Drive-Thru Diagnostics

Drive-Thru Diagnostics

Crank position sensor failure is common on VR6 engines. Mine died on my R32 while I was stopped at the drive-thru.

Today in the shop, we have my 2004 Volkswagen R32, and for the first time out of many Volkswagens that I’ve owned, over many, many years, it actually broke down and left me stranded.

With every failure of a car, there’s a story behind it, and most of the time it’s pretty boring. However, this one was unique because the car broke down right as I was pulling up through a local drive-thru. Just before I ordered, the car just completely cut off. So, I had the pleasure of pushing my car through the drive-thru lane and off to the side to wait for a tow truck.

Fortunately, I was able to pull the fault in my instrument cluster and found out that it had an engine speed sensor fault. See Figure 1. This is actually a super common VR6 issue. So, today, we’re going to diagnose the fault, and I’ll show you how to replace the sensor.

Figure 1

The engine RPM sensor, or G28 as Volkswagen identifies it, is the sensor that monitors the rotation of the crankshaft. In most Volkswagens, if the ECM does not see this signal coming back from the crank position sensor, it’s not going to start. There are a couple of easy ways to diagnose it, even if you don’t have the instrument cluster where you can pull up the fault. If your car won’t start and it sounds like it should start, like you don’t have any crazy low compression sounds, the easiest thing to do is crank the car and see if you have an RPM signal. The tachometer should move. You should see it rotating. You can also do this with a scan tool with better accuracy.

So, if you crank the car and you’re not seeing any RPM signal, odds are that is the place of the failure, but that does not mean that the sensor is bad. It just means that, that’s probably the place you have a failure. I also found that there are a few fuses if you pull them, that you also lose RPM signals. So, keep that in mind.

So now we’ve tried to start our car, we don’t have any crank signal, so we hook up our scan tool and confirm we have a G28 code. What do we do next?

You’ll want to verify the code by cranking with the scan tool and monitoring RPM. You might even want to hook up the oscilloscope and scope that RPM signal to see if you’re getting it out of the sensor. We know the ECM’s not seeing it, but we want to see if it’s the sensor side or is it wiring to the ECM or, something I have never seen, the ECM actually not seeing that signal?

Now, replacing the sensor is pretty straightforward, especially if you don’t have a huge belly pan like I have, but you’ll need to get the front of the car off the ground and, if you have a skid plate, remove it. Our crank position sensor lives right at the front of our engine. The bracket holds the secondary air pump on and our sensor is located just behind this bracket. See Figure 2. If you peek back up through, you’ll see the sensor; the wiring wraps behind the oil filter and then goes up.

Figure 2

We’re going to remove the bracket, this is a good opportunity to check (and replace if necessary) your secondary air pump mounts, which over time, like mine, do come apart. So, we’ll remove the two #5 Allen bolts and the #6 Allen that’s tucked up behind.

Before you start removing the sensor, it’s a good idea to clean the dirt out of the area, that way you don’t run the risk of it getting in our block. You don’t need to do an engine rebuild-level cleaning here; just spraying it with some brake parts cleaner should be good enough.

We’re then going to remove the #5 Allen bolt that holds the sensor in. I prefer to use the flat tip style socket rather than a ball-end socket, that way we don’t risk rounding this out. See Figure 3. You don’t want to have to extract a bolt here. Normally they come out pretty easy, but just as a precaution, don’t use a ball-end if you don’t have to.
Now, sometimes these do get kind of frozen in the block. Instead of trying to pull it straight out, try and rotate the sensor back and forth, and then usually you can grab it at the bottom and wiggle it out. If you need some assistance, sometimes a flat blade screwdriver will help. Just try and rock it out; you really don’t want to break this. It’ll be an un-fun sensor to have to drill out.

