120 pin ALH Tdi analog tach install

I thought this info would fall under the "google it" catagory since it seems so basic.... I i didnt want to ask a question that has been asked a million times so I tried searching it and didnt come up with much. So here are my findings. I realize its elementary but maybe it will help someone.

Ive been using a tiny tach on my ALH swap but really wanted an analog tach. So I looked at my pinout and found that #27 pin of a 120 pin alh ECU is the tach signal. The book says Green/brown wire) It looks like the AHU, and 80 pin ALH have a wire in the harness for tach signal but the 120 pin ALH does not.



The 3rd hole in from the left in the row above the blue wire is #27. This is the larger of the two ECU plugs.






When I disassembled the plug I found the pin to be in place with a small piece of insulation (brown & Green) not sure if this is how all ALH's are or if someone cut mine... I have two harnesses and they both are like this.
Knife is pointing at #27







Here is the pin removed.







I stole a pin and a length of wire from a spare harness......






I put everything back together and hooked it up to the tach. This one has adjustable dip switches but I didnt have to mess with them at all. It was dead on right out of the box.





I compared the new tach with my old tach and VCDS. All is good! Dont worry this engine is in my 71 year old Jeep and will never see 4000 rpm. I chose this tach because its very easy to read from 1500-2550 which is where i usually live.





The final product!

https://youtu.be/9jLoXL-lgGA



Hopefully this can assist someone who is trying to get info on this procedure. Thanks!

Awesome write up! Now I know where to point someone

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Pin 27 deffo tacho signal in all 121 pin ECU's, PD included, looks like someone cut the wires in your looms. You can take an analog signal from one of the windings in the alternator before it passaes through the diode, but if you are using the VW alternator it will only read correctly on a VW analog diesel gauge, this is what I have done in my ASZ converted B3 passat.

Nice!

That’s what I thought too... several people told me that I would have to add the wire. I was told the 1.8/vr6 have the wire but the tdi does not. I have two harnesses and they are both like this. I removed one from a stock car myself. So I’m hearing and seeing conflicting info.

CasaEd said:

Pin 27 deffo tacho signal in all 121 pin ECU's, PD included, looks like someone cut the wires in your looms. You can take an analog signal from one of the windings in the alternator before it passaes through the diode, but if you are using the VW alternator it will only read correctly on a VW analog diesel gauge, this is what I have done in my ASZ converted B3 passat.

Click to expand...

Now, that's what this forum has been missing!

Nice list @jimbote, but I also like to use pin 18 which is the earth switch wire for the ECU control relay

CasaEd said:

Nice list @jimbote, but I also like to use pin 18 which is the earth switch wire for the ECU control relay

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which ecm are you referring to? pin 37 on the 121 ecm is the wake up wire, then the ecm sends "earth" to relay 109...i'm not short-cutting the factory pathways... can you clarify your approach?

Pin 37 is the feed from the ignition switch which then lets pin 18 go to earth which energizes the ECU relay which then sends power to the ECU to get it up and running, and yes I'm talking about the 121 pin ECU's, the 80 & 68 pin ECU's also power up the ECU in the same way, but the pin numbers are different,On the 68 pin ECU Pin 38 is the ignition feed & pin 42 earths the EC relay, on the 80 pin ECU pin 47 is the ign feed and pin 33 is the EC relay earth wire. Hope this helps

CasaEd said:

Pin 37 is the feed from the ignition switch which then lets pin 18 go to earth which energizes the ECU relay which then sends power to the ECU to get it up and running, and yes I'm talking about the 121 pin ECU's, the 80 & 68 pin ECU's also power up the ECU in the same way, but the pin numbers are different,On the 68 pin ECU Pin 38 is the ignition feed & pin 23 earths the EC relay, on the 80 pin ECU pin 47 is the ign feed and pin 33 is the EC relay earth wire. Hope this helps

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still confused by your post, if maintaining the stock pathways why would you need to touch pin 18? it's function is undisturbed and not really worth mentioning unless there is damage to the stock wiring.

Ok, pins 18,42 & 33 is what the ECU sends to earth to enable relay 109 or 53 to send main battery power to the ECU when you turn the ignition switch on i.e pin 37 on the 121 pin ECU. Relay 109 is not turned on by an ignition feed, it has permanent battery feed at pin 30 and when pin 42 is earthed it switches the relay on which in turn sends
power through pin 87 of the relay to pin 23 of the ecu which then turns it on.

CasaEd said:

Ok, pins 18,42 & 33 is what the ECU sends to earth to enable relay 109 or 53 to send main battery power to the ECU when you turn the ignition switch on i.e pin 37 on the 121 pin ECU. Relay 109 is not turned on by an ignition feed, it has permanent battery feed at pin 30 and when pin 42 is earthed it switches the relay on which in turn sends
power through pin 87 of the relay to pin 23 of the ecu which then turns it on.

Click to expand...

yes, i understand that relay 109 is not directly turned on by the ignition switch, but is grounded by pin 18 of the ecm, so why are you concerned with pin 18? unless you are bypassing the ecm grounding ? and grounding the relay yourself to excite relay 109?

