109 Relay: Just do it!

I think the 109 relay should be replaced along with the timing belt service, just to get a fresh one every 100K miles.

When I first got my TDI, I was on here all the time. But as I got the car dialed in and other projects caught my interest, I was on the forum less and less and clicked off around 80,000 trouble free miles. Somewhere during that time, the 109 relay just slipped my mind.

Going to dinner last week, the car bucked hard once while cruising, and the glow plug light flashed for a fraction of a second. Went another 2 miles to the destination, enjoyed a nice dinner and the car started normally.

A mile later as I took off from a stop, the car died and would not restart. No Glow plug light and it just cranked but no fire.

After waiting 70 minuted for AAA to show up and being told it would be another hour, I cancelled the call and we took an Uber home.

Next morning I called a tower I had used before, and agreed to meet him at the car. While waiting for him, I hooked up VCDS. Was not able to access engine controller. I was thinking the ECU was dead and wondering how much hassle it would be to get my tune onto a new ECU.

Anyway, a few minutes searching on the forum and the dreaded 109 relay comes up and seems to match my symptoms. If you are not familiar, this relay is energized all the time the car is running and supplies power to the engine control unit. My memory jogged, I looked in the glovebox for the spare relay I had stashed years ago, but found it was the wrong one.

Get this, the local VW dealer does not stock it! Parts Authority said they could get me one the next day, so I went with that. I also ordered an OEM grey one from Cascade German through ebay.

The part did not show up the next day as promised, so two more days of downtime. Not a big deal for me as I have too many cars, but the Golf is my favorite and my DD.

When the part did show up, I popped it in and the car fired right up. Next day the OEM one showed up, so I swapped that in and now I have a proper spare.

My point is this though, carrying a spare may still get you stranded in a bad place. It may be dark, cold, and you may not have a torx to remove the panel roadside.

So, don't be like me! Spend 15 minutes and $13.50 and save yourself some big time inconvenience.

I think several of us have been burned by the 109. I know I have. I don't think it's really necessary to change them out as part of preventative maintenance but I wouldn't take the car anywhere without a spare 109 in the glove box and a T20 and T25. A basic driver set with bits and a basic socket set can really save you.

Good post, great idea! And, if you really don't want to get stranded, carry a "plug bolt" to thread into the oil filter housing where the turbo oil supply line attaches.

Just leave the cover off the bottom where the relays go, no need for tools, 10 sec job to change out the 109 relay.
Done this to all 5 of my TDI's.

Funny, in 15 TDIs I've never had one fail. And of course I have plenty available.

I have owned mkivs specifically since about 2009.. between my and my wife probably 6 different one's and never had a 109 fail.

The first of the MKIV TDIs had defective units, they were black, The dealer replaced mine a long, long time ago. They must have corrected this in the early to mid 2000s.
Now they get blamed for many problems, much like the MAF. Like 3turboz says, they provide power to the ECU, so when they go bad, you get pretty specific symptoms.

out of the seven mk4 tdi's in our family we have had three 109 relays fail.

BobnOH said:

The first of the MKIV TDIs had defective units, they were black, The dealer replaced mine a long, long time ago. They must have corrected this in the early to mid 2000s.
Now they get blamed for many problems, much like the MAF. Like 3turboz says, they provide power to the ECU, so when they go bad, you get pretty specific symptoms.

Click to expand...

Well, mine is a 2000 with 238,000 miles and it still had the black relay. Perhaps the updated one will last even longer, but it does get warm after a drive across town.

The whole thing about telling new versus old by color is long gone. They're all black now.

I have had 2 Relay 109 failures. Now I have a spare in the glove box and the cover off of the relay panel. Of course now that I am ready for failure the one in there has lasted 11 years.

I had a engine fault yesterday, and finally dug out the 109 to check it, but as IBW said, they're all black now. This wasn't a VW part, and it turned out that it wasn't at fault, but like already was said, there are specific symptoms.
It's also pretty easy to check a couple of the fuses for power, that are supplied by the 109 relay.

I used a meter and verified that I had power at 32, 34, and 43, which eliminated the 109 being the cause of my non-start condition.
Once AAA got my car back home where I had my VCDS and laptop, I found codes pointing toward the Quantity Adjuster, but that's not related to the 109 relay discussion.

It can't hurt to have one since they're so cheap, but I don't see the need to replace them on a schedule.

So if I purchased this for my car, it would be black?
Good to know!

Maybe not all black. That one is obviously gray. But they're all "updated."

I once had what was presenting as a 109 failure in my ten years of living with TDIs. Turned out that the socket into which it goes has four (five?) wires; one of them was being pushed out the back when the relay was pushed in and was giving intermittent failures.
So. There's that.