Alternator or ECU computer or some other sensor?

1990 Civic DX, 4 Dr., 5 Sp., 1500cc

I've got the Battery light on in the dash pretty much all the time now.
It was intermittent, not coming on for a half hour or so after startup,
and either coming on or going out under certain engine load conditions,
but now it's there all the time.

Honda Service Manual has an Alternator testing procedure that first
begins by removing the main connector from the Alternator and checking
for voltage to ground at two different pins inside the connector.

Unfortunately, mine fails the second of those voltage tests, and in that
case the test procedure refers you to the ECU troubleshooting section at
that point. Which is a bit of a problem because I don't have the special
interface cable needed to access the ECU circuits for test purposes, and
get this: the test procedure for the ECU says "Substitute known-good
ECU; if problem disappears, replace ECU" at various points in the
procedure.

Hmmm. I haven't priced it officially yet, but that thing costs in the
neighborhood of $250 bucks! I can't afford to buy one just for testing
and troubleshooting!

And if I *had* a known-good ECU, what would I need a testing procedure
for in the first place? Swapping it out would *BE* the testing procedure!

;-)

I think the problem is that the ECU has an economy circuit in it that
shuts down the Alternator under certain load conditions to save fuel.
Pretty neat idea, except when it malfunctions!

My question: How can I determine for certain whether it is the ECU
malfunctioning, or if it's some other load/condition sensor giving it
faulty data? And I suppose it still could be the Alternator itself...

Keep in mind I don't have a "known-good" ECU to follow the official
diagnostic procedure, and I'm not sure if I can go buy one, use it for
testing, and return it if it turns out not to be the culprit.

Any ideas?

;-)

Thanks,

--Charlie Henderson

 

I think you need a simple volt, amp, load test at your corner parts
store. Follow their advise. bob

 

Yes, perhaps. You're right, I didn't do the full field tests on the
Alternator, but remember, I was following the Honda Service test
procedures. They specified testing for certain voltages through the ECU
from the Alternator harness *first* - and that test failed.

And Bob, I appreciate the suggestion; believe me, I wish I could just
turn it over to someone with all the proper test equipment/new parts.
But I'll be lucky if I can afford the parts I need... there's nothing in
the budget for someone else's labor.

I was hoping these symptoms would be common, and someone here would know
where the likely failure is - the computer or some load sensor, etc...

Thanks,

--Charlie Henderson

 

Round here, they test 'em fer free! bob

 

Hey, you're right, Bob! I found a parts dealer who'll bench test the
Alternator for free. Or a *small* fee. So that'll take my diagnosis a
little further, with a degree of certainty. But if it tests ok, then I'm
still left with my original question:

That circuit in the FI ECU that shuts down the Alternator to save fuel?
How can I find and test the sensors that trigger that circuit *without*
replacing the ECU with a "known-good" unit?

By the way, Honda wants $850 for that ECU, and will *not* allow me to
return it if it turns out not to be the problem. I can get it
aftermarket for around $250, I think, but I can't return that, either...

Figures.

--Charlie Henderson

 

Actually, it makes sense, and that rule applies to all electric components.
If you plug anything the wrong way, or in a faulty circuit you have a very
good chance of burning it. Sometimes that is noticeable with the naked eye,
other times you need to run a full diagnostic to find out if the part
still works.

So nobody wants to take any chances on returned electric parts.

Cosmin

 

Only time I invested $100. in a junk yard ECU, it didn't fix the
problem. It's from an 89 LX-i. The ECU and the extra distributor for
that car are for sale, if anyone wants them. bob

 

Charlie,
I have a regular voltage testing machine but I was in Kragen's Auto Parts
store yesterday and saw a $9.00 item called "Auto Charging System
Analyzer". I believe they sell the same type of item at almost all auto
parts stores. You can use it to test the battery or test the alternator.
It comes with detailed instructions. I purchased it as a Christmas present
for someone. It may not tell you everything you want to know but at least
it will allow you to test the alternator and battery.

 

That circuit in the FI ECU that shuts down the Alternator to save fuel?

The ECU does not actually shut down the alternator, it just lowers the
output voltage. This is not based on any single sensor but a host of
conditions including engine RPM, vehicle speed, coolant temperature etc.
On my '94, stepping on the brakes will defeat this power saving mode and
bump the voltage up to normal.

Does you service manual outline what conditions need to be met for the
lower voltage to be engaged on your particular car? The one from Helm
does for my '94, but I don't know if Haynes and others go into that much
detail.