Carrera DME unit question...
 

In trying to teack down my fuel mixture problems, I'm starting to think it may be related to the DME unit. My car is an '87 and I am still using the original '87 DME with the newly installed '85 engine. DME units have different part numbers for each of those years, though the Autothority chip from the '85 fit in my '87 DME.

Ditto for O2 sensor. Different part numbers for each year. Could this be a/the cause of my rich running?

Richard,
Pretty much the only thing that changed was the idle speed ('85=800, '87=880rpm) and a little more advance. I forgot, what was your mixture problem?
-Chris

Doesn't the chip control all the engine functions? Unless the 87 DME box is wired differently from the 85, it would seem to me that the 85 Autothority chip would be better matched to the 85 engine than the stock 87 DME chip. Also, Richard I seem to remember we had a problem with the adapting/soldering of wires for the new O2 sensor when we put it in. Have you checked to see if the sensor is working? (Not that I know how you would do that.)

Do you have the car back now or is it still in the shop?

Car is still in the shop and Stan is having me compile all the info I can on the new engine and possibly mismatched components before he racks up labor charges finding more of them out for me. Dan, I have heard from others (including Wayne) that the aftermarket O2 sensor can be a problem. Marc has had no problem with his. I have a friend at the dealership who says any 3-wire VW O2 sensor will work. There's not much of a consensus on this and the Bosch one is an expensive gamble. Dunno about mine, but it's on the list to be checked. I may have to ask Rob to send me the original '85 DME to try out.

If you need one, I have a extra 85 DME unit. Its worked fine in three cars so far.

Joe

Thanks Joe. This gang never ceases to amaze me. Lemme do some more checking first before I go with your DME. Just out of curiosity though, in what cars other than a 1985 Carrera did your '85 DME work?

The '84 through '89 DME's are functionally the same. I run my 1989 "spare motor" with my 1986 DME. Are you using the stock '87 chip in your '87 DME?
-Chris

Chris, I'm using the Autothority chip from the '85 engine in my '87 DME. I've heard from others that DME from 84-89 are the same. Why do they have different part numbers then?

Richard they are the same. There are a few things different on the early dme. The Eprom for the new versions has 28 pins vs 24 for the old. However you can mod an early version dme to 28 pins easy, and it will function the same way as the later models. The only diffrences are that the old dme used onboard memory and the new ones (28 pins) use the memory on the eprom. Not that i,m so wise ;) i read all this on http://frwilk.com/944dme/ hope it helps.

Cheers,

Ed

The boxes are functionally the same but internally different.
'85 DME's usually have a 24pin/2K Eprom
'87 DME's usually have a 24pin/4K Eprom or 28pin/8K eprom.
See my chart DME data chart

I modified my '86 24pin/2K DME to use 24pin/4K ('87 style) Eproms and it runs fine with either my old stock 2K chip or my custom 4K chip. In other words, the '87 4K box will run the old '86 2K chips. I haven't looked at an actual '87 board so I am not positive that my mod to make my 2K box a 4K box is exactly the same as what the factory did.

I hope this helps,
Chris

Nice info on your chart Chris tnx :) i found an eprom programmer than handles the eproms of the later types. Now i,m looking for a way to real time edit one and make changes while driving. Maybe Steve can help me out with that one.

Does anyone know if there is any advantage using an 88 or 89
dme in an 87? the change from 24 to 28 pin eprom would to a novice seem like more "computer "power?

It would more like having the Declaration of Independence printed on 2 pages to get more political clout out of it.

The ROM just has numbers in it. It doesn't make any difference what is holding them.


Wayne

Power wise, probably not, though I have heard the advantage of the 28 pin eprom is a better idle. The 28 pin eproms were programmed with a idle speed of 880, while the earlier eproms were set at 800 - although one could program the earlier eproms to idle at 880 also. Don't really know if there is any advantage to this for earlier cars, it may have been raised to accomodate for the g50 tranny and clutch of the later cars. I will attempt to perform this mod , and program a new 28 pin chip identical in mapping to what I am running now, some time in the coming weeks, and see if there is any difference.

