can have a light and no fault is an open sensor circuit.
 

izzyandsue

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The abs code may not be on the obdii port, but on the code reader by the firewall near the wiper motor. A quick search might find the threads explaining how to read it, all you need is a jumper wire and watch the red light blink, like 3 blinks pause 3 blinks is code 33 and so forth. Might be a dirty sensor in a wheel for example.
 
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^x2, same here. Fortunately, you don't have a pre-'95. Those ABS sensors are made of unobtanium.

I'd check the continuity at the connectors, at all four wheels. Chances are that's where the fault is. If not, you'll have to work your way back up the harness until you find the open. AFAIK, the only way you can have a light and no fault is an open sensor circuit.
Wouldn't an open sensor send a code, such as a P1569, P1575, P1578, or something else ABS related? I am familiar with the jumper wire procedure but have not tried it yet. I was relying on my scan tool. Will give it a shot tomorrow.
 

izzyandsue

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Found it.

53804CB4-3657-4F5B-8667-BCFDE01A198D.png


4403836E-B1DA-4917-B62E-D0DA2ECE49DC.png


and the codes you read
9DD2FA10-888A-4A89-9C6B-514DB857D853.png

F9CDB647-8190-4904-9510-CED2F7FAF5EC.png

BB26C5F6-CE28-4114-8140-87E3531585E7.png
 

florida95fzj

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The jumper blink code procedure can also result in no codes as well. The FSM says that would still be a short but narrows down where to look. I have a short to track down myself. My ABS light is on with no blink codes. It just blinks every half second rapidly.
 
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The jumper blink code procedure can also result in no codes as well. The FSM says that would still be a short but narrows down where to look. I have a short to track down myself. My ABS light is on with no blink codes. It just blinks every half second rapidly.
I will investigate further tomorrow and provide an update. Thanks for the information; it will help!
 
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I will investigate further tomorrow and provide an update. Thanks for the information; it will help!
Okay, that worked. Getting a fault code 51 so that is the ABS pump motor circuit. Any thoughts on that?
 

izzyandsue

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There isnt much on the FSM, just this for that code. Maybe a bad ground in the actuator? Sorry, havent dealt with that particular one.
 

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I would love to see a theory of operation on the ABS system, especially in regard to page DI-224, specifically the Circuit Description, DTC No. 51, Fail Safe Function: "If trouble occurs in the ABS pump motor, the ECU cuts off current to the ABS control (solenoid) relay and prohibits ABS control."
If I am looking at that correctly, the ABS ECU can energize two relays within the ABS Control Relay unit:

1) Motor Relay:
The Motor Relay is activated when an NPN transistor in the Motor Relay switching circuit of the ABS ECU is 'signaled' to switch on or off. This NPN transistor has +12 volts present at its Collector just waiting for 'the signal' at its Base to 'switch' or 'change state'; it is essentially an 'open circuit' until that time. Once the signal to switch is received, the transistor is activated and the circuit becomes 'closed' allowing Current flow between its Collector and Emitter, energizing the potential (high) side of the inductor (coil) in the Motor Relay; in other words, +12 volts should now be present at the MR output of the ABS ECU pin 23, A13 and should also be present at pin 4, A6 of the ABS Control Relay unit. This causes the Motor Relay contactor to close (the contactor is normally open and becomes closed when the Motor Relay inductor is energized). Once this contactor closes, +12 volts from the battery is applied to the ABS Pump Motor of the ABS Actuator assembly; the motor is now energized (and assumed running). The opposite (low) side of this relay inductor is tied to R- (relay negative) via pin 3, A6 on the ABS Control Relay unit and routed to the ABS ECU via pin 24, A13; this completes the circuit as it supplies the final ground path.
Just think of this circuit theory as a water faucet: water pressure or potential on one side (Collector), waiting for you to turn the valve (Base) enough to allow water (Current) flow from the mouth (Emitter) of the faucet to the ground, if you will.

2) Solenoid Relay:
The Solenoid Relay works essentially the same way, except it uses the Solenoid Relay switching circuit of the ABS ECU. The main difference: this is a dual contact relay with one contact set being normally closed, the other normally open.
In its normal state (not energized) the normally closed contacts provide a 'ground' via pin 6, A7 of the ABS Control Relay unit and should be at 0 volts in this non-energized state; this ground affects both the ABS Actuator and the ABS ECU.
When the Solenoid Relay is energized, the contactor is pulled away from the first set of contacts, and therefore the ground is removed; the second set of contacts now applies +12 volts where a ground was present previously. This relay is energized when its inductor high side receives +12 volts from the SR output of the ABS ECU pin 11, A13 routed to pin 1, A7 of the ABS Control Relay unit. So, in summary, the solenoid relay applies either +12 volts or ground (0 volts) to the ABS Actuator depending on its energized state (and then on to various locations of ABS ECU). The inductor low side of this Solenoid Relay is also tied to the same common (ground) point on the ABS ECU at R- (relay negative) pin 24, A13 as with the Motor Relay.
This circuit does not operate the ABS Pump Motor but does interact with the ABS Actuator and back into the ABS ECU.

