Author Topic: Oil Pressure light & Tach ??? (Read 5840 times)

MadCityMike · « **on:** October 04, 2007, 01:10:07 pm »

This morning about a minute after starting out I noticed the tach needle dropped to zero and at the same time the oil pressure light started flashing. I gave the accelerator pedal a blip and the tach came back on, and the light went out. Then a few seconds later the tach died again and the oil pressure light started flashing again, so I parked it and took my other vehicle to work today. I've been wanting to hook up a mechanical oil pressure gauge anyways, so I'll be doing that this evening, but does it make sense that the tach would die whenever the oil pressure light was flashing, or am I looking at a dash failure of some sort? By the way, just to be clear, the actual engine RPM's are not changing when the tach needle is dying.

Reading some other posts, I gather that when the oil pressure light is flashing it means low oil pressure, and if I hear buzzing it's no oil pressure? Since I'm in the states finding AAZ specific parts is not exactly an easy task, so can someone post the part number for both of the oil pressure senders so that I can eliminate them as possible problems? Along those lines, can someone suggest a good online source for AAZ parts in the states??

Mike

Vincent Waldon · « **Reply #1 on:** October 04, 2007, 03:38:55 pm »

The dynamic oil pressure system pays attention to two different oil pressure sensors based on engine RPM.... so, if the control circuit gets confused about RPM it will watch the wrong sensor.... perhaps buzzing because it's looking for 2000 RPM pressure but the engine is only idling.

Flashing means the below-2000 RPM sensor is unhappy.... flashing *AND* buzzing means the above-2000 RPM sensor is unhappy.

Bottom line: you probably have an alternator issue first and foremost (tach is driven off the alternator output... tach going away means the alternator has gone away)... it sounds like a loose belt since it is RPM dependent, but it also could be a dying regulator or worn brushes and/or communtator and or bearing at that end of the alternator.

If you hook up a voltmeter to someplace convenient.... say the cig lighter... you should see the voltage fluctuate as the tach comes and goes. Or, the dome light at night will change brightness.

Check the belt tension... if it's OK the next easiest thing is to pull the brush/regulator assembly (2 screws) and look at the brushes and commutator.

One other outside chance... the "W" wire that comes off the alternator and feeds the tach/oil pressure cct has a break that is coming and going as the alternator flexes slightly under reving.

Let us know what you find out !!

Vince

MadCityMike · « **Reply #2 on:** October 04, 2007, 04:09:46 pm »

Thanks for the info. The alternator is definitely not happy as I'm only getting 11.5V while it's running. I know that I was previously getting some belt slippage/squawking but it's no longer doing that, so that tells me there's no more load created by the alt either. Is there a way to manually tighten up the belt, or do I have to replace the tensioner? If the alt is bad, are there rebuild kits available? I've rebuilt numerous older Ford alternators in the past, so if the parts are available I'm sure I can do it. I'd rather not spend the money on a complete one if I don't have to.

Mike

MadCityMike · « **Reply #3 on:** October 04, 2007, 04:39:55 pm »

Looking at different VW alternators, I see there is a 90 Amp version for the gassers of the same vintage. Mine is a 70 amp. Is the 90 amp a simple swap? Does a gasser alternator have the W terminal I need for the tach?

Vincent Waldon · « **Reply #4 on:** October 05, 2007, 12:56:30 am »

11.5V = bad for sure... is it that way all the time, or only when your tach stops ?

Didn't notice that you had a 94 ergo a tensioner... but yes, if the spring gets weak the belt can slip. Pretty rare though... I'd be including to look behind door #2: regulator/brushes/commutator.

In general parts are *not*available for the alternator other than the brush/regulator assembly... although at most places it's more than halfway the price of the entire unit. Sigh.

90A was the stock size for diesel alternators AFAIK... a gasser alternator won't have the w-terminal although you can easily add it yourself.. especially if you are used to pulling alternators apart.

MadCityMike · « **Reply #5 on:** October 05, 2007, 12:53:52 pm »

I saw in a post of yours a long time ago that you have a schematic to convert a gas alt to have a W terminal, but I haven't been able to find it. I understand the concept of where to make the connection, but a picture would be much nicer if you still have it available.

Mike