Allen Wiggs

My 94 968 started drove 30', died no spark.Injectors firing, dme relay, coil, fuses, ignition relays, speed sensor and wiring, alarm, cap, rotor are all good I tested most parts on my 944. There is 12v on term. 15 of coil and there is suppose to be 12v on term. 1 but there is only 4v with green wire connected and 12v with it disconnect, which is where I believe my problem lies. I know the green wire goes to term. 1 on the control unit, which is the triggering for the coil. The resistance from the green wire disconnected from the coil through the connected control unit is only .6 ohms my 944 resistance is 4.3k ohms. Can someone give me their 968 resistance? I believe the ignition triggering circuit in the control unit is bad. With the ignition key on I noticed the control unit getting very hot to the touch, is this normal? It gets hot fairly quick and I don't think it should be that hot, heat kills. It's probably already dead, $1,850 at Zims. I'am out of time today to check the heat of my 944 control unit. I have not found a thread that covers this low voltage issue. Everyone states that the term. 1 voltage should be 12v but never talk about what is the problem and fix if it is not. Any input would be appreciated.

Allen Wiggs

I did check the resistance of the green wire disconnected from the coil and control unit, it is clear no shorts open line.

Tom Carson

Allen...the ignition ‘final stage’ resistance for the 944 and the 968 is very similar but with the leads connected to different pins of course. Pin 1 is the same, but the ground on the 944 DME is on pin 5 (& others) and the ground on the 968 DME is on pin 10 (& others). My notes agree with your findings…approx 3 to 4 K ohms for the 944. For the 968 DME it was little more at 4 to 5 k ohms…nothing really much different.

Since the ignition ‘final stage’ is a semiconductor, reversing the meter leads should be very telling. You should have a very high resistance. On an analog meter, I had almost infinity. (btw…you probably know the 944S2 DME ‘final stage’ is external to the DME but the resistance readings are similar on pins 1 & 2 of the external unit.) I'll bet you have the same bad reading in both directions.

If yes, then it does sound like the final stage is shorted. I have never opened a 968 DME but I bet it’s just a big power transistor that any TV/Radio repair shop should be able to test, look up the specs, and replace it with something close that should work. Let us know what happens.

Allen Wiggs

Tom,thanks for the reply.I have the control unit with the cover off in my hand.Pins;2,3,10,14,19,24,26,30,44 are all grounds but they are not all internally tied together. Resistance between pin 1 and pin 10 is 90 ohms measured in both directions but resistance between pin 1 and various other grounds (not all)in both directions is .6 ohms in which is what I'am getting when control unit is plugged in. .6 ohms is almost a dead short which may explain the heat. I did not know the 944S2 DME 'final stage' is external to the DME is the 968? If so where is it located? I don't have a schematic for the DME and don't know what the specs for the transistor should be, do you? With the cover off and facing you, is your transistor clamped to a metal bracket (but insulated) on the far right? What you are saying is to desolder it and test? A transistor can be checked with an ohm meter I have a book that discribes this procedure. Have you done this before with success? Sure would be cheaper than $1,800. Thanks.

Tom Carson

The difference between the various ‘ground’ pins is probably because the DME is unplugged from the cable, which supplies the ‘chassis ground’ and as you said they are not tied together internally…so no big deal.

The 968 DME ‘ignition final stage’ is not external like the S2, although there are many similarities in the DME’s. btw, IIRC the S2 external final stage is near the coolant expansion / filler tank.

ok, I took my 1st look inside a 968 DME today…a single pc board and roomy. With the cover off, if you hold it up in front of you and have the plug on the right, the suspect part in question is mounted close to pin 1, to a 3” long heat sink (that looks like it is made for 3 of these semiconductors but with only 1 equipped). It’s on the right-hand side of the heat sink…the black thingy with 3 wire legs connecting it to the printed circuit board.

Clamping this device to the long heat sink is a push-on metal and plastic cover. You can use a screwdriver as a lever and pry one side up to expose the Bosch #30014…whatever that is.

