Measuring '91 Tach signal

[StefDS]

Hi all,

I recently installed a kit (https://van-quality-parts.fr/compteur-et-pieces/1511-pcb-de-remplacement-pour-bloc-compteur-.html) to replace the blue foil.

Everything works, except the tach (which was working fine beforehand)

There are 3 wires going to the tach: brown, black and green. Between the brown & black I have +13v.

I briefly shortcutted the green wire to the bolt on the seat, and it gave a spark, so I guess the green wire also has good connection?
Is there a way of measuring the green signal wire?

Anybody got an idea what could be the next step of diagnosing the issue?

Thanks!

[DuncanS]

You are supposed to have 9 ish volts. That's what the voltage stabilizer does. It's either blown somehow or bypassed. Are you getting continuity from the coil to the green wire at the tach?

Duncan

[StefDS]

Thanks for the reply.

Yes, I have continuity from to coil to the green wire on the dash.


The voltage regulator is built in the new box together with the PCB, as seen below (don't mind the red circles, I copied the image from the instructions)

I'll check this weekend on how to test the voltage regulator which was supplied.

Thanks

[MarkWard]

Take a test light and clamp it to ground in the engine compartment, and start the engine. Probe the negative side of the coil for just a moment. The test light should flash. Now repeat the test at the cluster wire connector. Should be the same result.

The problem is the coil negative side has more energy than the 12 volt test light was designed for. It can pop the test light bulb if you take too long. You have to be quick. All you want to see is that it flashed on and off.

On a points ignition back in the day, you could leave the testlight connected indefinitely. If you get the flash up front, your problem is not the path to the cluster.

[StefDS]

I just found some time to spend in the garage:
The test light flashes when connected to the coil, as wel as testing at the cluster.

Is there a way to test the tach itself?

[MarkWard]

Tach needs 3 things to work. A good ground, 12 volts when key is in run position, and the pulse signal from the coil. Unfortunately my memory of the back of the cluster is foggy. So, I can’t really be of much more help. There were changes in the cluster connector depending on years and some vanagons came with a clock instead of a tach.

Might be worth checking with GW. I recall their board is a fits all models solution. When I went from an early clock cluster to a later tach cluster, I did have to make a couple changes in the cluster plug connector. I used a pdf from a website from Old Volks Home. No idea if it’s an available still.

See what GW says.

[wesitarz]

[StefDS]

The blue foil got replaced with the PCB kit.

If I jump directly to the tach, it also doesn't work

I have a feeling that the Voltage regulator might be the culprit (the kit came with a built in one, see the picture above).

Any idea on how to test this?

[SLOWoody]

Having replaced my clock with a tach recently in my 83.5 Westy, I had to trouble shoot the tach signal along with the tach itself. To measure the tach signal on the green wire, an oscilloscope was used to measure the pulsed signal. I don't remember the exact settings for the oscilloscope but could ask my friend if you need them. This allowed me to determine that there was an issue with the signal and discover an issue with a connection of the green wire in the new EFI wiring harness. Once the tach signal was fixed I also discovered that the tach itself was damaged and the small chip on the tach needed to be replaced. From what I have read and experienced, the small chip on the tach PCB board can easily be damaged if the wrong voltage is run through it. If the tach wires were connected improperly at any point during the foil replacement, the improper polarity/voltage could have fried the small chip on the PCB board. You might want to confirm the functionality of your tach before you start replacing item on your new foil replacement board.
_________________
83.5 Westfalia, GW 2.3L w/ GW EFI, 4 speed

[wesitarz]

[TrashPanda]

[StefDS]

Indeed, thank for the diagram.

I might have found the discrepancy; the supplied instructions with the PCB kit say that I should connect the 3 wires on the tach in the following way:

looking at the back of the tach, from left to right:
1st pin: green
2nd pin: black
3rd pin: emtpy
4th pin: brown


This is the exact opposite of the diagram shared above, where the following is shown:
1st pin: brown
2nd pin: empty
3rd pin: black
4th pin: green


I can only test this weekend on the bus, but I suppose the diagram is correct?

[wesitarz]

Looking at the back of the tach from above, their instruction is correct. I rewired my cluster and the signal wire is correct as shown in the diagram as I recall.

[StefDS]

Coming back on this; I've measured again, and I got correct 12v, ground and signal at the 3 pins that go into the back of the tach.
So the issue must be the tach itself (I'm thinking I might have 'overloaded' it when connecting the new wires at the tach incorrectly)


Is there any way of testing the tach internals?

[MarkWard]

From racing, I have an old autometer tach. It come in handy from time to time. I also have an old tach and dwell test meter. Actually used it recently on a BMW 700 2 cylinder for diagnosing a problem.

I’d think you could come up with something similar as a last test before condemning your tach. It sounds like you have what the tach needs to work and it’s rare to get away with hooking up things backwards.

Another option is to send the tach out. We have a couple shops in Florida we use for that. Sucks when you follow directions and they end up not being correct.

[StefDS]

Just came across this clip on youtube, quite interesting


Link

[DuncanS]

[crazyvwvanman]

This is not true.
The cluster voltage stabilizer is only for the fuel gauge and the temp gauge.
To keep the gauge needles from reacting to variations in the dash voltage they get a their power from the voltage stabilizer, which is actually a 10 volt regulator.

Mark

[DuncanS]

Thanks for correcting me, Mark. It's been a couple of years since I did this and got confused about what the voltage stabilizer fed.

But I am right about reversing the polarity on the tach and burning it out. It, or at least mine had no reverse polarity protection.

Duncan

[DanHoug]

just for information... the tach signal comes off the negative side of the coil and provides a pulse every time the ECU lifts the ground on the coil, as there is always 12v resting on the other side of the coil. the spark occurs when the ground is lifted.

a single cylinder fires once every 2 crankshaft revolutions. so if the crank is turning at 1000rpm, a single cylinder is firing 500 times a minute. multiply this by the number of cylinders, four, and the total coil firings at 1000rpm = 2000 sparks per minute or 33 times per second. an individual cylinder is only firing 8 times per second.

the IC chip on the circuit board on the back of the tach is a Low Drop Out (LDO) regulator or 'pulse shaper' and can be replaced if damaged. it converts the voltage pulses from the coil to a current output to drive the tach motor. the below photo [credit: Alika T3] shows 2 different part numbers either both should work. replace the electrolytic caps while you're in there!

part # SN29736P or SAK215 LDO Regulator used in pulse shaper circuit

_________________
-dan
60% of what you find on the internet is wrong, including this post.

'87 Westy & '89 Westy both 2.1 4spd

Past projects can be found at--
www.thefixitworkshop.com