I did'nt see pin 18 on your list
https://forums.tdiclub.com/showpost.php?p=3110590&postcount=4

CasaEd said:

I did'nt see pin 18 on your list
https://forums.tdiclub.com/showpost.php?p=3110590&postcount=4

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It doesn't need to be on the list... i only listed wires and pinouts that require attention when making a standalone harness. The wire coming from pin 18 is NOT one of those because it's path remains undisturbed between the ecm and R109. If you are disturbing that wire, you are taking an unnecessary step.

jimbote said:

It doesn't need to be on the list... i only listed wires and pinouts that require attention when making a standalone harness. The wire coming from pin 18 is NOT one of those because it's path remains undisturbed between the ecm and R109. If you are disturbing that wire, you are taking an unnecessary step.

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Do you delete the relay from the harness?

So if you make a stand alone harness without using pin 18 but want to maintain OEM wiring how are you going to power the ecu up via relay 109 or any other relay for that matter ? @jimbote

CasaEd said:

So if you make a stand alone harness without using pin 18 but want to maintain OEM wiring how are you going to power the ecu up via relay 109 or any other relay for that matter ? @jimbote

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??? ....the wire from pin 18 to relay 109 is untouched and intact so why mention it? it's like mentioning the wires from ecm to pump, they need no attention when paring down a harness so why include detail about them?... i'm not building a harness from scratch. If you want to cut this wire and replace it (only god knows why) that is your business. I don't touch it in my standalone harnesses aside from removing the bulky connector from it's path so it's not worth comment unless i wanted to confuse folks on what is necessary.

To me a stand alone harness means it can be used in any vehicle, not all vehicles have a VW fuse box so the relay has to be wired accordingly, so are you just switching a relay on to power the ECU ?

CasaEd said:

To me a stand alone harness means it can be used in any vehicle, not all vehicles have a VW fuse box so the relay has to be wired accordingly, so are you just switching a relay on to power the ECU ?

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i use the factory VW relays (glow, 109) and the neccessary factory paths are intact for simple diagnosis, and it's still a standalone harness with one battery power wire, one key/on start and a few grounds to get running.

So from what I can gather from your information is:- the ecu relay has it's own earth, you switch it on with the ignition feed which then sends power to the ECU ?

CasaEd said:

So from what I can gather from your information is:- the ecu relay has it's own earth, you switch it on with the ignition feed which then sends power to the ECU ?

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If you read and understand the factory VW diagrams then you will understand how my harness works. I think you're making this a lot more complicated than it needs to be. Factory relays, factory fuses, factory wiring pathways, very simple.

I understand the factory diagrams which is why i'm on about this pin 18 as it earths the ecu control relay from the ecu when you switch the ignition on at the ecu, and i use this method in all the PD conversions I do here in Portugal

a peek inside my standalone relay/fuse box

This is what I'm trying to explain, in this ECU (AGR) it's pin 33 that earths the relay


https://imgur.com/yQKLpPl

CasaEd said:

This is what I'm trying to explain, in this ECU (AGR) it's pin 33 that earths the relay


https://imgur.com/yQKLpPl

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ok? so what are trying to do? we are talking about 121 pin ALH ecm's here. this is an AGR. The EC relay is functionally the same as relay 109 but uses a different pin to ground said relay. If i were building a standalone using the AGR harness i still would not mention pin 33 because i would leave that wire intact and only focus on the bare minimum of interfacing wires to get the thing running. And those would mainly be the key on/start "wake up" wire, grounds, and power wires. This pin 33 (AGR) or pin 18 (ALH) are part of the existing harness that needs ZERO attention when doing a standalone, unless of course, you've somehow hacked the harness to bits removing it or somehow severed it during the paring down process. Don't really know how else to explain it, but basically "if it aint broke, don't fix it" ...or "keep it simple stupid" (no offense intended or implied) ... I understand the importance and functionality of this wire and i assure you it's utilized in my harness, but so are ten dozen other wires that need no comment because they are part of system left intact during the paring down process.

CasaEd said:

I understand the factory diagrams which is why i'm on about this pin 18 as it earths the ecu control relay from the ecu when you switch the ignition on at the ecu, and i use this method in all the PD conversions I do here in Portugal

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are you cutting the wire to pin 18 (33 AGR) and earthing it with a switch? if so, why? why not leave it intact and wake up the ecm as VW intended? with the sw/li key on/start wire?

You missed my point, I put the AGR loom up as an example. I wire up 121 pin ECU's into mk2 golfs and the looms for the car side of the conversion come mainly from Mk4 golfs, seat leons and ibizas, these looms don't come pre-wired to the 109 relay they terminate into 5 plugs, so you have to do it yourself and yes, i'm KISS these looms !

CasaEd said:

You missed my point, I put the AGR loom up as an example. I wire up 121 pin ECU's into mk2 golfs and the looms for the car side of the conversion come mainly from Mk4 golfs, seat leons and ibizas, these looms don't come pre-wired to the 109 relay they terminate into 5 plugs, so you have to do it yourself and yes, i'm KISS these looms !

Click to expand...

I'm starting with complete harnesses, underhood and under-dash, there's no way i would try and piece together from just the engine side so i won't be including the wire from pin 18 in my chart... I understand now what you're working with but it doesn't apply here.