Figure 3

Now, if you can look through that little hole see Figure 4, you would see the sensor wheel on the crankshaft. So that is what our G28 is looking at. You’ll notice our wiring harness follows around the oil filter. There is a retainer that you have to pop off. Then your plug is actually up quite a bit higher. Getting to the connector might be the hardest part of the whole job, especially when it’s properly secured in the bracket. So we’ll unplug that and here is our sensor.

Figure 4

Now we have our crank position sensor hooked up to the multimeter set to Ohms, which as we know, resistance is not really the best way to test. However, I think this is going to be able to let me show you a pretty accurate representation of what I think happened. The wires coming off the sensor are shielded inside of a loom, and what I’ve found typically happens is right where the wiring loom goes into the sensor body, see Figure 5, the wires start to come apart and when they get hot, they start to lose contact. Typically when this fails, if you let the car sit for about a half an hour, the car will end up starting again.

Figure 5

The G28 is an inductive sensor. As soon as the engine turns, the sensor wheel will move past G28 and that generates AC voltage. The frequency and amplitude both change with engine speed.

Now, there’s also one section on the sensor wheel where it’s missing some teeth. See Figure 6. This is the spot that the ECM uses to identify where in the crank rotation the crankshaft is. Now, this spot may not correlate exactly to TDC piston cylinder one, but it’s what the ECM uses to calculate where it’s at. And for good or bad, if it doesn’t see this signal from the crankshaft, if it doesn’t see that this is rotating based on the reading of our sensor, you’re not going to get a car that’s going to start. This is done to help protect the engine in case there’s a problem. And so if we install our sensor, we should be able to pick up an AC signal.

Figure 6

Let’s open up the wire loom and check out the wires. Normally, the wires and/or sheathing shouldn’t come apart, even at the connector end. Mine is just completely crumbling apart. No wonder I had a failure. I’m actually surprised I didn’t have a failure sooner than I did.

So now, let’s go ahead and get our new sensor in and our car wrapped up.

Our new sensor has fresh sheathing as well as a seal, which you’ll want to lubricate with some dielectric grease or clean engine oil. Also, make sure to clean out the sensor port. We don’t want to create an oil leak that we didn’t have before. Then install the sensor. It should go in nice and easy. If it doesn’t, take it back out and recheck it. Tip: Don’t get it halfway installed and then run the bolt in to pull the sensor in the rest of the way, that’s going to install the sensor crooked and will probably tear the seal, which could lead to an oil leak or worse, a sensor that sits cockeyed instead of flush and won’t read our crankshaft, leading to a car that may not start.

Then we’ll put the bolt back in and make sure that we route the harness back where it needs to go, including making sure to get the wire in the loom, up behind the filter. I usually plug it in first and then get it locked into place. Make sure that it makes a good connection. Also make sure you get it back up where it’s supposed to live, way up in its bracket. Then tuck the wire back in the loom on the side.

Now at this point, you can lower the car down and try and start it and just make sure everything starts. You might get a check engine light for secondary air because we still don’t have our secondary air pump hooked back up. If you’re confident, go ahead and put that secondary air pump back on, get your belly pan back on and see if your car starts.

That was a pretty sad sensor. However, a pretty straightforward, easy diagnosis and a pretty easy repair. Also, something I need to be really clear on: If you’re having a cam position sensor fault or a crank position sensor fault, and your car still runs (even if it runs poorly or maybe it’s sporadic), you might want to consider not just putting a sensor on it. Oftentimes, especially on the turbo Volkswagen and Audi cars, that cam position sensor fault is because there actually is an oil pressure issue or there is a timing issue. These cars are pretty good about not having those kind of problems, but remember, we don’t want to just throw a sensor in every single time.

Even on this car, we did a bit more diagnosis than just throw in the new sensor. We monitored for RPM on the instrument cluster while cranking and with the scan tool. So if you’re having some kind of other issue with cam position or crank position, do not forget to check timing, especially if the car runs poorly or doesn’t sound right when it’s starting, because you might have another issue. TS

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