McNally EGT/Boost Gauge Wiring Question

Another issue I'm hoping the fine people at TDI Club can help me out with. I researched how to hook up the gauge in my 2003 Jetta and found a w/u saying to wire the power and ground into the dimmer switch. Big mistake only works with headlights on now. Second issue is it only works sometimes as in when it doesn't work it flashes so bad I feel like a army of Policemen are behind me and it makes it a nightmare driving at night. Its like its stuck in its startup sequence. When it does decide to work the boost gauge doesn't really work the EGTs work fine but boost keeps creeping up to 30 unless I romp the throttle hard I grounded the sensor to the battery. And have all wires hooked up correctly as far as I know just wanting to know where the best part under the dash is to hook up the power and ground for the gauge that has power as soon as the car is turned on. Thanks in advance

PS I also looked up the McNally installation guide and it tells you to find a good place to hook it up not a specific wire to tap into or anything like that.

Any exposed metal screw that's screwed into any of the metal frame under the dash is will be a good ground... headlight dimmer not so much.

In terms of power, there's a stud labeled "75X" that's a perfect terminal for aftermarket accessories as it's in sync with the ignition switch.

Good idea to run an inline fuse to anything you power, of course.

I ended up using the TCM terminal on my car...it's a 5 speed so no TCM here but the switched power is there...

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Where and what is the TCM and where is that 75x stud?

Powerstroke9773 said:

where is that 75x stud?

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Google "VW terminal 75x" and look at the images... first one.

Transmission control module, if you have a 5 speed you don't have one, it's one of the fuse ports on the side of the dash, I used a 10 amp fuse so I have a fuse in line and switched power all at once

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75X or just X, circuits have Black/Yellow wires. The X circuits are "off" during the Start Mode but are back "on"(hot) when the ignition is turned back to ON (run position).

So, look for some Black/Yellow (Black with Yellow stripe) wires and follow to a terminal at or on the Relay Panel above the clutch pedal (under dash)...... Yeah, it's a real back breaker to work up under there.

OP, is your Boost Gauge mechanical?

No the boost gauge is electric. And y'all are saying there should be yellow and blacks coming out the back of the fuse panel? And that those are powers. And then the stud is the ground?

The picture I've told you how to Google shows both the 75x power terminal and a good ground terminal.

Dunno how to be any clearer than that.

AndyBees said:

75X or just X, circuits have Black/Yellow wires. The X circuits are "off" during the Start Mode but are back "on"(hot) when the ignition is turned back to ON (run position).

So, look for some Black/Yellow (Black with Yellow stripe) wires and follow to a terminal at or on the Relay Panel above the clutch pedal (under dash)...... Yeah, it's a real back breaker to work up under there.

OP, is your Boost Gauge mechanical?

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There may be an X circuit on the back of the Fuse Panel ... but, why look there when it would be much easier to find the lug labeled 75X as Vince has referenced?

Fuse 31 or your fuse panel, take a 10 amp fuse or 5 amp fuse and bend the wire around the side without a spade and stick it in the slot... viola switched power

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Vince I apologise I didn't realize that their was a ground and power hook up in that picture I really appreciate y'all help and apologize again for the confusion.

WildChild80 said:

Fuse 31 or your fuse panel, take a 10 amp fuse or 5 amp fuse and bend the wire around the side without a spade and stick it in the slot... viola switched power
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Click to expand...

If this method makes you hesitate, I found some nifty fuses with a wire attached to a
leg at the local auto parts store. Just be sure to install them so that the wire is fused.

So this morning prior to going to work I got under the dash and cut wires I spliced into the dimmer switch to eliminate the flashing blue light in my face while driving at midnight. I also looked for that Block with the 75x and man that thing seems like the way to go I just need to buy some ring connectors big enough to fit on those studs. that definitely will be where I hook it to. But on a side note what does the 2, 30 connectors do I saw in the picture?

Not sure about the 2. The engine and wiring harness for the ALH TDI in my Vanagon is from a 2002 Jetta. During the process of doing the install, not once did I see any reference to 2 circuit. If I could see the pic you are talking about, that would help.

30 is always hot in VW wiring schematics. (it is always hot if the car has a battery in it)
31 is always a ground.

Its not "2" I meant to say the 02 as in number of connections listed 30. If you look up in the post there's a post referring to a google image and you can clearly see them there they are bigger red wires.

As I previously stated, the "two" 30 connections are always hot.

In the pic, it appears someone has tapped into the power source on the 75x terminal as I don't think it has a solid red wire from the factory. As I stated earlier, X circuit wires are Black with a Yellow stripe...... which indicates that they are X circuits. X circuits have no power on them during the Start Position of the Ignition. They do have power in the ON Position of the Ignition.

Also, either circuit (X or 30) is not fused... thus, if you tap into any of those, you should use an in-line fuse holder such as this.



A multi-meter would be of tremendous help in determining the status of any circuit, ignition ON or OFF.

^Excellent information! Hadn't done enough 'lectrical to know about the color coding.

Yeah that's awesome I greatly appreciate the help with this. I'm waiting for Saturday to wire the gauge and try and fix my power issue