1987 DME in a 1984 Carrera

[Synchromesh]

I've been having some issues with my car and one of the things that needs to be tested is the DME computer itself. My car is an early '84 (built 8/83) and I have a friend who has an '87. So I was thinking swapping DMEs for a few days to see if that cures my issue. Will this work? I know chips are different in pin count but I figure if I swap the entire computer as a unit it should be possible. Thanks.

[4flyboy]

Ingo Schmidt is your guy for this.

ischmitz here on the boards helped me tremendously with time, advice and even a loaner when I had serious issues with my DME unit.

He'll be able to answer this question.

Good luck,

Scott

[proporsche]

Hi I have in my 1985 1988 control unit and it works;-)

Ivan

[ChrisBennet]

To test this, put your DME in your friends car.
I wouldn't put his DME in your car just in case your car has something wrong that is damaging the DME.
Your 84-86 DMEs are programmed to idle at a slightly lower RPM than your '87-89 so it may hunt a bit at idle with the non-matched DME. The engines "natural" idle speed is easily corrected with a screwdriver and a temporary jumper wire (no reprogramming required).

[ischmitz]

First off, what are the "issues" you are having? I am assuming you are swapping DME to see if these stay with the car or move with the DME. My suggestion: Start with a good problem statement and then go from there.

As far as swapping DME that is fine. As long as neither car is hard down swapping is fine and won't do any damage. Contrary to what some here post with a car hard down swapping a good DME into it can damage that DME. And swapping the questionable DME into another car can damage that car. So neither is entirely safe.

In your case just keep in mind the later model DME (with the later software) has the idle speed programmed to be 880 RPM instead of 800 RPM to account for the larger load of alternator, compressor, etc. If you get hunting idle it means the idle stop needs a slight adjustment. Just pointing this out so you don't get to wrong conclusions if one of the cars develops "hunting" as result of the swap.

Ingo

[Synchromesh]

Thanks guys.

Issues I'm having are quite complicated. I've written about them before but here is the gist. There seems to be a problem that the engine gets cut. This is somewhat temperature related although the exact pattern is hard to figure out. When letting the car idle after sitting overnight the engine sometimes will just die and refuse to start for a while. Then it'll start and problem will persist for some time with engine dying momentarily and restarting again. After the car warms up well the problem disappears. However, it does pop up again if I drive it on the freeway for a while then get off and let it idle (ie at a streetlight). Lugging the engine during this time make it die more often. It also seems to be more of a problem with weather outside. When it's nice and warm it's less of a problem but when foggy and gloomy outside it's somewhat worse.

So I went to a Porsche mechanic. He cleaned some contacts as he told me and that made it somewhat better. At this point: all 3 sensors on the left side of the engine, played around with the ICV valve (which seems ok), replaced the DME relay and did some additional testing by connecting to the fuel line. It turns out the fuel system loses pressure immediately when this happens meaning injectors are shut down by the computer.

We opened the computer up with a friend and found no obvious damage, in fact looks like new inside. So now I'm thinking to try it with a different DME to see if that cures. Because I ran out of ideas of what else this could be.

[ischmitz]

I'm assuming when you talk about "all three sensors on the left" you are referring to the speed sensor, reference sensor, and cylinder head temperature sensor".

What was done to those sensors (replaced, played with its connectors, other things)?

The last statement doesn't make any sense: fuel pressure is a result of the fuel pump running and the pressure regulator working. As long as the DME "sees" pulses on the speed sensor it will keep the pump running and fire the injectors. With both taking place the pressure in the rails will hold between -37 PSI and -25 PSI.

If the DME stops receiving pulses from the sensor(s) the pulses to the injectors stop immediately and the fuel pressure needs to remain above 20psi for another 30 minutes.

If the DME continues to issue fuel pulses but the fuel pump stops pumping up fuel pressure in the rails will drop quickly and the engine eventually will stall lean.

Please clarify if your mechanic saw fuel pressure dropping in the rails. If yes I would start looking at the fuel pump.

Ingo

[Fly911]

I had the same problem with my '87 Carrera with an aftermarket ECU (not Motronic). It turned out to be the RPM sensor on the engine. Since my ECU has online diagnostics, I could see the fault codes. Changed all three sensors on the front left side of the engine (RPM, TDC and Temp), and everything has been OK since. The DME relay is also worth checking. There are two relays inside, one is power to the ECU and one is for the fuel pump. Any of those fail, and your car is dead. I have installed a spare DME relay next to the original one, and it's a 30 seconds job to switch from one to the other. A mod well worth it, with the flimsy reputation of the DME relays.
As far as the Motronic ECU, the multi pin plug is identical on '84 to '89, so all interface signals are identical. However, the 84 to 87-1/2 have a 24 pin chip, while the '87-1/2 to '89 have a 28 pin chip. Still, the ECU's should be interchangeable.

[larrym]

YES - I have 3 ECU's for my '89-3,2 - one 28-pin with Wong chip, 2- 24-pins with various chips for testing

all interchange & drive just fine

most '87s still used a 24-pin and many had the "said best of the stock chips" xxxx-302 chip

p.s - one is available FS

Originally Posted by Fly911

As far as the Motronic ECU, the multi pin plug is identical on '84 to '89, so all interface signals are identical. However, the 84 to 87-1/2 have a 24 pin chip, while the '87-1/2 to '89 have a 28 pin chip. Still, the ECU's should be interchangeable.