TheSamba.com :: View topic - VW Dealership Diagnostic Plug Block Port Socket

Sun Jun 09, 2019 10:52 am

TL;DR
-The diagnostic plug began in early 1970s and was used by VW dealership service centers to 'diagnose' and check various parameters of the engine and electrics using a special computer (pictured above)
-No one has a VW diagnostic computer anymore, but the diagnostic plug on early-to-mid 70s ACVWs can still be useful, actually (if the wiring isn't all hacked up by a previous owner)
-It connects to approximately 19 items directly. Some are useful even without the VW diagnostic computer, some are not.
-The remainder of the plug terminals are sort of a mystery. Could be additional 'testing' circuits for the old VW diagnostic computer to use to 'assess' or 'calculate' additional things, but the wiring diagrams only talk about what is connected directly to the plug block in the car. No diagrams (AFAIK) exist which explain/outline this diagnostic system from the computer side of the plug.

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Pictures of the plug in the car

Pictures of the computer end of the plug

I've been doing some research on whether or not the old VW Diagnostic plug in my beetle is useful in any sort of way. It's not that I dislike it being present or that it has caused any issues, but when I discovered that I can tap a 12v power source and a good solid ground out of it, I wondered.......

What all does that thing connect to? :-k

So onwards into learning and adventure we go!![/i]

Resources:

SpeedyJim
http://www.speedyjim.net/htm/plug.htm

Samba search with lots of short, fragmented discussions:
https://www.thesamba.com/vw/forum/search.php?searc...=titleonly

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When did this port show up?

The Diagnostic port was introduced around the 1971 model year and continued at least until 1974 (VW just quietly seemed to do away with it in the later 70s).

What did it originally do?

It was used by the dealership to 'diagnose' many issues (some were more of 'estimations' that the VW computer would give rather than every 'diagnosis item' being from a separate sensor.)

Pictures of the VW Dealership set up

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:?: So...many people ask:
Quote: If nobody has the VW computer to plug into, shouldn't I just get rid of it?

:!: Answer:
Well, you can- if you want

BUT

If your wiring loom & car is fairly intact you may find that plug useful!!

For example, without disconnecting anything, through the diagnostic port:

---One terminal in the plug block provides 12v (+)
---Another terminal provides a solid ground
---Yet another can be used to hook up your tach/dwell meter
---You can check that the ignition coil is getting power to the (+) side (#15 post on coil)
---You can compare the voltage drop/quickly check for resistance or corrosion across important portions of the wiring (battery ground strap & Gen/Alt D+ wire)
---You can even operate the starter from the diagnostic block (just as the computer could do so in the 70s)

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Diagrams of the known connections to components:
Diagram with plug numbers:

Some of SpeedyJim's helpful diagrams which illustrate what diagnostic plug terminals connect to what components:

Wiring diagrams for each model year can be found here:
https://www.thesamba.com/vw/archives/info/wiringt1.php

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My assembled list of what diagnostic plug terminals go to what component (if I could find it listed or explained somewhere):

AlmostHeavenWV_VW wrote:
1
2
3
4
5 Rear window defogger relay, power output to defroster grid(terminal #87)
6 Voltage regulator, power feed from gen/alt (Terminal D+)
7 Battery electrolyte level probe
8 Battery, negative (direct connect to post) (Pin 8 & 17 were used by computer to estimate piston compression)
9 Tail/brake light (+), rear left (71&72)
10 Turn signal (+), rear left
11 Turn signal (+), rear right
12 Tail/brake light (+), rear right (71&72)
13 Coil, negative (terminal #1)
14 Gen/Alt Raw output, (D+ terminal)
15 Coil, positive (terminal #15)
16
17 Ground strap, battery to body ((Pin 8 & 17 were used by computer to estimate piston compression)
18
19
20 "Ignition timing sensor pulse transformer" (?inductive pick up?)on spark plug wire #1
21"Ignition timing sensor pulse transformer" (?inductive pick up?)on spark plug wire #1
22
23 ??Used by VW Diagnostic to determine approximate TDC??
24
25 TDC Sensor ('74 and up)
31. Ground
50. Starter solenoid (Terminal #50)
(+). Starter motor/Positive feed from battery (connected to starter) ('71 &'72)

Just thought this would be nice information to assemble and get out there for all the ACVW-ers out there.

Hope this helps down the road!

Happy Bug'n

Sun Jun 09, 2019 2:02 pm

I leave them intact but I can’t tell you how many I’ve seen hacked for no better reason than to make things look cleaner. My 74 still has the plug and I worked around it when I installed the new tar boards. :D guess I’m a purist

Sun Jun 09, 2019 2:17 pm

Zundfolge1432 wrote: I leave them intact but I can’t tell you how many I’ve seen hacked for no better reason than to make things look cleaner. My 74 still has the plug and I worked around it when I installed the new tar boards. :D guess I’m a purist
Guilty as charged.

