Installing aftermarket rev counter / tacho into a petrol T25

[937carrera]

I spent this afternoon installing a cheap Chinese rev counter into my T25, here's how I did it.

Thanks to slowcoach in this threadhttps://club8090.co.uk/forum/viewtopic.php?f=39&t=155394 who gave me the of idea of the best way to achieve this.

There'll be a few posts, you'll know when I'm done

First of all remove the top of the dash console by pressing on the two recesses where the clips are on the back, then pulling forwards. Next remove the plastic shield over the brake / clutch reservoir



At this point you might want to disconnect the battery. I didn't, taking the view that there was unlikely to be any potential for a short, plus I could carry out circuit testing easily.

Next disconnect the speedo cable by squeezing the clips together and pushing to the front.

Undo the three philips screw that fix the instrument cluster to the dash top and remove the various switches from the left and right of the instrument cluster by squeezing the tabs (easiest when you know how to do it) or pulling the electrical connectors off.

You can now lift up the instrument cluster so the dash looks like this



Remove the cluster (don't forget the main connector on the right hand side) and you're now ready to start the wiring of the tacho

[937carrera]

Here's a photo of the connector onto the instrument cluster.

If you put a small screwdriver into the void at the ends of the connector block you will me able to lever the end of the connector bluck up, then open it wider to give access to the connectors.



I now went away and studied the wiring diagrams, having come to the conclusion that all the connections I would need would be on this connector block.

They were, note that the connections on the wiring diagram have to be counted from right to left as you look at them. Connections are as follows

t1 - lighting, blue/grey
t3 - earth, brown
t8 - 12v feed, black
t9 - tacho signal, green



The connections from the tacho are only thin cable so I figured I could simply insert them and clamp in place. That worked for 8 and 9, but not for 1 and 2 so I secured them by using some small table ties to prevent movement. That worked, and in the end I also put cable ties on 8 and 9.

[937carrera]

Now at this point I thought I should check the wiring to the tacho (I had already tested the instrument by making temporary connections in the engine compartment, to terminal 1 on coil for signal, carb cut off for 12v and earth)

I switched the lights on first - all ok, dimmer worked as well.

Then I started the van, the tacho didn't work. What had I done wrong ?

I rechecked the connections, they seemed OK and then I went back to the wiring diagram to start tracing the wires. I spotted that the green wire went back to the fuse box, through an internal connection and then back the the coil with a 7 pin connector in the engine compartment. That seemed a decent place to do some tests.

Next I opened the cover on the connection box on the left hand side of the engine compartment and this is what I saw



Could it be that easy. A couple of continuity tests later I had discovered that this broken green wire was the tacho signal wire. It was a very clean snip, almost as though it had been removed at factory. I can''t understand why someone would cut it afterwards.

So what this meant was There was no need to run a separate cable to the engine compartment, as others have. Fantastic

I simply installed a bullet connector to join the cable

[937carrera]

So now all the connections were made and it should work, time for a test



Looking good

Now it was simply a matter of reassembly. I had to do this a few times as the cable attached to the tacho was just a little short and I had to re-route them a couple of times to have the length to install the gauge where I wanted it. Handy having one of those oddment trays that all campers seem to have. I don't mind screwing things into that.

This is where it will finally be installed. The cable is routed under the master cylinder, over the speedo cable and up to the left of the instrument cluster clip.



I'm happy enough, though the illumination is a bit bright. Mind you, I'm unlikely to be doing much nightime driving in the camper, it being a summer vehicle. I suppose I could always insert a small resistor to dim down to '80s illumination.

Total materials cost for the job:

Tacho £10 + £2 delivery ebay
Two pieces of cable shrink wrap to keep the cables tidy under the dash
One bullet connector

Ridiculously inexpensive

[AngeloEvs]

Cutting the tacho wire may have been done to prevent the DOPs buzzer from sounding if oil pressure is low at given RPM.

