Nct testing battery/alternator?

lalababa · 17-04-2019 3:12pm #1

I've stuck in to other threads on ncts this question before but haven't got an conclusion.
During my nct two leads/clamps were put on my battery during the revving stage.
What is this for?
Many thanks in advance!

17-04-2019 4:04pm

rev counting

lalababa · 17-04-2019 4:38pm

Aha thanks, seems to be attached to the battery..how does that work....Volts to revs?

tossy · 17-04-2019 4:55pm

lalababa wrote: »

Aha thanks, seems to be attached to the battery..how does that work....Volts to revs?

It's to boil their kettle, NCT are notoriously stingy with staff electricity privileges, if they get a big car in they'll do a roast. That's why they are always hapyp to see big fancy cars.

Kenny Logins · 17-04-2019 5:39pm

BrownFinger wrote: »

rev counting

How does that work? Do they not use OBD?

barneygumble99 · 17-04-2019 7:20pm

Kenny Logins wrote: »

How does that work? Do they not use OBD?

They can use a magnet, OBD or clamps

lalababa · 17-04-2019 9:25pm

How does it work from clamps to battery? AFAIK in my v. limited experience you hit a max voltage around 14.5 at around 3k revs. Or should I just read a book on it?

zilog_jones · 18-04-2019 4:07pm

Your alternator does not produce perfectly smooth DC power, there is a voltage ripple relative to the alternator speed, and that will match engine speed.

If you looked at the voltage output on an oscilloscope you would see. Also, sensitive audio equipment which is being powered by your car's alternator would also pick up this as noise (depending on how good the isolation is).

Kenny Logins · 18-04-2019 5:01pm

zilog_jones wrote: »

Your alternator does not produce perfectly smooth DC power, there is a voltage ripple relative to the alternator speed, and that will match engine speed.

No it won't. Alternators rotate at a much higher RPM than the engine.

Cerco · 19-04-2019 5:50am

Kenny Logins wrote: »

No it won't. Alternators rotate at a much higher RPM than the engine.

Yes the alternator speed is much higher, but it is directly proportional to engine speed. Therefore it should be possible to derive engine speed from speed of alternator.

grogi · 19-04-2019 8:23am

Cerco wrote: »

Yes the alternator speed is much higher, but it is directly proportional to engine speed. Therefore it should be possible to derive engine speed from speed of alternator.

But the relation is not consistent between cars. Even same model with same engine might have different ratio.

Cerco · 19-04-2019 9:55pm

grogi wrote: »

But the relation is not consistent between cars. Even same model with same engine might have different ratio.

My point is that I agree with zilog Jones that it is possible to calculate the engine speed from alternator speed. I expect equipment would be calibrated and adjusted in line with car manufacturer & model specifications. Then within that there would be tolerances.
I do not think the same model with same engine would be designed or produced with different ratios. Of course there could be minor variations but they would be within a narrow specified acceptable range.
The ripple pulse frequency would be directly related to alternator speed . Either equal to or twice depending on the rectifier design.
I do not know the equipment used or car design details so I cannot comment much more than that. This is just my opinion.
Perhaps you or others have more information?

Kenny Logins · 19-04-2019 10:36pm

Or just use the OBD port...

Truckermal · 19-04-2019 11:43pm

tossy wrote: »

It's to boil their kettle, NCT are notoriously stingy with staff electricity privileges, if they get a big car in they'll do a roast. That's why they are always hapyp to see big fancy cars.

My Alternator packed up 2 year's ago during the NCT but they put a jump pack on the Battery so they could complete the test! Yes they were miffed as when they seen the 3.0L they had a rack of lamb pulled out of the deepfreeze...:P

20-04-2019 9:32am

zilog_jones wrote: »

Your alternator does not produce perfectly smooth DC power, there is a voltage ripple relative to the alternator speed, and that will match engine speed.

Your alternator dosent produce any DC power at all.
They produce AC!!

demanufactured · 20-04-2019 1:42pm

They are not connecting it to the battery.
They clamp into the engine block, it's a method of capturing RPM during a smoke/emissions test.

CarService · 20-04-2019 2:52pm

This is not accurate and can be couple of hundreds rpm different than actual rpm. They calibrate it by idle rpm only. It needs at least 2 calibration to be the right, but they can not to do the high rpm calibration. The easiest way are to take the rpm from obd2. It get the signal from cam/crank sensor. It is easy to read these sensors directly, only the access may bit hard.

zilog_jones · 23-04-2019 10:22am

Cerco wrote: »

Yes the alternator speed is much higher, but it is directly proportional to engine speed. Therefore it should be possible to derive engine speed from speed of alternator.

That's what I meant, it proportional to the engine speed and can be derived.

BrownFinger wrote: »

Your alternator dosent produce any DC power at all.
They produce AC!!

Ah come on now, the output is DC - every car alternator has a rectifier. You'd have to disassemble the thing to get AC from it. If you're getting AC at your battery terminals then something's very broken.

And the point regarding "why don't they just use the OBD-II port?" - not every car doing the NCT is OBD-II compliant, and it wasn't a requirement for diesel cars sold in Europe until some time in the mid-2000s. It would have been much more the case when the NCT started, but the methodology has not changed.

CarService · 24-04-2019 7:47pm

Some of alternators have the AC terminal, it is marked the letter W.

mgbgt1978 · 24-04-2019 9:29pm

W211 is back :eek:

Nct testing battery/alternator?

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