Relay 109 - Main Power Supply Fuse

Turbo Steve · August 8th, 2001, 16:27

(Due to frequent e-mail demand from new members, here it is again.)

Since prevention is much better than redemption, it makes sense to replace the weak Relay 109 ECM Circuit Fuse before it gets too hot and leaves you stranded. The upgraded version of the new Relay 109 circuit has heavier contacts which can withstand the heat better.

This relay switch (whatever name you want to call it) basically sends power to every sensor in the car as well as through the ECU. Failing ECM Load Reduction Power Relays have been known to damage ECU's when they go bad, not to mention the safety factor involved when they decide to die while you are idling in traffic or traveling on the highway! [img]images/icons/shocked.gif[/img]

Here's what I suggest you do:

Contact VW Parts.com (AKA Impex World Distributors) at VW Parts.com / (800) 736-3550 and order VW Part #1J0906381A for $15.85 plus $4.42 shipping. (Note - This part that retails for $19.58 fits most A-4 late-model TDI's.)

I also recommend ordering two of these critical parts in order to keep a second newly-designed circuit as a spare.

Installation is literally a snap. After removing the driver's side under-dash panel (two T-15 Torx screws), simply use a flashlight to look-up where the fuses are and remove the largest fuse - Relay 377 - and carefully set it aside.

Directly above the empty slot (where Relay 377 used to be) is Relay 109, which can now easily be removed and replaced. Everything can now be put back together in reverse order.

Note - The newly-designed Relay 109 is gray in color and slightly smaller than the older black model, which tends to fail under hot temperatures.

Here are a couple of diagrams (courtesy of MIGOLFGLSTDI) relating to replacing this Relay 109 ECM Power Fuse:

See also my old thread on this same subject.

[ August 18, 2001: Message edited by: Turbo Steve ]

Turbo Steve · August 8th, 2001, 16:36

Other excellent Relay 109 threads:

http://forums.tdiclub.com/cgi-bin/ul...&f=17&t=002470

http://forums.tdiclub.com/cgi-bin/ul...17&t=000411&p=

TGOLF · August 8th, 2001, 19:17

Thanks Turbo for the post.
(New Member, but Fred TDI reader since '99)

I just replaced 109 relay and will get a spare. The replacement cured some of my problems that the Dealer won't / couldn't replace since May!
I have been on "109 relay" post so won't bore.

One other point on replacing the "...PT-109..." Relay.
Maybe my hands are to big, but I also remove with three screws the panel below the steering. Additional to the diagram MIGOLFGLSTDI shows.
Be careful as TurboSteve noted that the numbers on relays you pull or replace can be inverted. Speaks volumes to the quality...don't you think?
My pull of the 377 relay that Turbo notes was inverted and the new 109 relay went in inverted from original.

Anyway I find it interesting you note: ..."Failing ECM Load Reduction Power Relays have been known to damage ECU's when they go bad, not to mention the safety factor involved when they decide to die while you are idling in traffic or traveling on the highway!"...

I'm wondering if my ECU is now damaged. How can Dealer check if ECU is damaged?

Can this relay damage other electronics?

I am now a believer, it is easy to think that these TDI's are Diesel juggernaut's and that these posts of electronics '...won't happen to me...'
Know the weak points and be ready.

On my way to the treads...TurboSteve
Thanks to MIGOLFGLSTDI too.

MOGolf · August 8th, 2001, 20:17

Here is an image of the footwell panel that TGOLF refrenced. Mine, in a 2001 Golf, is held in by 3 T-20 screws. (Some vehicles have T-15 screws.)

Edited since picture was incorporated above.

[ November 06, 2001: Message edited by: MIGolf ]</p>

Turbo Steve · August 12th, 2001, 18:59

"I just replaced 109 relay and will get a spare" (TGolf).

Good advice that should serve as a friendly reminder, especially in the middle of a hot summer.

MOGolf · August 15th, 2001, 07:09

If you're wondering what relay 109 supplies power to, I just examined the wiring diagrams for the 99.5-2002 Golf/Jetta (A4).

