Dougs951S

I've been noticing that lots of people have been having issues with their ISV not working, and not being able to find a solution. It could be in your DME! It was for me, and here is how I fixed it for 4 dollars.

http://postimg.org/image/wtrrm2lnn/


Those two components circled are type ON588 darlington transistor pairs. They control the ISV by pulsing the ground to the valve. Pulse width modulation is what controls how far the valve opens. They can be replaced with TIP31 transistors off the shelf from radioshack for 2 dollars a piece. You will be able to locate them on the board using the white etched component ID codes on the board. Simply desolder the old ones, resolder the new ones, and enjoy a beautiful factory idle.

This is the DME schematic, the two isolated rectangular circuits in the top of the diagram are the control circuits. They are off pins 33 and 34 of the DME connector.

http://www.the944.com/files/951power.pdf

mtnman82

Thanks Doug!!! I'm sure I'm not the only one who'll make use of this.

URG8RB8

Interesting, good find!

What happened to your board, lower left hand corner near R528?

Tom M'Guinn

Great post. Thanks!

Dougs951S

URG, that actually is not my board, but a stock image of the DME I found that had a good view of the transistors in question. I just circled them in red to highlight. I would have posted images of my own repair as it was actually more involved and an improvement over factory if I had remembered to before I installed the DME. The change I made was to incorporate a BTA24 diode pair and a pair of 2 amp fuses inline to protect the DME from damage in the case of a short in the driver circuit. I did this a few months ago and had been meaning to post this since then. The recent rise of gremlin ISV issues is what prompted me. I hope this helps some people.

Doyle623

Good timing! I'm just about to attempt this repair myself. The wire covering came off around the connector to my ISV, causing all 3 wires to touch. I'm pretty sure that shorted out these transistors. I'll post my results when complete.

lee101315

Good find. Its about time we get some good tech back on this board as opposed to the " does this turbo cause lag " posts...

Dougs951S

Yes that will absolutely destroy both transistors because the short is directly across the ground leads of the ON588. I will post some more info on what I was talking about with my modification. In that case the diodes would limit the current across the ground and the worst case scenario is that you would blow a fuse and your dme would be undamaged.

Wald944

Very timely folks! I have been hunting a gremlin all winter. All the theories and electrical tests pointed at the ISV. Failed in an open position and the override didn't work. So blocking the hose to the intake in order to idle.
So I finally took the Intake off, the ISV is fully funcitonal.

I guess I have to open the DME now because it is obviously an electrical problem. Would have been a quicker fix!
Hopefully back on the road next weekend!

Is the test port ICV override the best way to TEST before re-assembling?

It did allow for more learning about my car and discovery of more lazy work of PO. Venturi Delete = Just cut the CV-WG line. (DELETING and cleaning)

Doyle623

So, I ran to RadioShack, bought the TIP31 transistors, watched a youtube video on replacing transistors, went to work, and now I have perfect idle.....for the first time in about 2 yrs. Thank you Rennlist!

Dougs951S

Great to hear! Glad I could help, isn't it a great feeling to have something as simple as a nice idle? Also it was just brought to my attention that the TIP120 darlington transistor which is a better replacement for the ON588 than the TIP31 is also available off the shelf at radioshack for 2 dollars. I don't know if this is a recent thing because they were not available at my local store when I performed this repair over 6 months ago. Regardless, Motoxxx the TIP31 works well and should give you years of reliable service. I intend to leave mine as is.

mtnman82

Awesome! Wish we still had a Radio Shack... My DME/KLR are back under the rug for the first time in over 2 years so I'm a little less motivated to pull everything up again ,but it must be done eventually.

eman930

How do you know the difference between the ICV or the tranistor being the problem?

Wald944

You can follow procedure at:
http://www.texasblake.com/porsche/tech/icv.html

I already had my intake manifold off because I confirmed that my ICV failed in the open position. This was done by plugging the hose that goes to the TB intake pipe.
If you don't have the ICV out or disconnected from the hoses, you will have to listen for the quick sound of the solenoid opening/closing the valve.

Also, I don't have the PowerProbe used, so I just ran 2 wires to my battery terminal for the 12V.

Wald944

My ICV was in working order so I have the DME on the table ready for dissection and replacement of those transistors.

You can also check the continuity of the wiring harness from DME to ICV per Tom's instructions https://rennlist.com/forums/944-turb...e-issue-2.html