mm88

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?

PureDrifter

iTrader: (10)

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.

Yamae

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.

dicer

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.

Yamae

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.

dicer

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.

PureDrifter

iTrader: (10)

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

dicer

How far off topic are we?

mm88

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

PureDrifter

iTrader: (10)

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.

Yamae

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.