anti lock brake light

[wig]

after running awhile, the light always comes on, seems like after it warms up, doesn't have to be moving. Anti lock diagnostics show no codes. Lately stays on until it cools down. Can't find no shorts, got an anti lock solenoid relay and ECU cheap from junk yard, same problem. Running out of options here, any ideas out there. Also vehicle reguarly serviced, all fluids, etc.
90 model SR5 4WD M/T 3VZ V6.

[Brian R.]

Check the connections on all your wheel sensors.

[wig]

'90 has no wheel sensors, but thanks for the reply.

[Brian R.]

OK, I believe it. Can you tell me how the brake system knows one wheel is locked up to know to apply the anti-lock system?

Not being argumentative, I just don't have a clue...

[wig]

2 ways, 1 speed sensor over differential that reads serrations on ring gear. 2 Deceleration sensor in front of console mounted solidly to a brace. Then the actuator working off the power steering pump pulses brakes. There is also a relay that controls this through signals from anti lock ECU, which is seperate from the engine ECU or Toyota calls it something else for the engine. All these check out, no codes. Sounds really like a short to ground to light, but could be anywhere. Havn't found any frayed or bad connections so far. Also the '90 has only rear anti lock, what good is that?

Appreciate the come back, no problem with asking questions, I don't take it wrong in anyway.

[Brian R.]

Never too old to learn something new. I knew about the rear anti-lock. I just assumed it was a standard system. Never really looked.

You didn't say your anti-lock system wasn't functioning. Have you tried it to see if it is functioning as it should? Maybe the fault is in the light, as you suggest. How about the fluid level in the master cylinder?

[wig]

Appears to be functioning fine. I can let it sit idling standing still and it will come on. All fluid levels are fresh and at level they should be. The light function is all I haven't figured out yet, how it grounds to come on, same positive feeds all the guages and lights, but looks like the anti lock light is a little different. I suppose it could be just about any relay, solenoid anywhere grounding out. Still have not figured out actuator yet either, wondering if the electrical part of it may be grounding out. No codes though. You would think a code would set, of course it may not be as sophisticated as I might think, there may not be a code for everything in that anti lock circuitry. I'm still plugging away at it though, still reading and researching. Dealer has no clue, been to two different ones. I have the wiring and fluid diagrams for the antilock, but there is no real description on how the current flows or functions, It's only a supplement to all the other shop manuals showing accessories and anti lock. I'll never give up, just not in me.

[Brian R.]

Usually, the switch is on the power side and not the ground side. The lights usually are always grounded and the light comes on when power is fed, not when the light is grounded. There is probably a switch being activated when the light comes on, probably in the ECU. Further down the circuit, there is possibly a switch activating either normally or incorrectly or a bad sensor that triggers the signal in the ECU. I don't think it goes any deeper than that.

[wig]

Actually you are close. The anti lock light is fed positive all the time according to electrical diagrams and fed a ground when something is wrong via either the ECU or the anti lock relay. The relay controls the ground via a signal from the anti lock actuator, that triggers the light via ECU or the relay. Acutually the relay can be positive or negative depending on how it's triggered. That's only a general description, gets really complicated.

Anyway, I've talked to a dealer that agreed to let me use a new relay to see if it fixes the problem, if not then no charge. The antilock for '90 was very poorly documented and they are just as confused as I am. Since I do a lot of business with them, they are willing to work with me at a discount, since I'm providing a lot of the logic. So, still plugging away at it, hopefully to a resolution, the next step would be the actuator (Toyota calls it a solenoid) and the relay is a solenoid relay, confusing I know. The actuator controls the brake fluid on one side and power steering fluid on the other. This is how it pulses the brakes by turning off and on the solenoid or actuator. We are 99% sure the ECU is not bad, since they could find no records of ever having to ever replace one and they are built really tough, besides they retail for $250.

It also turns out to not be a short, but an open, indicating the relay, then the actuator, then the wiring. I've checked all the wiring and it's OK. The relay is cheaper, the actuator is big bucks, unless the electronic portion is replaceable sepearate from actuator, but I doubt it. That's a big job replacing that unit, requires a special control box from Toyota to regulate the unit after replacement to bleed air, but I bet you probably could do it without it. Takes longer though.

I got 3 relays from the junkyard and tested them and they are all bad, so maybe I'm on to something. The manual tells how to test them and none of them test out, that's another reason the dealer is working with me to find out if the relays could be a problem, seems they have other customers with the same problem. By the way only that relay was used in '90 and '91.