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Discussion Starter · #1 ·
Can anyone tell me what voltage I should find on the alternator W terminal ? I now have oil light problems ! I've tested both oil pressure switches/ tested the wiring as far as I can, I used Penbryn's guide as posted here but can find no fault . Both switches are new yesterday and my symptoms are loss of speed signal. Voltage at the terminal is 7.12 volts which seems a bit low . Trying to avoid removing fuse board to test onwards but will if reqd !

Naff
 

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The w terminals have a square wave output with less than 50% duty cycle - the peak voltage will be close to output voltage, typically 14V ish. Because you are reading an average voltage, you will not see that peak voltage.

14 x 0.636 = 8.9v and when you allow for the lower duty cycle, the 7v you see might be right.

Do you have a Hz measurement on your meter? I cal VW revcounters to read 5000rpm at 1190Hz so at 850 rpm you should see around 200 Hz
 

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Discussion Starter · #4 ·
Reg , I've got my Fluke 78 working again and that has Hz ranges - AC and DC. I presume you're just metering from the W terminal or speed signal wire to ground on a DC range ? Or am I being as thick as only I can be, I know it's true, my missus tells me quite often !

Nathan.
 

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Yeah measure from W terminal to ground - DC hertz

I've never actually done it though, I use a bench frequency generator to simulate the W signal when I build diesel revcounters from petrol ones.

I have had reason to hook an alt up to my oscilloscope though and know what the output looks like
 

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Discussion Starter · #7 ·
It also supplies a speed signal to the instrument cluster to let the DOP monitoring system (2 oil pressure switches) what rpm is driving the oil pump so that the second(higher pressure) switch can be monitored. If it was only required for the tacho signal it would not be there on T4s WITHOUT tachos. Look at Elsawin and Autodata current flow diagrams the circuit is fairly easy to see once it's been pointed out to you - as it was to me !
Basically it allows a comparison of rpm to expected oil pressure at that rpm and operates the oil warning lights/buzzer if oil pressure is low. There is an excellent post by Penbryn on here explaining it far better than I can.

Naff.
 
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