First off i'm sure there is not a single dealership that has one or would know how to use it.

Secondly I don't think it work do much for my engine.

Sun Jun 09, 2019 2:52 pm

Great post, AlmostH, if for no reason other than laying out clearly what each connection does!

Have you actually tried any of the ports - say to check the voltage at your coil or to turn the starter over?

Sun Jun 09, 2019 4:19 pm

Sun Jun 09, 2019 4:39 pm

i wish my 79 had one. be cool to look at the fuel injection system on it.

Sun Jun 09, 2019 4:45 pm

Was there, did that...

Sun Jun 09, 2019 5:26 pm

Impressive post, thanks for taking the time to round all that up, well done! 8)

Mon Jun 10, 2019 1:46 am

Here is the VW Diagnosis Punch Card Booklet.

Mon Jun 10, 2019 4:58 am

vamram wrote: Great post, AlmostH, if for no reason other than laying out clearly what each connection does!

Have you actually tried any of the ports - say to check the voltage at your coil or to turn the starter over?

I haven't tried my port yet to test anything, but it appears mostly undisturbed. I can't find the coil voltage wire, but I did find the 'tach/dwell' wire (although it had its spade connector snipped off :roll: )

I know the main 12v + feed works and I've got that big fat ground too- I've used those via some 8 gauge 'jump leads' I made to plug into the socket and connect power to my timing light.

Mon Jun 10, 2019 5:25 am

AlmostHeavenWV_VW wrote:
I know the main 12v + feed works and I've got that big fat ground too- I've used those via some 8 gauge 'jump leads' I made to plug into the socket and connect power to my timing light.

Got any pics of that hooked up?

Mon Jun 10, 2019 5:47 am

vamram wrote: AlmostHeavenWV_VW wrote:
I know the main 12v + feed works and I've got that big fat ground too- I've used those via some 8 gauge 'jump leads' I made to plug into the socket and connect power to my timing light.

Got any pics of that hooked up?

I don't but can try to remember to take a picture when I get a chance. Nothing complicated. Just some wire and two crimp "blade" terminals (what looks like the part that the normal crimp terminal slides onto. Then just connect timing light clamps.

I'm working on possibly crudely replicating the computer plug for the block so that I can have a "harness" of sorts. We will see if I'm successful. Lol

Sat Jul 06, 2019 10:26 pm

My alternator light sometimes doesn't come
On at start up. Does the alt. Dash light wire connect or go through this unit? Wondering it could be the place it is shorting out? Any thoughts?

Sun Jul 07, 2019 1:07 am

On the BMW2002faq forum, someone had built a box that plugged into the diagnostic port of a 2002 and did some basic measurements (tach, dwell, voltage, etc).

It should be possible to do something similar or even more sophisticated.

https://www.bmw2002faq.com/articles.html/technical...nual-r133/

Sun Jul 07, 2019 6:10 am

Mine was intact and left it that way even though the original engine wiring has been changed.

I felt there were too many wires going to the port to remove it.

Sun Jul 07, 2019 9:05 am

johnboy715 wrote: My alternator light sometimes doesn't come
On at start up. Does the alt. Dash light wire connect or go through this unit? Wondering it could be the place it is shorting out? Any thoughts?
No, the warning light is not part of the test network.

Find the blue wire in the back (either on your alternator or on the regulator if you have one) and ground it, if the dash light still doesn't work suspect the bulb or circuitry on the back of the speedo.

Thu Jul 25, 2019 5:06 pm

Can you guys check tach/dwell, ignition voltage, generator ouput, VR input,system voltage & start the car without getting up from your chair?

I can with my control box for the VW Diagnostic Block :D

Thu Jul 25, 2019 6:00 pm

AlmostHeavenWV_VW wrote: Can you guys check tach/dwell, ignition voltage, generator ouput, VR input,system voltage & start the car without getting up from your chair?

I can with my control box for the VW Diagnostic Block :D

No and HOLY SH1&+ is that for real?? :shock:

Thu Jul 25, 2019 6:14 pm

AlmostHeavenWV_VW wrote: Can you guys check tach/dwell, ignition voltage, generator ouput, VR input,system voltage & start the car without getting up from your chair?

I can with my control box for the VW Diagnostic Block :D

That is awesome! I want one.

Thu Jul 25, 2019 6:53 pm

Is that all?, I demand resistance values for every major light circuit as well as comparisons of voltage drops on grounds while cranking :?

Just kidding, Nice work! 8)