I’m guessing it is some sort of high power switching transistor in what looks like a ‘TO 220’ case. And I’m betting it’s an ‘NPN’ transistor. But all of this is an educated guess…I have never repaired a DME…never had to. (And I don’t fix TV’s either…when they break they go in the trash.)

It’s possible this Radio Shack part…Catalog #: 276-2020; Description: TIP3055 Transistor; Price: $1.69…just might work:

http://www.radioshack.com/product.as...Fid=276%2D2020

the specs for which might be close to these at:

http://www1.jaycar.com.au/images_uploaded/TIP3055.PDF

but that would be too good to be true…but hey, ya never know!!

A $1.69 part! …wouldn’t that be funny !

Allen Wiggs

Yea, it's definitely shorted out. Radio Shack could not cross reference it. The TV guy can probably get one but he needs to know what the triggering voltage is. I don't know either NPN or PNP if the trigger voltage is real low it maybe a FET. I'am going to check trigger volts with an o-scope tonight.I've been on the net looking for Bosch Motronic repair. There is a place in the UK but states they can no longer take anything from the US or Canada etc. I'am going to call them later. Like you said $1.69 part. These Motronics look like they're a problem on a lot of other cars, from what I've read on the net. And it's the spark circuit too. I'll make some calls tomorrow, one to Bosch and see what they say. Does anyone know where to get the correct transistor or a repair job? I'll check on that Radio Shack #.

Tom Carson

That final stage serves the same purpose as a relay but can do it faster, better, cheaper. The TIP3055 is a common, general purpose, NPN switching transistor that can handle up to 15 amps and is basically an electronic relay. There's not much downside.

Allen Wiggs

A guy at Vertec claims it's not a transistor but a Darlington. I see in my book that a Darlington amplifier is used used to amplify a lower gain transistor. I don't understand why the DME would need to amplify a ground path? Might be a good salesman, they repair them. Later in the day, as I stood with the TIP3055 in my hand, I descided to just send it to them. If I would tried it and it didn't work I would be wasting a day and my dry road days are numbered in Alaska. It was about half the size of the one in the unit. I realize if it didn't work it was already broke, but time is my main factor. $1850 was a little high $580 for a repair is better, even if it's a $1.69 part. I love driving this car, it eats other cars up and it gets the looks. Vertec can put one of their chips in it for another $169, I'am going to talk to one of their techs tomorrow and decide. The salesman claimed a 7% gain, but said he didn't think it raised the rev limit. From what I've read the chips gain some here but lose some there, the biggest gain came from raising the rev limit. Have any input on the best chip or chips in general? And one last thing, Programainic.com also repairs DME's. I didn't see 968's just 944's. I'am tired of this feul infection saga, thank you Robert Bosch.

Tom Carson

hmmm...I went to the FR Wilk website and downloaded a 944 DME schematic (not a S2 or 968...didn't have one there).

The 944 DME is a Darlington but I'm sure those 944's are the 'top hat' type TO3 case or package. Like I said earlier the 968 looks like a TO220 'package'.

Something like a NTE263 is a NPN Darlington and can handle 10 amps (15 peak)...dunno if it will work though. specs:

http://www.nteinc.com/specs/200to299/pdf/nte263.pdf

And at $4.27 CDN still might be worth the try.

http://68.146.192.140/mro/products.asp?PN=NTE263

(but if you do get it repaired let us know what the replacement was)

Mike

Different car, different DME, different device, same problem:

https://rennlist.com/forums/showthre...threadid=76706

The device switches a lot of current, and a Darlington does a better job (higher gain). I repaired my 951 DME with a non-Darlington (2N3055) and the car sputtered and puked >4K RPM. Once I put in the "right" part, all has been well.

Tom Carson

Mike, read your old thread...good stuff, and the BU941ZP (TO220 package version) Darlington has even better specs then what I guessed at (the NTE263). It sounds like the DME repair you did with the BU941Z (TO3 package) is still good 3 years later!!

see:

http://us.st.com/stonline/books/pdf/docs/1008.pdf

I just tried to order from Digi-Key but they only have the BU941ZPFI a lower power version...I'll keep looking