Author Topic: tach reading incorrect (Read 4612 times)

fillmore · « **on:** November 21, 2007, 05:34:07 pm »

the tach on my AAZ 1.9td is no working correctly, when the car is idling the tach is reading around 2200 rpm, when accelerating through the gears the tach will hit red line and the oil light will flash once. i took a closer look at the wiring to the alt and found a 2 prong plug aswell as the main power source for the alt. i tried unplugging the 2 pin connector and the tach did stop working but the oil light wold flash constantly so i plugged it back in..

what causes a tach to jump around and read incorrectly? the serp belt and tensioner are new and working fine, all the wiring looks fine, no corrosion or breaks anywhere, could it have pulled out of the back of my cluster perhaps?

bigblockchev · « **Reply #1 on:** November 23, 2007, 09:05:26 pm »

If you pull the cluster and reseat the two plugs you may find that things clear up. It is dirt simple to get the cluster out. the light switch just pulls forward with finger pressure and the little cover on the other side does the same no prying with screwdrivers or anything. Take out the two phillips screws holding the cluster surround trim and pull out with both hands on the sides of the trim , you may have to pry slightly at the top to get clearance. There are two more phillips screws holding the cluster in after that . Remove them and tilt/wiggle the cluster forward diagonaly to partially remove it. the plugs have little retainer clips which must be pushed in on the sides to allow the plug to be removed. Loosen the plugs off and push them back in. You will be able to determine if this helps even without completely re installing by starting the car and checking the tach to see the rpms. Maybe it works maybe not but it takes 5 mins tops and costs nothing. Cheers Dan

fatmobile · « **Reply #2 on:** November 24, 2007, 03:10:25 am »

I don't know much about the '93 diesels but how's the voltage regulator doing? Mine is messing up and it throws my tach off.... '91 Golf with a gasser tach converted to work with the W terminal.
I think the tack is driven by the same voltage regulator as the fuel and temp gauges.

bigblockchev · « **Reply #3 on:** November 24, 2007, 03:41:41 am »

The MK3 tach is a different animal than the MK2 as far as how it works, I tried to find out how to convert a gasser tach to diesel but all the info that I found was for the MK2. I tried a gasser cluster in my MK3 and everything worked as far as lights and speedo went (except for the glow plug light) but the tach was not compatable. It may be that the voltage regulator is having some effect on the tach if the fuel guage or temp guage is wonky that could be a sign of where the trouble lies. Cheers Dan

fillmore · « **Reply #4 on:** November 25, 2007, 02:30:13 pm »

im curious to know if the oil pressure light has anything to do with the tach? like i say at idle the tach reads 2000 so when im accelerating the tach will hit readline and the oil pressure light will flash on. Im assuming the light automaticly comes on once your at readline but where im only actually reving the car to roughly 3000RPM it makes driving difficult. when i unplug the 2 prog connector from the alt the tach stops working but the oil light also flashes constantly so im unsure what to do :? i will try reseating them and see what happens.

Vincent Waldon · « **Reply #5 on:** November 25, 2007, 02:50:32 pm »

Quote from: "fillmore"

im curious to know if the oil pressure light has anything to do with the tach?

The answer is "yes".

A2 and A3 VWs use a dynamic oil pressure warning circuit... paying attention to one sensor below 2000 RPM and a different sensor above 2000 RPM. On diesels both the tach and the oil pressure warning system get their engine RPM signal from the W terminal on the alternator.

Th good news is that if you are experiencing simultaneous wonky behaviour of both your tach and your oil pressure lights it's a great clue that points to an issue at what they share: the W signal from the tach.

Check the wiring at the tach closely... particularly the two smaller wires... is the connector corroded ?? Wire broken under the insulation due to vibrations (has happened to me twice) ?? Belt nice and tight ??

burn_your_money · « **Reply #6 on:** November 25, 2007, 03:05:40 pm »

Quote from: "Vincent Waldon"

A2 and A3 VWs use a dynamic oil pressure warning circuit... paying attention to one sensor below 2000 RPM and a different sensor above 2000 RPM. On diesels both the tach and the oil pressure warning system get their engine RPM signal from the W terminal on the alternator.

So all mk2s are wired for a tach? What happens if you install an alternator with no W terminal?