99.5 Jetta no start..Ecu problem/question

Andykbob · Feb 24, 2017

Hey
So the Jetta sometimes wouldn't start but then I would let it sit for a hour then it would . Eventually it got worse and now won't start at all.

I brought it to the shop and got it hooked up to every scanner they could try and it cannot be read. So the shop is assuming that the ecu is dead!

Now I'm trying to source a 99.5 Jetta ecu and I'm trying to figure out the correct part number .
Does anyone know the part number I need ?
Does it have to be a 99.5?
Will other years ecu work on it?

Thanks in advance for all your time ?

BobnOH · Feb 24, 2017

I don't like their assumption. They need to be sure, rule out the dumb stuff.
Does it have aftermarket radio? If it's not wired just so, it will disable the ability to connect a scanner.
Relay 109 is famous for various problems, it's main function is to provide power to the ECU. There are also fuses.
If your 109 is black, just replace it, it's a known defect.
Only ECUs from your make/model will work. Just look it up at an on-line dealer using your VIN. It's also printed on the ECU.

wonneber · Feb 24, 2017

Does the glow plug light come on with the key, even if it's just for a second.
If not start checking electrical.
BobnOH listed the fuses & such.
Bad grounds as well.
The one under the battery box has been mentioned many times.

Rich W.

Owain@malonetuning · Feb 24, 2017

So they've gone over every other aspect of the car? These ECUs are pretty bulletproof. Unless the car has been modified and has sockets fitted that are coming loose it should be fine.

You need an ECU from a 1998-1999.5 ALH, the newer 012 code ECUs and wiring are different. These ECUs are getting harder to find so we've started hoarding a lot of them

Vince Waldon · Feb 24, 2017

There's a version of those ECUs, if memory serves, that have a known "won't start when hot" issue in the firmware.

You seem to indicate you can't start at all now, so perhaps your problem's progressed... but if the car starts fine after sitting overnight a bit of searching on "hot start issue" might be of help.

BobnOH · Feb 24, 2017

car-part.com is an excellent place to get a used ECU, roughly 250-600 USD.
It's a part many yards will pull when the cars come in and the website's database covers all on North America.
But I would bet money that's not what you need. Try the trusted mechanic list or the local TDI Club forum for a better mechanic.

Owain@malonetuning · Feb 24, 2017

Vince Waldon said:
There's a version of those ECUs, if memory serves, that have a known "won't start when hot" issue in the firmware.

What happens is the startup fueling comes on at a higher RPM with increased coolant temp, putting more strain on the battery and starter. You can quickly test this by unplugging the coolant temp sensor.

It will usually result in cranking over 3-5 times before firing, but it should fire. Highly doubt the problem is actually the ECU, but they're readily available for around 250 US and do not have an immobilizer.

Vince Waldon · Feb 24, 2017

Ah yes...that's it: "hard starting when hot", not "won't start when hot".

turbocharged798 · Feb 24, 2017

Relay 109, almost guaranteed.

bbarbulo · Feb 24, 2017

ok so when you say every scanner they could... did they have VCDS / VAGCOM? and did it say controller not responding? were they able to connect to any other controller, like the airbag, central convenience, etc?

really it sounds like this isn't the right shop for the job. where in ontario are you?

Andykbob · Feb 26, 2017

Wow!!!
Thanks for all the responses everyone.

I will definitely follow up on everything you all said.

I'm up in sudbury, tdi mechanics are hard to find up here unless I go to the stealership.

I will get back to you with my findings.

Once again thank you all.

wonneber · Feb 26, 2017

You didn't mention if you had an aftermarket radio installed.
Known issue if its wired wrong.
That's why I ran new power wires to mine when I replaced it.

Rich W.

Andykbob · Feb 26, 2017

wonneber said:
You didn't mention if you had an aftermarket radio installed.
Known issue if its wired wrong.
That's why I ran new power wires to mine when I replaced it.

Rich W.[/QUOTE

Hey
No it's all stock

wonneber · Feb 27, 2017

Andykbob said:
wonneber said:

You didn't mention if you had an aftermarket radio installed.
Known issue if its wired wrong.
That's why I ran new power wires to mine when I replaced it.

Rich W.

Hey
No it's all stock

Click to expand...

OK, rules 1 thing out.
I would still check power sources (fuses & relays)
Relay 109 is known to be a problem.
If the glow plug light comes on it usually rules the relay out.
I would check the ground wire under the battery tray. Mentioned many times and found dirty by some.
A voltage drop test from the battery negative post to the engine block (with the key and lights on) then from the post to the body should not be more then a few tenths of a volt.
When cranking the ECU must have at least 9.6 volts to it.

Rich W.

Andykbob · Feb 27, 2017

The engine cranks over no problem, just doesn't fire up.

Andykbob · Feb 27, 2017

Andykbob said:
The engine cranks over no problem, just doesn't fire up.

Where exactly is fuse 109?

BobnOH · Feb 27, 2017

Relay 109.
If you remove just the lower dash panel above your feet you can se the relay panel real good. It's adjacent to the fuse box. Sometimes times the relay is labelled 601

If it's black, it's almost surelt the problem.

steve6 · Feb 27, 2017

Andykbob, definitely swap out relay 109 as said, theres a million threads about it just google a bit youll figure it out.

The reason people mention the radio is there is a communication line within the car for internal electronics, if this line gets shorted (because of aftermarket items) it causes communication issues. So keep that in mind, be sure to see if there is any odd aftermarket accessories tapped into the electronics. This may correct your ECU communication issues, or the shop people were just idiots.

The communications line at the radio looks like its a 'constant' 12vs and some people use it to run add on's.

Andykbob · Jul 3, 2017

Well better late than never.... I ended up buying another vehicle so the shop I brought the Jetta to said the computer was bad so we all kind of forgot about it. Fast forward a few months, I bought the new fuse and replaced it and it started right up!!!

Thanks for all your help?

sisyphus · Jul 3, 2017

Andykbob said:
Well better late than never.... I ended up buying another vehicle so the shop I brought the Jetta to said the computer was bad so we all kind of forgot about it. Fast forward a few months, I bought the new fuse and replaced it and it started right up!!!

Thanks for all your help?

Which fuse did you replace? I'm having the same problem.

Andykbob · Jul 3, 2017

Relay 109
Search "relay 109" and there is a few detailed threads. Very easy fix!

sisyphus · Jul 3, 2017

Yeah, well words have actual meanings. A relay is not a fuse. That's why I asked.

Andykbob · Jul 4, 2017

Good catch!!!!
You go girl!!

sisyphus · Jul 5, 2017

Andykbob said:
Good catch!!!!
You go girl!!

Pretty easy catch when you know what you're talking about.

99.5 Jetta no start..Ecu problem/question

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