Steve W.
When you get this done I would be interested what the outcome would be to see if it works.

keith epperly

It won't be possible to reprogram the EPROM while underway, if at all- It would require a FLASH memory chip to do that. Not invented yet in this era...:cool:

It's possible. You just use an Eprom emulator.
-Chris
Link to Xtronic emulator

I wouldn't reprogram the eprom while driving anyways. There is a lot thought and time involved with remapping eproms and I would not remap a new chip in real time.

I can a buy a sytem here that does just the thing, but it costs 2000 euros :( Offcourse Steve is right about the thought and time needed to program it. I was thinking of making a number of configurations at home and try them out. I guess stopping the car while doing this is the best thing to do Steve you,re right about that :) What i meant was writing it in flash like Chris said, sometimes my english is just not good enough to explain what i mean ;)

Ed

With an Eprom emulator setup you can "tune" without opening the box and changing chips. Take a run, look at your A/F curve and/or power, adjust the maps and take another run. You wouldn't do this while underway.
-Chris

Chris, et al;

Agreed. I guess I took Dutchie too literally... didn't mean to hijack the thread...sorry.:cool:

Richard,

The 87 control unit is not interchangeable with the 85 because the wiring harnesses are different. I already had this discussion with my P-wrench.

Regs,

Joe

Your wrench is quite mistaken.
-Chris

Chris,

I verified the differences looking in my Haynes wiring diagrams. They don't seem to be pinned the same.

Regs,

Joe

Joe,
Do you rember which pins? I run my '89 motor in my '86 911 with an '86 DME without any problems. When I tried the '89 DME the motor ran but not great so I assume(d) that my '89 DME had some problems. Perhaps there is a harness difference.
-Chris

Chris,

I have a late production 86 3.2 that came with the 87 control unit. That's how I found out that that the early ones would not interchange when it came to smog time...it failed because the timing was too far advanced. That's when we found out that the factory started using the 87 engine on some late production 86's. The referee station had to modify their data after I proved to them of this fact.


Regs,

Joe

stlrj,
Yes, the timing advance is slightly different and the idle is 880 on the later DME's instead of 800rpm. (See earlier posts in this thread.) I don't understand how that would effect smog exactly. I'm curious, what does the sticker in your engine compartment say the idle should be?
-Chris

Joe:

The pin-out difference is news to me. Do you know which pins were different? The only difference I remember seeing on DME harnesses was a different plug for the O2 sensor.

Tim

Here's an informative link on '84 - '89 DMEs.

http://members.rennlist.com/messinwith911s/DMEHacking.htm

Chris is right:
I have an acquaintance who ( surprisingly, we find) runs a late DME box on his USA spec '85 Carrera....ended up having to buy a 28 pin custom chip instead of the normal 24 pin chip his car should have.
--- no problems----- the box fits and his car runs...empirical evidence prevails.
---Wil Ferch

Chris,

No sticker since the engine came from an 86 wreck.

Joe

I just checked my Haynes Manual (not sure if there is an update) and their diagrams only include the 84 and 85 model years. I did find what I think is an error in the Bentley manual. From 84 through 87, they show DME pin 21 (BLK/VIO) going to one lead of the tach and DME pin 4 (YEL) going to the ignition, where on the 88 through 89 diagram, they show pin 4 going to the tach and pin 21 going to the ignition switch. I believe this is an error in the manual and not actually a physical change. This might be what Joe was refering to.

Tim

I just checked the factory manual schematics and pin 21 goes to the tach, at least it does for 1985, 1986, 1987.
I looked at 1988 and 1989 but frankly I couldn't decipher them. The '88 and '89 seem to have a lot more stuff hung off that pin.
-Chris