I have an ECM from an '86 2.5L 5 spd. Would it work in an '84 SE 4 spd?

They are different numbers.....but sometimes that does not matter.

86 5 speed = 1226864

84 4 speed = 1226156

the PROMs are different also.

I saw the numbers were different in the GM '84 to '88 Fiero parts catalogue. The catalogue sometimes shows pictures of parts that don't match the actual parts. I don't know how accurate it is...
I could swap the PROM...

[This message has been edited by David Hambleton (edited 04-09-2015).]

I have an 87 5sp with the 86 4 sp with no issues!

The 84 ECM is has "card edge" connectors for the harness - only year I think.
Not sure if the 86 has pin type connectors - someone with 86 Duke will have to confirm.

Top ECM in pic is from 84, bottom is an 87 V6 ECM.

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Calgary time/temp

3.4L Supercharged 87 GT Click me
Super Duty 4 Indy #163 Click me

84-86 engine will work but:
use 84 engine harness. C500 is different.
use 84 dist and coil or splice in new ICM and coil. Second option maybe easier to find ignition parts.
gm change throttle cable and TBI connection. Use 84 TBI or find correct cable.
5 speed need shifter cables, maybe shifter.

84-86 ECM different # but does matter. Prom have some minor changes but will work.

w/o headaches, 87-88 DIS engine is allot different wiring and can't use 84 engine wiring harness w/o issues because 84 is only made to use "old type" l4.
switch dis motor is easier for 85-86. Later years are made for use w/ different power trains as plug and play items for C500 and C203.

also See my Cave, Duke Quick Ref

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Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should.
(Jurassic Park)

The Ogre's Fiero Cave (It's also at the top and bottom of every forum page...)

[This message has been edited by theogre (edited 04-09-2015).]

A couple of years ago, I replaced the rattling 2.5L in my '86 with a 2.5L from an '87.
Today, I replaced the 31 year old 367,600 kilometer ECM in my '84 SE 4 speed with the surplus ECM from the '86.
The hope was it would solve the issued discussed here: //www.fiero.nl/forum/Forum2/HTML/135078.html
The engine started right up and seems to be running normally.
As I suspected, the shift light comes on to suggest 5th gear, but since I don't have one, I tilt the wheel so I can't see the light.
The light goes off over 3,000 RPM; about 100 kilometers per hour or 62 mph.
I presume if I swapped the PROMs, that issue would disappear.

Thanks for posting, guys. I appreciate it.

The engine has been running fine with the '86 ECM, just the shift light looking for 5th gear when there isn't one.

So, I swapped the '84 prom in to match the 4 speed trans and the idle went to 2500rpm with a continuous check engine light.
Went for a 4 km test drive; check engine light stayed on and idle at 2500 rpm.

Swapped the '86 PROM back in and am back to normal, (except for the shift light). Hmmm...

Before the cracked block engine was replaced in '87 under warranty, the idle was 900 rpm. Thereafter, mostly 1400 rpm. It would only drop to 900 rpm when it was raining. With the '86 ECM, it idles consistently at 900 rpm. Another hmmm...

Put a piece of tape over the light.

I don't usually resort to such highly technical solutions. The tilt wheel obscures the light; I still know it's on. The same issue would exist with the tape. And I'd have that non-stock tape there. Can't do it.

I have a 1985 v6 4spd ECM for sale if you (or anyone) are interested. I had it all boxed up to sell but another member who was going to buy it dropped the ball and disappeared off the planet and never sent payment. And now he won return any emails or PMs and its been two weeks. I also have a black one that I think may be from an 88.

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===Always trying to find time to work on cars===
Louis Duet
Baldwin, Long Island, NY
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"Blue" <= '85 Fiero GT
3800sc series 2 swap in progress
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"Boo" <= '81 Delorean DMC-12
VIN #5835
Stock PRV engine
Peugot 604 Intake manifold
Exhaust headers
Anti-3rd brake light