My 1994 SC400 throws a code 31 (Low voltage reading from the Mass Airflow sensor). The car coughs at freeway speeds and dies often at idle, especially on warmer days. Symptoms improve with MAF unplugged -- I drove like that for a while, but got 8 to 13 mpg per tank, and now am overdo for smog inspection so need to get this fixed asap.

I have swapped 2 other MAF sensors (hence tried 3) without a change in symptoms. All MAF sensors measure 2500 ohms (THA-E2) at room temp - precisely within spec.

I believe I have something screwy with the wiring to pin 5 (E1) of the MAF connector, which is a brown wire that I believe ought to be a ground. It certainly does not behave like a ground, but I am hoping to get feedback from this forum -- both on whether my conclusions are solid, and on how I should fix it. This may or may not be the source of my problem, but after spending quite a bit of time on this, this theory for the cause is the best I have.

First, some preliminaries. I have found a few web resources helpful while troubleshooting this. First is this thread here on the Club Lexus Forum:

https://www.clublexus.com/forums/sc-...on-issues.html

A simplified schematic appears there:



I also found this more detailed schematic (for an SC300, but wire colors and pin-outs that I've looked at seem to match), with the MAF at the top-right:



On the first diagram, you'll notice that E1 is depicted as ground. On the second diagram, it is shown as going to a triangle labelled "EC" which goes to ground. My first question: What does that little triangle depict? And what does "EC" stand for in that triangle? (I've thought of "Engine Controller", "Exhaust Computer", ..., but I'm thinking if that were the case, then why isn't it shown as connecting to a pin on the ECU box on that same diagram? -- I think it must be something else).

If pin 5 really is supposed to be a ground, then my expectation is that the voltage difference between that pin and battery ground ought to remain very close to zero. This is not so in my case. For example, with connector unplugged from the MAF, key on, it reads 1.00V. With connector unplugged from MAF, engine running it fluctuates between 1.1V and 1.6V.

Here are my questions for you:

• Is my interpretation that pin 5 (E1) should essentially be a ground correct? Do you agree that these measurements implicate that wire?
• Is it safe (without risk of damaging the computer or something) to short this wire to battery ground? Would it be safe to short E1 and E2 together? Temporarily and/or permanently?
• If I splice a new wire in here to repair this, where should I ideally hook the other end of the wire to?
• Where physically is that EC-triangle component located?

My thanks in advance for any help on this.

 

On the SC400, pin 5 of the MAF should ground to ED, which is the ground lug on the rear of the RH cylinder head.

I would say you are probably correct that there should not be a difference of 1v between that wire (pin 5) and the battery ground wire.

What is the resistance between pin 5 and body ground/battery ground? What does a continuity test indicate? 0 ohms? Open?

I would locate the ground lugs on the cylinder heads, remove them, wire brush or sand the contact surfaces and reinstall. Then recheck your voltage and resistance/continuity between them.

I think you are on the right track.

 

Thank you Stockhatch. That information is incredibly helpful, especially the information that the other end of that wire connects to the ground lug on the rear of the cylinder head.

One thing I didn't mention, for the sake of keeping it tolerably short, was that the problem first appeared on my second drive after having had some major engine work done, including the removal of the heads. The mechanic who did the work felt this MAF problem was unrelated to that job. I've been looking for some explanation for how this problem might be a result of that work, and this could be it -- perhaps he forgot to reconnect the ground wire!

The wire itself does not appear to be a purely open connection. Here are a few more measurements (with connector unplugged):

Key off: Resistance from Pin 5 to battery ground: 250 ohms.
Key off: Resistance from Pin 5 to a bolt near the back of the head: 0 ohms
Key on: Voltage from pin 5 to battery ground: 2.0V (*)
Key on: Voltage from pin 5 to same bolt near back of head: 1.6V
Eng on, idle: Voltage from pin 5 to same bolt near back of head: fluctuates 1.6-2.3V

(*) Voltage to battery ground the other day was 1.0V, but today was 2.0V.

It is extremely difficult to see the rear side of the cylinder head, even with a mirror, but I'm inspired now to hunt this down. I'll repost when I figure something more out.

 

Clean your battery ground terminal and check the cable connection on the block. What is the resistance between battery negative and the block or head?