Questions about DME (Computer)

[PorscheGuy1981]

Hey guys, I usually lurk and take in the info but I've ran into some issues I have questions on. This is my second 944, my first turbo car. I know from my first 944 about the reference sensors and dme relay. Those have all been replaced. Been fighting an intermittent stalling issue following an electrical smell. Finally traced it to the dme (computer). Pin 5 and 19, especially 19 show signs of getting real hot. Also the circuit board has some damage. First questions would be, if I get a new computer after I find and fix the problems, should I just be able to swap my chip into the new computer and all my tuning settings be there? I guess what I'm asking is if all my tuning settings are stored on the chip? Now on to the problem. I've looked up what those pins control, both being grounds, one a chassis ground and the other a shielding ground for the reference sensor. Anyone ever ran into this issue or know anyone who has?

I'll post up a pics of the the car for those curious. I bought it pretty much the way it is. I've just replaced sensors and done some paint work. Its an 87, upgraded turbo, rebuilt motor, custom grind cam, fabspeed exhaust, bilstein fire hawk cup coilers. Put down 347hp on the dyno. Thanks for any help in advance guys!

[pmar911]

That is some serious current flowing through that ground wire #19 to cook the PCB trace like that.

I don't have a lot of experience with the DME computers so I cant tell if this is a well-known problem caused by aging of a component inside the DME, but I would be careful just plugging in another DME is you have a short outside that pulls a lot of current through the DME.

I guess the correct way to check it is to measure the current through each of the DME wires to check for excessive current draw. If it is on the 12V permanent then the problem is inside the DME, if it is on one of the sensor / injector / spark wires - look outside.

You have the 24 pin DME eprom. As far as I know, this 4Kbyte chip holds all the settings, while the actual program is inside the intel MCU. On the 28 pin DME, both code and settings are on the eprom. All your settings should be on the eprom chip and you can safely move it to another DME.

/Peter

[kdjones2000]

My guess is that you have low impedance injectors installed without some ballast resistors.

Stock are high impedance ones.

Looks like you are still on the stock engine management, but must have new injectors installed to make that kind of power.

[PorscheGuy1981]

Thanks for the replies. I am going to go through all the grounds in the car just to be safe and redo/clean any that need it. I am hoping I find a problem when I do that because I would hate to plug in and fry another computer as mentioned. Unless the problem was inside my current dme then I may not find any grounding issues but the car is 30 years old now so it won't hurt to go through it. I've already replaced a few grounds and the car only has 98k on the odometer, but it doesn't have age on its side.

The stalling issue was very random before I pulled out the dme. I could drive it for a few days and it be completely fine. Then some days I could fire it up and only get a mile and it shut off. Sometimes it would crank and crank but finally fire back and sometimes it fired right back up with no hesitation. I'm betting on something outside the dme being wrong. Can't imagine why it would just randomly have an internal failure not caused by any outside circumstance although it's not impossible.

[DannoXYZ]

Check, measure and replace if necessary the ground connectors at back of engine. Also ground cable from battery to chassis. If those grounds aren't sufficient, starting current goes through DME.

Also remove injectors and measure voltage on injector connectors. With key ON, you should have +12v on one terminal and ZERO on the other. If you measure ANY voltage on 2nd connector, there's a short on injector harness that causes extra drawn when DME grounds injectors to fire.