Hello,

My brother was driving his 98 ls400 a couple days ago, when the check engine and vsc light on the car came on and the car seemed like it did not want to accelerate. I had to stop by to pick him up, but when i started the car a few minutes after this happened, it started and drove just fine. He stated that a few weeks ago, the car suddenly stopped accelerating for a quick second and started to work again. When i review the error codes on the car, the only error i see is p1633, which according to the service manual, is caused by a ram error, ecm cpu malfunction, or ETCS power supply malfunction. I believe all of these components are in the ecm, so is the ecm going bad?

It should also be noted that the car also has an issue with low idle. it idles around 400 rpm when warm, but if i turn on the ac or headlights, it goes up to ~600 rpm ( i think this car should idle around 700 rpm). A couple years ago, i changed all the spark plugs on the car, cleaned the throttle body and maf, and did the capacitor replacement on the ECM (the capacitors did not appear to be visually bad when i did this).

Any ideals on what could be going on? Thank you for looking.


PS: is there a way to troubleshoot or test an ECM, or is there a procedure for it at the techinfo site or service manual?

 

first and foremost, check battery and alternator voltages (engine off and at idle for charging). I've seen this code pop up here/there when the car was low on voltage. Also don't be surprised if after loosening/replacing the battery the car gives you VSC-Off errors, it's supposed to. They'll go away.

 

There is no troubleshooting manual for the ECU. We need to analyse it all by ourselves or simply ask a dealership to fix. Dealerships just replace it and charge you some big money though.

I don't have any experience regarding P1633 and I can't give you any particular ideas.

These are just the basics of the troubleshooting. The 1st step is to check the regulated low voltage DC lines. Are the voltages all correct and well regulated? Aren't there any voltage ripples? The low voltage lines use electrolytic capacitors of low voltages and you can assume which lines are so.

Next comes the power up initial reset signals and power down reset signals. Are these generated properly? So far is the basic troubleshooting.

In your case, you also need to check the signal from the throttle position sensor. I worry that it might have some continuity or glitch problem. Since there is no manual and you have to be patient to do the reverse engineering deeply if you don't like to visit any dealership.

 

Yamae, could a person build a test bench of sort for the ecu? And could you just power it up and not need all the other connections made to it, to check what you have just described? It just could be a lot of trouble to check such things while its in the car especially if there is no garage or shop to put it in and its raining outside.

 

It depends. For many people, it is almost impossible but it may be possible for some persons. I usually test different kinds units at my bench after the intensive analysis and basic confirmations. That's why I have different measuring equipments and power supply units that costed more than a new Celsior.

 

Since its a 12v system in the car isn't that all a person needs to power the ecu? Do you ever use a logic analyzer? I suppose a person could use some variable resistors to simulate some of the sensors.

 

stay on topic guys, trying to help this guy with his issue more than discussing benchtop ecu diagnoses.

 

How far off topic are we?

 

Thanks for the replies.

I replaced the battery with a new one late last year, but i havent checked the alternator yet. I'll give that a shot along with the Throttle position sensor and throttle control motor. Could a loose or bad accelerator cable do this as well?

Thanks again

 

Unlikely. In 98+ the cable is a backup device, the throttle pedal position sensor senses throttle input and via the ECU actuates the throttle control motor.

 

I don't think that a loose or bad accelerator cable causes your problem. I'd rather think that the throttle position sensor or the related circuit causes it.

Other than that, check the low voltage power supply circuits in the ECU. Attached photo below is a part of those. See the metal film resistor at the bottom end and compare the color of it with the one 100Ω at nearly the top. Those 2 resistors were originally colored same. The one 2.7Ω at the bottom is obviously semi burnt and the resistance is bigger than 2.7Ω and this causes malfunctions of the ECU when the battery voltage is lower caused by too much loads or poorly working alternator. I sometimes find this problem among Gen2.5 Celsiors.

 

Hi yamae,

Getting the ecu repaired for my 98 LS400, replacing capacitors etc. My guy noticed that this specific 2.7 ohm resistor (R862) is partially fried on my board, slight discoloration and reads 3.1 ohms. Do you know the wattage for this resistor? Or can you point towards an adequate replacement resistor?

Best regards

 

The resistor R862 is a 3W metal film resistor used for the voltage regulator. It heats up and it increases the resistance a little by a little as it ages. It would be better to replace it before the resistance reaches 3.0 ohm. One idea is to use a 5W resistor instead of 3W since it can be mounted and stronger against the heat.

 

An update:
ECU repair is finished and the 98LS400 made it 1000+ miles to its destination.

There were no 5W 2.7 ohm resistors available from digikey in a reasonable quantity to purchase. They did have two different 3W resistors available, see below.
https://www.digikey.com/en/products/...BVXDm6xbuOjQZE
https://www.digikey.com/en/products/...Cbz6sDvtOGl3xw

We went with the MOSX3CT631R2R7J resistor as it looked more like the original resistor

 

Jims98LS400, I think we have this same issue. Is there a way to deal with this problem short of breaking out a soldering iron and buying 3W resistors? Most repair shops will likely giggle with this approach.... while intellectually right on the mark!! We've been thru 3 reman alternators and 1 new alternator now, and the shop is stumped., works for about 5-10minutes, then "no charge".. 1998 Lexus LS 400 with 270K+ miles.... great car.... but starting to get electrical gremlins..... dash instrument gauges, etc.