sjc2467

I'm a diagnostic technician for Mercedes, but I am having a bit of trouble diagnosing a problem with my Lexus (I have no Lexus training, and no experience on similar systems).

I have a '93 SC400 with 33,500 miles on it (really). Until last year, the car was rarely driven (garage kept). When I started driving it, last year, it had about 23,000 miles. Other than a cracked coolant reservoir, which I just replaced, it has been trouble free until this week. Brake fluid appears surprisingly clean given the age, but I have been planning to flush it as soon as I get a chance (I bled the front brakes pretty well, when I did the LS400 caliper mod, last year, but other than that, the fluid was pretty old (it was always dealer serviced, which may have included a brake flush at some point, but not recently). Now I suddenly have the trac warning light turning on after I start driving most of the time and the traction control is inoperative (once the warning illuminates). Codes are 43 in trac indicating a 'fault in ABS or trac and 54 in ABS indicating 'TRAC pump motor operation frequency excessive' which is set when 'TRAC motor relay On is output continuously 10 times at intervals of less than 24 sec. when traction control is not being executed.". I believe the code 43 is simply a result of the other fault, and can be ignored. I can hear the pump cycle for 10 short bursts, as soon as I start driving, right before the light comes on. I live in NY, and we did recently (2 weeks ago) have some heavy snow, which resulted in some traction control operation, but nothing crazy, and it seemed to perform as it should (this was before the problem started).

I believe the problem is due to an internal leak in the hydraulic actuator or an electrical fault in the control unit, but I do not know how to verify (I don't like throwing parts at a problem, to see what sticks, but that may be my only option).

I found several other reports of this exact problem (on this forum and elsewhere), but most of them had no resolution (one person had luck bleeding the accumulator and hydraulic actuator, but that did not work for me). I thoroughly bled the accumulator and actuator by manually powering the TRAC pump (which seems to be working fine). I visually inspected the lines from the pump to the accumulator and from the accumulator to the hydraulic actuator (even with the pump running), and I cannot find any leaks (and the fluid level has not decreased). Once or twice, the pump seemed to build pressure when manually powered (when I first tried) but now it just runs without slowing much. It seems that it should build pressure pretty quickly and slow down (but I don't know for sure). As such, I think the most likely fault is an internal leak in the hydraulic actuator, but if it's normal for an open valve to prevent pressure build-up when the car is off, it could just as easily be the control unit (I know some of them have issues with the capacitors...)

Does anyone have any thoughts or diagnostics advice?

If I cannot narrow it down, I am going to replace the actuator. Even at my price (dealer cost) the actuator is fairly expensive, so I think I'll be taking my chances with a junkyard part (unless someone on here has one...?) Does anyone have any tips on bleeding the actuator, once it's installed? Obviously I would pre-fill it as best I can and bleed from the bleeder screw, as I have already done, but what about the air trapped in the vertical lines coming out the top, and the air trapped inside the unit? I assume this is not part of the normal hydraulic brake circuit, so bleeding at the wheels probably wouldn't help much... Is there a procedure that uses a dealer-level scan tool (or something else) to actuate the unit and help bleed it? Should I just bleed at the bleeder and the wheels, then go activate the system a bunch with some wheel-spin, and re-bleed?

Thanks for your help!

sjc2467

Just wanted to follow up in case anyone else has this problem and finds this thread. I found a description of operation document for the TRAC system on a mid '90s supra, which is very similar (although not identical) to that of the Lexus. Pump pressure in the TRAC system is regulated by a spring loaded relief valve in the TRAC actuator, which should hold pressure in the accumulator and line (if the pump keeps running, excess fluid is relieved back to the reservoir, but once the pressure drops, the valve closes and holds pressure). My system was not building or holding pressure (when manually actuating the pump motor), and since I had no significant* leaks in the system between the pump and the TRAC actuator, the problem had to be internal to the actuator (I suspect the relief valve failed or became jammed open by debris). I replaced the actuator, bled, cleared the codes, and the problem is resolved.

I did find a very slight leak in the high pressure hose between the TRAC pump and accumulator. My searching indicated that I am not the only one to have problems with this hose, so this is a good thing to check before replacing anything. For some reason, the hose has broken down, and is in poor shape, and seeping fluid. Based on the condition of the outside, it is possible that a small piece broke from the inside and plugged my relief valve, although that is just speculation. I just picked up a new hose; I'll be installing it tomorrow.

As for bleeding: it's pretty easy. I was particularly worried about the 3 lines on top, which seemed like they would trap air. One is a return to the master cylinder, which should self-bleed when the pump runs. The second is the normal rear brake line, which will bleed as part of the rear brakes (either rear wheel should be sufficient, but I choose to bleed both; I only got bubbles from the first one). And the third is the line from the pump/accumulator. This is bled by opening the bleeder on the unit and activating the pump motor (I believe that opening the bleeder then starting the car may work to turn on the pump, but I chose to simply disconnect the connector and apply fused battery power manually, with test leads). As for the two lines out the side: they both go to the ABS unit: one is part of the normal rear brake circuit, which will bleed from either rear wheel, and one is a return to release pressure that the TRAC system has applied to the rear brakes. It shouldn't matter, and should self-bleed, eventually, when the system operates.

In short: bleed from bleeders on accumulator then TRAC actuator (by actuating the pump), then bleed from either rear wheel (both recommended) and you are done. If you replace the high pressure hose, not the accumulator, you can just bleed the accumulator and TRAC actuator, but it's probably a good time for a full brake fluid flush, anyway, so you can follow up by bleeding at the wheels.

And for anyone who's curious about the function of the actuator: basically, it is normally a pathway between the master cylinder and ABS unit (for the rear brakes), but when TRAC is activated, it blocks off the normal brake circuit from the master cylinder and applies regulated pump pressure to the ABS unit, which takes care of the fine-tuning of pressure to the rear wheels (returning excess fluid back to the actuator). On modern cars, this functionality is simply included in the ABS unit.

pishta

so the problem was bad accumulator? Well, that is the same issue Im dealing with. My pump would growl for 10 seconds after start and then stop; the TRAC light would come on 7 seconds later. I checked the pump and its working, I pulled the bleeder on the accumulator and nothing came out. I took the bleeder hose from the accumulator and looped it to the M/C reservoir (yeah , I know do not reuse fluid...) and jumped the TRAC motor relay pins in the fuse box (strap across the 2 top pins to manually cycle the motor) I got alot of air bubbles and then a clear stream. I buttoned everything back up and bled the smaller port on the opposite side and found alot more bubbes then clear fluid and a noted labor in the TRAC motor. Started back up and found TRAC light on and that the pump no longer energizes with the starting of the motor? I ran the codes and I got the 43 and a few ABS codes....I DMM'd the output of the harness to the pump motor and only saw a tick of power on startup and then nothing. I think the stored codes stop the entire process. once I cleared the ABS/TRAC coded (Tc jumped to I and pump brake 9 times within 4 seconds) the TRAC code 43 and ABS 53 came back within 1 minute...but the TRAC motor did energize for only about 2 seconds this time. TRAC code 43 is unknown.....

sjc2467

Yes. And good luck getting one. They're not available anymore. Had to get a used one which is already on borrowed time. Might be able to find someone to rebuild one...

If you open the bleeder without activating the pump does much fluid come out? It should. Those bubbles you saw were probably all the nitrogen from the accumulator that had leaked out into the fluid.