ECU question. What's the difference?

[D1 MAD]

The ECU fitted to my 1987 350i is marked; 4CU 83986B.
I have just been gifted another ECU marked 4CU 84477B. I am told this latter one came off of a 25D reference V8 engine, which looking at the Rimmer Bros chart was a 1996 3.5 EFI Auto unit, 9.35-1 compression.

My question is, do the ECU reference numbers give away any useful information on how the ECU performs, and would I be taking any risks fitting the latter just to try it out?

Reason for asking is that I am curious to know if the gifted item is any good, and whether it may be any better than the one on the car at the moment. Since overhauling the top end and fitting a 214 cam I have very little engine braking under 2000rpm, which I have been told by a local V8 tuner is a feature of the cam and can only be sorted by remapping the ECU.

Cheers.

[adam quantrill]

As someone else has said before, the AFM's are quite often matched to the ECU's. It will probably run OK, but the fuelling will probably be worse, or possibly better.

[D1 MAD]

That's interesting, I'd better go back and see my nice neighbor then as he offered me the AFM as well. In fact, as he is going to fit carbs to his engine he has said I can have the entire EFI setup, manifold, plenum, injectors, loom the lot. He even said I could have a nice clean looking 35DLM8 dizzy that came spare with his job lot purchase, all for nowt as it is in his way! Think it only fair that I buy him some beer though.

[350zwelgje]

CU... is a flapper ECU. CUX is for hotwire. Would be surprised that a '96 ECU can be a flapper type ECU. And as said before, seems that ECUs were matched to AFM. And the issue is that if it is a flapper ECU adjustments are not easy. The later CUX (hotwire stuff) can be rechipped with a chip like tornado and even be 'fine tuned' during a rolling road session. So for a matching process a CUX is preferred, and you are very limited and would need the help of the likes like Mark Adams to be able to setup/adjust a flapper ECU. No DIY options on flapper stuff unfortunately. You can still use it as a back up (I have spare ECU for my flapper, that seems to work ok for example).

Rob

[D1 MAD]

Thanks for that, numpty me should have typed '1986' not 1996 Doh!
I will do my best to blag the AFM and the rest of the bits this weekend and will try the ECU out with my original AFM and the new one. Well it would be rude not to at the price
I was guessing that the numbers may have identified the ECU as being set up for a particular cam as I was thinking a Range Rover would require different fueling to a Vitesse, or am I way off track? I also wanted to make sure I wasn't going to do any damage by experimenting with it.

Cheers

[Wedg1e]

D1 MAD said:

I was guessing that the numbers may have identified the ECU as being set up for a particular cam as I was thinking a Range Rover would require different fueling to a Vitesse, or am I way off track? I also wanted to make sure I wasn't going to do any damage by experimenting with it.

Cheers

No, you're about right. All 4CUs are the same (with a couple of provisos*) but there are a selection of resistors that fine-tune the ECU to AFM and fuelling to the car. Note that some of the resistor values are VERY specific and Maplins are unlikely to stock them.
ANY 4CU is worth having as a spare because you can always copy the resistor values from your original one.

• US-market V8s (as used in the TR7/8) had lambda control circuitry fitted, I've never seen a UK-market ECU with any of it. One of the two circuit boards in the ECU has a lot of missing components, that's where the lambda control would be.

[TVRleigh]

remember the get the same or better torelance resisters, will be gold silver or bronze I think. as they could vary by 1-2%

[adam quantrill]

Alternatively, measure the resistors on the old one with a decent DVM and replicate the values exactly by using resistors in series. That way you can get to within +/- 0.5%

[TVRleigh]

Also the better Q ones will suffer less from heat. also do the compare cold. or at same temp.

[D1 MAD]

Cheers all, I'm learning fast

[Wedg1e]

TVRleigh said:

remember the get the same or better torelance resisters, will be gold silver or bronze I think. as they could vary by 1-2%

I think you'll find that some (if not all) of them are from the 2% tolerance range anyway.