TDC Marker Unit
by Richard Atwell
Ever wondered what that unused wire lying in your engine compartment is for? That rubber plug near the air cleaner with the weatherproof cap?
It's a diagnostic plug that leads to a sensor at the rear of the case behind cylinder #1 called the TDC marker unit. It's an inductive pickup that produces a signal when each of two dowels attached to the back of the flywheel pass the sensor and it's used for measuring the engine timing electronically.
My apologies that you had to look at that GEX sticker for even half a second. Luckily that engine isn't mine and hopefully it's not yours either.
The plug has a 3 prong male connector and is shown in the 68-79 baywindow bus fiche as item #32 in illustration 94 (113 919 101).
This connector was also used with the 80-91 Vanagons and the same part number is listed in addition to the number for just the male plug (item #15 in Illustration 132, 113 919 113). The VW 411/412 sedan also used this sensor but I can't confirm whether or not this sensor was used in the Porsche 914 via its parts catalogs. I found several other VW/Audi/Porsche vehicles that utilized this sensor.
The sensor is actually shown in the wiring diagrams. Here's the diagram for my 78 bus (see G7 on current track 63):
This plug isn't to be confused with T20, the large diagnostic connector in the engine bay above the battery that hooked up to another diagnostic device. Its purpose was to be able to read various voltage values and aid with engine diagnosis. It was introduced in Jan 1971 but must not have been that helpful or contributed too much to the cost of building the bus because it was dropped after the 1977 model. I suspect the test equipment was probably too expensive for the average dealership and weren't VWs becoming more reliable anyway. ;-)
The VW dealer used to connect a special tach dwell meter that had a cable that hooked into the TDC marker plug to read the ignition timing via the inductive pickup. One such unit was the VAG 1367 made by Grundig Electronic of Germany. You can see the unit in this 1979 Brochure (it's the top unit depicted - second row, right photo).
Tach dwell meters can be bought for $15 on eBay and a good timing light is under $50 so what's the big deal? Well, the metal or plastic timing scale isn't the most accurate scale since it's marked in 4 degree increments and the reading depends on its mounting location on fan shroud and upon maintaining its original shape. In addition to this problem, try aiming a second timing light and see what results you get (variations in timing lights).
For example, one time I had a nice long argument with the smog referee when I said my timing was 7.5 BTDC and he claimed it was 9 BTDC. After I got home I set the timing to 6 BTDC and sure enough the engine ran better. So much for the accuracy of the scale. It's really hard to judge where the timing scale should really be so don't take my timing values, apply them and wonder why your engine is running hot. The difference between these positions on the timing scale is minuscule.
Enter the VAG 1367: set your timing accurately every time or at least once to get idea of how far off your plastic scale is by borrowing one (if you can find one).
The Grundig unit has three displays: a 4 digit display for the engine rpm; a 3 digit display for reading the dwell and timing angle in 1/10th of a degree; a display for indicating the selected number of cylinders.
You can see in this simple schematic how the unit connects to the engine.
In addition there is an inductive pickup (VAG 1367/7) for testing Vanagons. I initially thought that it looked about the same size as the drive plate inspection hole but it's smaller and only fits Vanagons. I was hoping there was a chance that the tester could be used on engines with a cut/removed TDC marker and implicitly know the position of the pickup but alas it's not the case.
Probably the coolest aspect of this unit is that you can check the total advance without using the timing light. In other words you can check the advance WHILE you are driving.
Two buttons on the left let you set the max rpm in thousands and hundreds of rpm and if you slowly increase the rpms when that value is reached the unit will record the timing angle and dwell at that rpm. This is very useful for checking the advance of the distributor to confirm that it is working. The check the mechanical advance, disconnect the vacuum line from the vacuum can and subtract the initial advance from the distributor specs.
For example, the specs for my engine are as follows:
I set the VAG to record at 1600rpm, and the reading I get is 20 deg. I reset it for 3400rpm and the reading I get is 30 deg. Both values within spec once you subtract the idle rpm.
To check the vacuum advance, I hook up a vacuum gauge between the vacuum can and the connecting hose and rev the engine until it reads 8" of vacuum which happens around 1700rpm. Because it does, you can subject the idle rpm and the mech advance @ 1700rpm (previous test) and see if your vacuum can is working.
You can also hook up a MityVac directly to the vacuum can and apply 8" of vacuum and watch the timing advance (be careful not to over advance the timing at idle!)
The last time I set the timing using the Grundig, I got the advance at idle right on at 7.5 BTDC. Although the computer conveniently shows you the timing in 1/10th of a degree the harder part is turning the distributor so it only changes by this small amount. You need to acquire a feel for the tightness of the locking clamp nut that the technician at the dealer had in order to be able to nudge the distributor just enough.
I have a new timing scale and this one reads just over 8 BTDC when the computer is showing 7.5 BTDC. Much like dwell for points having to read 50 degrees for the most power, the timing also has to be exact. For some reason the VW manuals don't show you the necessary advance required through the RPM range in detail. When I've seen these advance charts for other makes it's illustrative to discover how the engine needs x degrees between 3k rpm and 4k rpm. When you start with timing that's off by at least this much, you've just lost some acceleration ability.
These engines lack computerized engine management and they are very sensitive to tuning. I make it a habit of checking it before the required maintenance interval at 15k miles to get the most out of my engine.
If you've got the flywheel off and you're absolutely obsessive about your timing and don't have a VAG1367, you can paint a mark at say 7.5 BTDC on the back of the flywheel and aim your timing gun through the hole used to reach the bolts for the drive plate. The drive plate bolts were only for automatics but the hole still exists for manuals and is accessible behind a plastic plug (ignore the oil leak in my photo of the opening). You'll also need to paint a reference mark on the case to line up with the mark you put on the flywheel.
Many new vehicles have 3 marks on their timing pulley. White for the timing mark and one green and one red mark 2 degrees to each side of the white mark. All 12V flywheels have 130 teeth so each tooth sweeps 2.679 degrees. By placing a mark a little closer than 1 tooth on each side of your painted timing mark you'd have a better idea how close you are to your white mark compared to using the plastic scale. I think this is a better idea than adding cheesy marks to your pulley especially permanent ones but it takes more work to pull off.
To find your spot on the flywheel you need a protractor and some string. The dowels are located at 12 BTDC and 18 ATDC. With the flywheel on the ground upside down, affix the protractor and secure some string to the center. "Zero" the protractor at the 12 BTDC dowel (rightmost) and sweep the string until you reach your timing position and make your white mark. If you have an engine stand you can use a plumb bob for greater accuracy.
04/28/04 - Created
05/31/04 - Added photo of Vanagon pickup
04/26/09 - Added links to repair services
09/07/11 - Fixed broken photos, added translate button, updated footer