1992 Lexus 400 - Possible Ecu Problem

[fiveofakind]

This problem is occurring quite frequently

Preface: this LS-400 has 267K miles on it. But runs good when it is operating properly.

Symptoms: 1. Anti-lock indicator light comes on. 2. Radio cuts out when I step on brake. 3. Interior dome lights & map lights are dim. 4. Headlights are dimming but I still have enough to drive. 5. Radiator & Low oil light will come on occasionally sometimes. 5. Car will idle erratically sometimes between 1,500 to 2,000 rpm. 6. When starting, battery seems to have low voltage in it sometimes.

The above mentioned symptoms are occurring quite frequently, however when I floor it from time to time, these symptoms will go away & the drive will drive normally for awhile but these problems will come back eventually.

This seems like it is electrical problem somewhere which will may be hard to trouble shoot or is it a possible ECU problem ???

I dont want to pour a lot of money into this as it is a very high mileage LS-400....

I am very mechanical with a set of factory shop manuals but this one has me stumped.....

Any advice that someone can offer on this problem would be greatly appreciated.

Thanks for reading,

Jeff

[RDM]

None of those symptoms are ECU related.

Have you checked the alternator and charging system, including the plugs on the alternator? Sounds more like that's your culprit. All of those problems can be explained by voltage loss, I'd also look at the battery (under load) to see if a cell is damaged.

[93ls400walt]

one simple issue is the wire harness located in the driver side trunk hinge under the black plastic trim. Remove the trim and pull the harness out. Follow it back to the different bends and inspect the wires for cracks or breaks. Splice in the same size wire if any areas appear to be damaged.

This problem is common and can cause many weird issues. Search the threads for more info on this.

Some of what you described may be ECM related. See my recent thread on ECM repair.

[fiveofakind]

one simple issue is the wire harness located in the driver side trunk hinge under the black plastic trim. Remove the trim and pull the harness out. Follow it back to the different bends and inspect the wires for cracks or breaks. Splice in the same size wire if any areas appear to be damaged.

This problem is common and can cause many weird issues. Search the threads for more info on this.

Some of what you described may be ECM related. See my recent thread on ECM repair.

93ls400walt....thanks for the advice.....i will check for cracks in the wiring where you described.....I will report later on the weekend....i have heard of this wiring check before but have never of this as being the culprit

I will advise......Thank you

[fsuguy]

one simple issue is the wire harness located in the driver side trunk hinge under the black plastic trim. Remove the trim and pull the harness out. Follow it back to the different bends and inspect the wires for cracks or breaks. Splice in the same size wire if any areas appear to be damaged.

This problem is common and can cause many weird issues. Search the threads for more info on this.

Some of what you described may be ECM related. See my recent thread on ECM repair.

93ls400walt....thanks for the advice.....i will check for cracks in the wiring where you described.....I will report later on the weekend....i have heard of this wiring check before but have never of this as being the culprit

I will advise......Thank you

fiveofakind,

What you are describing sounds like an alternator problem. My '92 showed similar symptoms about 8 months back, and thanks to information on in this forum, I suspected the Alternator. Battery would die after getting a full overnight charge, and all dash warning lights would suddenly light up etc. Anyway, took the alternator off and had it tested at an Autozone where they confirmed it was defective. Put in a new one and problem resolved.

HTH

Mike

[fiveofakind]

fsuguy,

I am starting to think that it is an alternator problem...I did have the alternator rebuilt about 3 to 4 years ago....Maybe it is shot again.....

I did take apart my ECM module to look at the boards.....no corrosion & no leaking capacitors......

Someone suggested the trunk hinge wiring problem.....I have not looked into that but I believe that is not the case....

Thanks,

5ofkind

[killerFatty]

If you floor it and the problems go away it would make sense for it to be the alternator because the higher rev's would turn the alternator quicker and therefore charge more!?!? The trunk hinge wires would be somewhat uneffected by the change in throttle. That would be my logic, although I can't say that with any kind of certinty. If I was you, I would just take the 5 or 10 minutes and check the trunk wires just incase though. It can't hurt.

[fiveofakind]

killerfatty,

revving it up in place only took care of the oscillating idle (1,400rpm to 2,200 rpm ) maybe. Once the the car warmed up & was run on greeway for 10 to 15 minutes or so....would the idle come back down to normal ( 600 rpm )...

Possibly idle speed control is suspect....not sure...

[steve2006]

Have you pulled the fault codes?

[VBdenny]

How old is the battery? first thing I would suspect since they give many of the things you described. The wires in the trunk break often but are easy to access and identify for repair. The usual result is the "R" (Reverse) light coming on when you apply the brakes.

[fiveofakind]

I will check trunk wiring this weekend....I dont suspect that this is the problem....