[itchyfeet]

This green wire at the dash is only present on late vans with the buzzer of doom. Early vans you nedd to run the wire to the engine bay.

you don't need to remove the dash to get at the connector, you can see it by looking up under dash and reach up from below, remove it and pull it far enough away to get to the wires.

personally I cut the wires about 50mm from the connector and use 3 way Wago 221 or 222 connectors to splice in.

[937carrera]

Useful information thanks, I was not aware of those Wago connectors, but even so I don't like cutting into original wiring, even scotchlock type if it can be avoided.

On the buzzer of doom that might be something to follow up on. It looks like my camper has had a little fire at some point, as some of the cabling is melted and the higher pressure sensor wasn't connected when I fixed the oil leak arising from the copper sealing ring. I did reconnect it at the time but it didn't seem to work correctly (can't remember what testing I did, maybe the chopped wire was the issue) so I left it as is, with just the low oil pressure switch connected.

Another little job to do - I'll have to get my head around the DOP system again

[itchyfeet]

I never understood why people won't cut origional wiring like its sacred
its just wire.

Scotchlocks are a bodge and poking wires in .........

[itchyfeet]

Are you going to keep the burnt wire too because its origional? maybe you could get it reinsulated

[937carrera]

The wires Ok, it's just the teminal block that's a bit manky, functional but manky. I'll replace it one day, if I can find an original VW one

We agree on the scotchlocks, on the terminal block it does have a clamping tab for each connection, which worked on a couple of the connections, so not entirely just stuffing a wire in. The cable ties do eliminate any movement, but of course people can choose what route they want to take. For me the learning was that all the connections are readily available to install a tacho, on the later vans anyway. It was easier than I anticipated.

[bigbadbob76]

Cheers for the writeup.
I tend to solder splice when I break into the wiring and insulate the splice with heatshrink sleeving.
Did you have to change any settings on the tacho for number of cylinders?

[937carrera]

Thanks, I've been receiving info for a while, time to give some back where I can, I'll need more help when I get back on the CU bodywork.

I have used solder splicing as well, and in fact used unwarmed heatshrink I already had to keep the 4 wires tidy as they were routed under the dash. I went for a non-permanent solution as it does facilitate easier disassembly / removal if required.

The tacho I bought has a little switch on the back that allows you to switch between 4/6/8 cylinders, I think they all do. Not suitable for some Audis and Ferraris

I've also been reading up on the DOP / buzzer of doom system operation, thanks AngeloEvs. Pretty sure that the wire to the 0.9 bar / high pressure sensor has been earthed thereby defeating the high rpm sensor and silencing the buzzer. Should be an easy job to reconnect and and then test proper operation of the system.

I feel happier knowing that I have a rev counter to confirm what my ear is telling me about engine speed when making adjustments. I now know that fast idle when cold is around 1300 rpm, which drops to 900 rpm when up to temperature, 850 rpm with lights on. I think that means everything is spot on

[937carrera]

Another little job to do - I'll have to get my head around the DOP system again

Not quite qualifying for the "It took you how long" thread, but I took a look at the DOPS wiring today. As suspected the yellow wire was connected to earth, thus defeating the switch.

I disconnected the earth and checked that the oil warning light was flashing with ignition off. All OK

I then connected the yellow wire to the fly lead I had installed from the "high" pressure switch and started the van. No oil light or buzzer of doom.

Leaving the van running I disconnected the yellow wire went to the front and revved the van up to around 3500 rpm. Cue flashing light and buzzer of doom. Allowing the engine to return to idle silenced the buzzer and extinguished the oil warning light.

Job done I think

[bigbadbob76]

My yellow wire is earthed too and has been since I got the van.
I've never found my high pressure switch, I suspect it may have a blanking bolt in the hole.
I know I have low oil pressure so I assume someone did this to silence the BOD.

[937carrera]

The "high" or better 0.9bar oil pressure switch is located at the rear of the engine, in the centre near the crank pulley.

As a picture is worth a thousand words, have a look here https://www.thesamba.com/vw/forum/viewt ... ?p=7140165

The problem with people bodging, is that when the job is fixed the bodge can remain in place. In my case the van has had a recon engine, but when it was fitted neither the "high" pressure switch wiring or the tacho feed were made good.