Hi,

There are two wires coming off the alternator . the big red one that goes to the battery to charge it and a smaller grey/black(i think?) one that from what i can tell goes to the ecu. I assume this is the field wire that tells the alternator how much to charge the battery. Is this a straight forward voltage to the alternator or is it pulsed from the ecu in anyway?

getting nothing from the big red wire to charge the battery so wanted to test the value on the other wire to make sure it was getting a signal to make it generate the charge .... (2005 grande punto 1.4 8v)

No it's not pwm controlled as far as I know.

It's the classic field energizing cable. The only difference is that it's not controller from a simple bulb on the dashboard (like old cars used to) but it's controlled from the main engine ECU.

Increase RPMs (2000-3000 or more) and check if it starts producing power.

no still no charge at any revs -

if its not pwm i guess i can apply battery voltage to that smaller terminal and see if it produces charge then

Yes you can, just a touch of +12v to d+ is enough to energize the field normally with the alternator spinning.

will give that a try at the weekend

Also check the earth wire between battery / to chassis leg to gear box

surreyrust said:

will give that a try at the weekend

Click to expand...

I would like you to make some more tests if you can

1) With the key to on position (but engine NOT running):

• Measure Voltage on D+ terminal - with the cable connected on alternator
• Measure Voltage on D+ terminal - Alternator side - with the cable disconnected from alternator
• Measure Voltage on D+ terminal - Cable side (ecu) with the cable disconnected from alternator.

2) The same tests with the engine running.

I would be VERY grateful if you could provide me that voltage readings.

Also, if you have an obd reader read any stored errors

The small terminal (wire) on an alternator with a built in regulator is the warning light. It is not normally connected directly to the battery, but via a small lamp (the "ignition" light on the dash) and the ignition switch (in the classic circuit at least). It supplies the initial voltage required to get the alternator field magnetised and the alternator producing power. On the older cars this meant that if the "ignition" lamp failed the alternator would not start working even thhough it was OK.
Don't connect this terminal directly to the battery with the wire connected, you might damage the ECU. Disconnect the wire and connect the terminal on the alternator to the batery positive with a small 12V bulb (sidelight or interior light size) in series. Then check the output with the engine running. If none it's the alternator, possibly just the regulator/brush pack.

HTH,
Robert G8RPI.

1) With the key to on position (but engine NOT running):
Measure Voltage on D+ terminal - with the cable connected on alternator
Measure Voltage on D+ terminal - Alternator side - with the cable disconnected from alternator
Measure Voltage on D+ terminal - Cable side (ecu) with the cable disconnected from alternator.

Click to expand...

ok

ignition on

a) 1 V (connected)
b) 3 mV (disconnected alt side)
c) 11.5V (disconnected ecu side)

engine running

a) 1V
b) 8mV
c) 11.08V

given that the ecu is supplying a field voltage i guess it must be the alternator? - looks a pain to get out of there!

Yes I think the alternator is dead.

well got the alternator off eventually! (for anyone else trying this just remove your air con pipes and then your air con pump and save yourself a lot of faffing about)

testing the resistance across the two slip rings (field windings ) i get 278000ohms!! so i guess my field windings have broken down ??

i take it that means a new alternator or can this be repaired?

Hi,
The field winding (rotor) should mesure just a few ohms, so there is something wrong. I take it you are checking at the rings themselves, not the brushes. The most common failure point is where the wire is attached to the slip ring. Have a good look at the connections with a light and magnifier. If it's a bad joint there you may be able to fix it yourself. Otherwise it's a recon alternator.
You mentioned disconnecting aircon pipes. You are not able do this yourself, the aircon gas must be removed and captured by an authorised specialist (tree hugger laws). You also need to seal the connections immediately to stop moisture getting in.

Robert G8RPI.

my aircon hasnt worked for sometime and there was no gas left in the system

i thought the R134a fluid used was environmentally friendly anyway?

I find the easiest way to remove the alternator with the car on a ramp. I hate AC related stuff.


And no the gas is not environmentally friendly. It's ozone friendly only.

how do you get to the upper alternator mounting bolt from below?
I tried but failed (ac pipes in the way), but then i was crawling under the car, might be easier on a ramp?

On my 1.4 16v it's impossible to remove it in other way. 10 minute job on a ramp

ah - mines a 1.4 8v
couldnt get to the top bolt for the pipes , couldnt even get the air con pump off without removing the pipes as the bolts that hold it on were blocked from fully coming out by the pipes! All the more annoying as the air con didnt even work anyway.