Suspecting either a faulty battery or intermittent faulty alternator.......I will specific gravity in each cell of battery..Battery is only 2-3 yrs old.....

The problem is intermittent faulty alternator is one minute they are working...the next creating havoc.....I would have to catch it the time all of these problems are going on to check it.

[curiousB]

If you have alternator problems you'd likely have other symptoms with it. Like: lights dimming, battery dying, and so on. It sounds like you are seeing these issues so it could be alternator. Get your hands on a digital voltmeter (DVM). It doesn't have to be an expensive one. $10-20 versions are good enough for what you are measuring. Do the following:

1. measure battery voltage at battery posts (measure at battery + and - posts, not to frame) with engine off and nothing electrical turned on. Should be around 12 volts. If it is much lower than this then you either have a bad cell or discharged battery.
   
2. start car but don't turn on anything electrical other than engine running. Battery should now be around 13.3 Volts. Be careful doing measurements wth moving parts under hood.
   
3. start turning on electric loads headlights, defogger, blower, bun warmers, rear defrost, headlamps. If alternator is good the battery should stay at around 13.3 volts if it sags in voltage then alternator is suspect. Try reving engine to around 2k rpm and see if voltage resumes to 13.3 if so its either alternator or slipping serpentine belt (would squeal if slipping).
   

If your alternator is bad the most common issue is brushes are worn out which is easily repairable. If they are worn or broken you can't get enough field current and then the alternator can't get enough power out. If its not the brushes then it is likely blown winding, fried diodes on rectifier plate, or bad voltage regulator. Generally these failures mean replacing the alternator with a rebuilt unit.

If you don't have a DVM get one. One of the most useful tools you can own and very inexpensive.

[mann777]

).

If your alternator is bad the most common issue is brushes are worn out which is easily repairable. If they are worn or broken you can't get enough field current and then the alternator can't get enough power out. If its not the brushes then it is likely blown winding, fried diodes on rectifier plate, or bad voltage regulator. Generally these failures mean replacing the alternator with a rebuilt unit.

If you don't have a DVM get one. One of the most useful tools you can own and very inexpensive.

Just to want to add to the curious list, as the symptoms quoted are Alternator, there is a rectifier with the Collar Adjusting Alternator, if this is fused, you would have the brushes getting worn out faster, and later the condenser blowing out. You may want to look at these also.

[fsuguy]

).

If your alternator is bad the most common issue is brushes are worn out which is easily repairable. If they are worn or broken you can't get enough field current and then the alternator can't get enough power out. If its not the brushes then it is likely blown winding, fried diodes on rectifier plate, or bad voltage regulator. Generally these failures mean replacing the alternator with a rebuilt unit.

If you don't have a DVM get one. One of the most useful tools you can own and very inexpensive.

Just to want to add to the curious list, as the symptoms quoted are Alternator, there is a rectifier with the Collar Adjusting Alternator, if this is fused, you would have the brushes getting worn out faster, and later the condenser blowing out. You may want to look at these also.

I would add one small suggestion to the most excellent testing write-up by curiousB, and the additional note by mann777:

Check to see whether your power steering fluid is leaking - this is usually a prime culprit for failing alternators - the PS is mounted just above the alternator and when it leaks the fluid drips right onto the alternator eventually causing failure. If you have a leak, fix it at the same time you fix the alternator. Also look up this forum for a picture of a makeshift shield (looks like a sort of plastic bottle trimmed to fit on the alternator) to protect the alternator.

HTH

Mike

[_mach_]

I had a very similar problem with my 1991, every time i hit the brakes, dash lights would come on, radio would cut out, and sometimes the car would just die. It ended up being a bad connection on the battery itself. The main terminal was ok, but there was a ton of build up in the wire itself (the battery terminal had been replaced with a clamp/crimp style end). I cut back the insulation, cleaned the crap out of the wire itself, and put a new battery terminal on. Completely fixed the problem. I would start there, check both positive and negative cables, if you've got a multi meter, you should check ohms resistance between battery terminal and the ground on the engine, as well as the positive to the distribution block. Hope this helps.

Good Luck

Grayson

[fiveofakind]

All problems have been resolved......alternator needed rebuilt...bad diodes in rectifier & bad brushes, ic regulator also was bad.....battery had a bad cell in it.....battery replaced.....all symptoms have vanished......

Alternator was rebuilt by reputable auto electric shop...$ 120.....I had the Lexus alternator rebuilt...the rebuilt ones from AutoZone or Kragen's are junk......battery was a 8 yr battery that went south after 3 yrs...prorated & replaced for about $ 50.....new fan belt $ 18....The labor for teardown & replacement was free.....my labor....

If anyone is interested in rebuilding their alternator...a good website for alternator parts is

www.transpo-usa.com

Thanks for everyone's help...

fiveofakind