Swapping Early and Late DMEs

[loftygoals]

I've read that you can switch DME's between early and late cars if you swap injectors, DME, and HFM.

 

I'm back working on my 924S project car. It's a 1987 924S. I've swapped in the HFM, injectors, and DME from my 1983 944 Spec race car. The car will start, but the idles really high. I've looked for vacuum leaks, but can't find any. I did notice that the idle control is different between the early and late cars. The late cars use some kind of DME controlled idle control pump. Could this be the issue?

 

Thanks,

-bj

[944-Spec#94]

Yes it is.

 

 

85.5 (including 924S) and up use a eletroniclly controlled idle stabizer valve (ISV). It is connected to a 3 prong connector under the intake manifold. It is an air valve that opens or closes based on the DME signals. The DME tells the valve how far to open base on temperature and if the A/C is on. I believe it also takes into account the O2 sensor.

 

The 82-85 uses a auxilliar air valve in the same location under the intake. This valve works by ambient temp is an bi-metallic. This means it opens or closes on it own based on tempeature. There is a 2 prong electrical connection to the DME, but this seems to be for reading the signal only. If the A/C kicks on there is a second valve use to add air to keep the proper idle speed.

 

 

So it is likely the car will idle different.

 

I have run for years an 87 924S fuel injection/control in my 84 944. I have used the 87 DME harness complete with 87 AFM (no the HFM as you type), 87 injectors, 87 ISV, and 87 DME. It runs fine like that. I have swapped in 88 block using the same 87 controls. Idle speed is 100-150 rpm higher with the 87 DME as compare to the 88 DME. I have also swapped the AFM and DME from my 84 944 and the car ran fine. It did not seem quite strong, but ran. I can't remember where it idled.

 

 

Reciently I have been having idle cut out issues. I finally replace my 87 924S DME harness that I had been using for 10 years with my only spare. It is and 84 harness. This works fine except it has not connetion for the ISV. I traced the wires and they are not even the same. Seems like the older harness does not even read those ports from the newer DME's. So for now I runs without any idle control. ISV is basicly stuck in semi cold start position. Not fully cold start, but not closed either. So when warm it idles high maybe 1200 rpm. Cold it works, but it is summer here so cold is relative.

 

I have considered swapping back to the 84 air valve as I have one of those, but I don't want to pull the intake to get to it. So for now I will just see how it idles. At worst I have to manually keep the idle up when cold.

[loftygoals]

Thanks Joe. I appriciate it. Sorry about the HFM/AFM confusion. In BMWland it's a HFM. Why can't these Germans agree on terms?

 

I just got a tach working, so I know what the motor is doing, now. It is hunting between 1300 and 900 RPMs. If this were a race car, I wouldn't worry about it, but this is going to be a street/DE car so I want it well sorted.

 

I have another lated DME and AFM I'm going to swap in. I think this DME is bad, but I'm not sure. I'll know shortly...

 

-bj

[loftygoals]

Good news, I swapped in this extra DME and everything works great. Idle problem is gone! So while putting a late DME in a early car may work OK, the opposite isn't as true. It will run, but not idle happily.

 

-bj

[Cory M]

If anyone needs a DME I have one I bought as a spare for my 1985.5 944. I tested it out and it works fine. I paid $200 but will let it go for $100. Send me a PM if you want it.

[Weston]

FYI... The DME harnesses and pin-out differs slightly between the early cars and late. Pin 28 is the altitude switch on early cars, but a ground on late (its alt. sensor is on pin 30, which the early DME doesn't use). If you plug an early DME into a late harness, it may think you're in Denver and lean the fuel out a bit.

[944-Spec#94]

I guess that means late DME in early harness will not pick up the altitude sensor and run rich at altitude.

 

Ok if the Alt sensor drops fuel from 100% to 94% then...

 

Early DME + Late harness = 6% lean all the time

Late DME + early harness = 100% below 3280ft(1000 meters) = perfect

Late DME + early harness = 100% above 3280ft(1000 meters) = 6% rich.

 

So for a late DME and early harness you need a way to tap in to the DME and give it the altitude sensor signal.

[cbuzz]

hmmmm might have to go to the dyno and do some test runs........

[944-Spec#94]

hmmmm might have to go to the dyno and do some test runs........

 

 

Great... Like you need to go any faster.