So having said all that, I am interested in the theory of operation for the ABS Fail Safe Function, or, in other words, how the ABS Pump Motor interacts with the ABS ECU and vice versa. Is the ABS ECU looking for a voltage drop, gain, or both on the high potential side of the ABS Pump Motor; or is it looking for a current drain above or below what would be considered normal for that motor when it is assumed to be energized? The interface point for this is MT of the ABS ECU (pin 6, A13). Is MT looking for a current drain/voltage drop as with a binding armature or shorted winding in the ABS Pump Motor, or is it looking for a lack of voltage drop or lack of current drain as with bad contact brushes, open windings, or open circuits in the wiring? Perhaps both? This would make trouble-shooting a lot easier.

It looks to me if you wanted to test your ABS Pump Motor while installed, you would need to disconnect the battery, disconnect A4 and A5 of the ABS Actuator, apply B+ (+12 volts) to pin3, A4 and B- (ground) to pin 1, A4. I would seriously recommend using a circuit breaker or fuse no greater than the 60 amp ABS fuse in the existing ABS power circuit. You might want to put a range correct ammeter in the circuit to see what amount of current is being drawn. Just guessing, but if the current drain approaches the 60a fuse rating (or blows the fuse), then the motor is probably bad (frozen) or a wiring short is present; if the current drain is about half the 60a fuse rating +/-, the motor is probably good; if the current drain is minimal or zero, then there is very likely an open or resistive circuit, perhaps at a ground point. You will likely have an answer in a just few seconds. Choosing the correct wire size is important: too small a gauge will go up in a flash and potentially start a fire or get hot enough to burn flesh, paint, plastic, etc., in no time flat!
Another point to consider would be the length of time you allow the motor to run in this manner, during trouble-shooting, due to pressure build-up in the system. USE EXTREME CAUTION! WEAR SAFETY GLASSES! I am not recommending anyone try this, but I will be doing so soon to test it out.
This is another area where a theory of operation would come in handy as it would be nice to know if the ABS Fail Safe Function is looking for extremely high braking pressures, and shutting down when that condition is sensed, to prevent hydraulic component failure/ rupture.

If I made errors or am in error, please let me know and I will correct it.
 

izzyandsue

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I am just a simple mechanical engineer, not an electrowizard, but I would try to power the motor by finding the Black with Red (3 on A4) wire that goes to power the motor, and ensure the White with Black wire that grounds the motors is actually grounded (1 on A4), or to the motor body. Also, instead of a big power source with 12v, mabye a simple 9V battery just in case, see if it moves.
 
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I am just a simple mechanical engineer, not an electrowizard, but I would try to power the motor by finding the Black with Red (3 on A4) wire that goes to power the motor, and ensure the White with Black wire that grounds the motors is actually grounded (1 on A4), or to the motor body. Also, instead of a big power source with 12v, mabye a simple 9V battery just in case, see if it moves.
9v transistor battery (assuming that's what you mean) will not stand a chance at moving a motor in a circuit that employs a 60 amp fuse; certainly not with hydraulic pressure to contend with. Those batteries are rated in milliamps. You will need something with the available amps of an automotive battery (and one is already available). That would almost be like trying to start your car with the 9v battery; it ain't happening. Thanks for the input though.
 

Zjohnsonua

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You should have a brake fluid level switch on your master cylinder. If the level gets too low the ECU will render the system inoperable and give you the lamp as a warning.
 
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Do you have a brake fluid level switch on your master cylinder? If the level gets too low the ECU will render the system inoperable giving you the lamp.
That was the first thing I checked before anything else. Yes, it has a fluid level switch and the fluid level is at max. However, a defective switch or low fluid level would give an ABS light, but not give a DTC code 51 during diagnostics. That code is reserved for the ABS Pump Motor circuit.
 
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I had the same problem and it was a dirty connector up under the ABS module on the driver's fender. I turned the key on, everything else off, and wiggled the connectors with the ignition on, not running. I think I heard a click, and the problem went away. There's some ABS relay up under there, and I think the connection just got dirty.
I had traced this problem using the wiring diagram, but once an intermittent connection goes away, you may not recreate it again.
Good luck to you @CALAN !
 
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Okay, as a follow-up, you cannot use a 1996 Toyota Land Cruiser ABS wiring diagram verbatim on a 1997 Lexus LX450; there are subtle but confusing differences (such as the ABS connector nomenclature is different). The bottom line is: Get the correct FSM for the variant FJ80 you are working on. It will save you time and money!
So, after disconnecting A5, A6, A7, and A8, I began tracing the ABS wiring between each segment from connector to connector, with the last segment being the test for continuity through the ABS Pump Motor circuit.
On the ABS Pump Motor circuit, I used an ohm-meter set at 2k ohms between pins 1 and 3 of the female (motor) side of A5. That circuit shows to be open. So, I must assume there is an open in the wiring between connector A5 and the ABS Pump Motor or brushes within this motor are not making contact with the commutator on the armature; perhaps maybe a field winding could be open, but that seems unlikely. More trouble-shooting is needed and I will post the results when found.
 

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