It supplies power to
- Glow plug relay
- Fuse 32
ECM
Fuel quantity adjuster
- Fuse 34
Cold start injector
Waste gate bypass regulator valve (N75)
Change over valve (N239)
EGR vacuum regulator valve (N18)
Mass air flow sensor (G70)
- Fuse 43
Clutch vacuum vent valve (if equipped)
Brake vacuum vent valve
Relays for preheating coolant (if equipped)
Crankcase ventilation heating element

The ECM supplies power (and sometime ground as well) for:
Fuel cutoff valve
Cruise control switch
Manifold (intake) air pressure/Intake air temperature sensor
Needle lift sensor
Engine speed sensor
Engine coolant sensor
Closed throttle position switch (part of "go" pedal)
Kickdown switch (part of "go" pedal)
Throttle position switch (part of "go" pedal)
Fuel temperature sensor
Modulating piston displacement sensor

The ECM also communicates to the instrument cluster electronics and transmission control module (on automatics).

As a result of this, you can see why if the relay starts to cause power to flicker, or voltage drops, for all of these components, the results can be "unpredicatable" and none of them desireable.

HoffTDI · August 16th, 2001, 09:45

How long or how many miles would it take before this relay 109 would go bad? I am waiting for my 2002 Jetta TDI to come in. Do you think my car will have the new gray relay or should I check and see if it has the old black one yet?

justme · August 16th, 2001, 10:11

It will very likely have the new gray one. There's not telling when/if one will go bad. I've got a total of about 164k miles across 3 TDIs (90k on the oldest one) and have never had a 109 problem. My sister also has had her TDI since '96 and hasn't had a problem. But a friend of hers has (also a '96).

Lug_Nut · August 17th, 2001, 08:56

<BLOCKQUOTE><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><HR>Originally posted by TGOLF:
Be careful as TurboSteve noted that the numbers on relays you pull or replace can be inverted. Speaks volumes to the quality...don't you think?<HR></BLOCKQUOTE>

Maybe that's why neither of mine failed. They aren't 109 after all. They are actually 601's!
And just this morning not only was my entire newspaper printed upside down but the fold was on the right! [img]images/icons/smile.gif[/img]

VelvetFoot · August 17th, 2001, 09:07

The title of this thread should be changed to avoid confusion. It should be something like "Relay 109 or Main Power Supply Relay" or something like that. The load reduction relay is something else.

TGOLF · August 18th, 2001, 08:26

Update since replacing the '109'.
I took my car back to clear codes and check what the Regional Rep. suggested to Dealer. They couldn't find any problems after clearing codes and didn't charge me the two hour labor they were going to.
Driven under all conditions which have brought on problems before replacement,have had no problems since install!

I asked Dealer if there was any way to check only the 109 relay. They didn't have an answer. (Dealer originally didn't know what a 109 relay was...601 may have helped)
Is there any way to check just the '109'?
I now have an extra '109', but I'm wondering if these newer 'gray 109's' are OK.
More importantly I would like some piece of mind that there isn't any other 'hidden' problems.

It seems to me from reading most of the 109 trouble is on cars with 2 years, 40K+, Maybe 109's should just be repleced at your 40K service...
I have petitioned with the NTSHA on 109 as saftey concern too.

[img]images/icons/grin.gif[/img] [img]images/icons/grin.gif[/img] [img]images/icons/grin.gif[/img]

mediko · August 18th, 2001, 15:00

I had starting problems, cranking well but not starting. The glow pug light was not coming on.

After replacing relay 109. It started in one crank, and good since then, 1 week.

99.5 A4 Jetta, 33K miles.
mediko, ca

TGOLF · August 19th, 2001, 11:47

That is one of the problems I had too.
See you have 99.5 too.

I think everyone should petition NHSTA on the 109 relay. I also, feel it is unbelieveable that VWofA doesn't look at 109 relay's, inform Dealer's and Tech's etc. My proiblem had been going on since May!

Thank goodness to all the support here on Fred's... [img]images/icons/grin.gif[/img]

red golf tdi · October 19th, 2001, 20:49

Relay 109 is for sure the problem.
The latest design is a grey plastic style replacing the old black plastic style. My friend's Jetta was intermittantly quiting on him and his relay smelled burned when we pulled it out. I bought a replacement for my Golf and keep the orginal one as a spare. It cost about $20 from VW. It's an odd looking relay unavailble from the parts stores at this time.

BRBarian · October 19th, 2001, 21:28

In addition to powering circuits on fuse locations; 32, 34, and 43, relay 109 also is connected to the unused fuse number 28.

So as an emergency road repair, you can jumper the fuse connector from fuse 12 to fuse 28 and bypass relay 109 altogether. Just pull the wire when you shut off your car...

Fuse 12 is the unswitched power source for DLC.

All you need is a short piece of wire with male fuse size pins at each end... If you don't have nice pins, you could probably just jam a wire into the fuse recepticles.

Quick and easy...