General 90 EPROM ECU in a 91?

[Ben]

I have a 91 5 speed GSX with an e742 EPROM in it. My car has 91-94 coils/power transistor. I didn't even notice that an e742 was for 90 models until I started ordering my Keydiver dsmchip today. does this affect anything?

I also have the correct EPROM ECU for a 91, but it's nasty on the inside, and the 90 EPROM looks brand new, so I want to use the 90 EPROM ECU if possible.

I did a little searching, and I found where someone said that you don't have to do anything to put a 90 ECU in a 91 car, just swap 2 pins on it to fix an idle issue. is this true? What idle issue is it? Of course I drive a DSM, it doesn't idle perfect but if this is the cause of a small idle issue that would be AWSOME.

Thanks for the space.

 

[Ben]

I did some more searching and found the same answer in several other threads. I went on to vfaq and found the ecu pin out. now I'm going to go swap em and see if my idle is a little more consistant.

 

[Ben]

noticed no difference what so ever.

 

[dsm-onster]

There is no idle issue swapping between 1990 and 1991+ ecus and cars. There is a tachometer accuracy and 'bounciness' issue.

 

[Ben]

I guess I hadn't noticed a bouciness issue.
How does it read? as in does it read higher than it should, lower than it should??

 

[silver bullit]

I have put a 91 ecu in my 90 and my tach dont work, other than that its the same, i did not chage anything but the ecu.

 

[steve]

There is no idle issue swapping between 1990 and 1991+ ecus and cars. There is a tachometer accuracy and 'bounciness' issue.

Sorry Matt but I'm not sure you know what your talking about. The pins used for the Idle Position Switch and the MAF reset line change positions between 90 and 91. If the pins aren't swapped the ECU wouldn't see the IPS work and won't be able to reset the MAF air counter

If you don't notice a difference then perhaps the IPS isn't working to start with. I can sure tell the difference when I'm testing them.

There is no issue with the tach when using a 90 ECU in a 91+ car.
The issue with the tach is when using a 91+ ECU in a 90 car.

 

[dsm-onster]

Sorry Matt but I'm not sure you know what your talking about. The pins used for the Idle Position Switch and the MAF reset line change positions between 90 and 91. If the pins aren't swapped the ECU would see the IPS work and won't be able to reset the MAF air counter

If you don't notice a difference then perhaps the IPS isn't working to start with. I can sure tell the difference when I'm testing them.

There is no issue with the tach when using a 90 ECU in a 91+ car.
The issue with the tach is when using a 91+ ECU in a 90 car.

Yes! My IPS is not working. Thanx for catching that! My ISC is removed, too. After tweeking w/ it, she seams to idle fine w/out them.

As for the MAF counter, I drove it around for several miles, on and off boost, with no noticable effects. I was logging w/ the scanmaster, and I never saw an issue. ??? So, I'm now confused. I'm definately not the only one who has swapped and had the car drive fine.

 

[Ben]

If you don't notice a difference then perhaps the IPS isn't working to start with.

smart guy right here!!! the part of the throttle body that holds the IPS broke off, and I havn't had one for a while. I just ran a wire inside the cabin, and I ground it myself. I'm lazt most of the time and don't though. That's funny though. I was running my 91 eprom for a while, and grounding that wire made a difference. then when I swapped ecus tp the 90 epom, grounding that wire no longer affected anything. and the scantool never showed it to change when I grounded it.

 

[steve]

If you not swapping pins 6 and 14 at the ECU harness then the IPS isn't running to the correct ECU pin when you switch ECU types.

 

[Ben]

yeah, when I put the 90 eprom in, I didn't switch the pins. I just switched them yesterday. Now on to another idle problem.
when I turn my headlights on, the ilde drops down low. right at 500-600 RPM. And with 272s, and polyurethane motor mounts, that low idle is ROUGH!!!!!

I need to rig up some kind of IPS. I'll probably get a WOT switch, and mount it somehwere on the pedal or TB to ground when the throttle it closed.

 

[dsm-onster]

yeah, when I put the 90 eprom in, I didn't switch the pins. I just switched them yesterday. Now on to another idle problem.
when I turn my headlights on, the ilde drops down low. right at 500-600 RPM. And with 272s, and polyurethane motor mounts, that low idle is ROUGH!!!!!

My setup idles rough w/ fp2Xs at 500rpms. These have 1 degree LESS duration. Your 272s need a healthy 750 rpms+ to idle very smooth.

 

[tqmx1]

So if you ran 2 jumpers in the engine room between the IPS and the MAF reset. If I get this right 2 pieces of 18 to 20ga wire 6 spade connectors 3 female 3 male. You could use eather ECU without trying to switch pins in a verry confined space and if you are using a MAF T you are down to 1 jumper seeing as the MAF reset is disabeled.

Or am I missing somthing?

 

[Siral3x]

So if you ran 2 jumpers in the engine room between the IPS and the MAF reset. If I get this right 2 pieces of 18 to 20ga wire 6 spade connectors 3 female 3 male. You could use eather ECU without trying to switch pins in a verry confined space and if you are using a MAF T you are down to 1 jumper seeing as the MAF reset is disabeled.

Or am I missing somthing?

I can't really follow you, but if you're tring to say what I think you are, then NO I would not want any wires hanging in my engine bay , do it at the ecu...it will take some time, but it's worth it.

 

[tqmx1]

I can't really follow you, but if you're tring to say what I think you are, then NO I would not want any wires hanging in my engine bay , do it at the ecu...it will take some time, but it's worth it.

Just have the jumpers follow the wire looms that go behind the engine around the battery box and around the spring pirch and ending up at the 7pin connector for the MAF. cut the MAF reset wire put a female spade going into the plug and a male plug on the wireing harness on the IPS you all ready have the spade termanels. So all you wold have to do is hook up one jumper from the IPS wire to the MAF reset unit and the other would go between the MAF reset wire to the IPS switch.

Presto chango for the 91 to 94 hook up the jumpers to run a 90 ECU and if you went back to the 91 or later ECUs you would disable the jumpers and hook the wires up as normal.

 

[steve]

It's takes a whole 30 seconds to release the pins and swap them in the ECU connector. Unless you switch ECUs more than I do there isn't much of a reason to make it any more complicated.


Having a working ISC greatly improves the quality of your idle and the ability to react to changing loads.

 

[tqmx1]

It's takes a whole 30 seconds to release the pins and swap them in the ECU connector. Unless you switch ECUs more than I do there isn't much of a reason to make it any more complicated.


Having a working ISC greatly improves the quality of your idle and the ability to react to changing loads.

The only reason I brought it up is under dash you have to be about the size of the GYCO gremlins to work under there. In the engine room you can stand homoerectis (well almost) with full size hands with plenty of room to work.

Just another option to pin switching.

 

[fourreGsixty3]

Or just take the ECU out, and swap pins on a work bench.

 

[tqmx1]

Or just take the ECU out, and swap pins on a work bench.

Correct me if I am wrong the pins in the ECU are fixed. The pins in the connector are the removeable ones. And unless you take a pair of dikes to the harness they stay tight and cozy in the car.

 

[Ben]

Yeah, you have to swap the pins on the connector. sericously, it doesn't even take a few minutes. and very easy.

I have a known good IAC motor. I forgot what my base IAC count was though. I know it's not where it needs to be cause my car idles too rough at 750 RPM.

 

[dsm-onster]

Or just take the ECU out, and swap pins on a work bench.

The pins can be swapped at the harness, not the ecu .

 

[fourreGsixty3]

O yea I forgot about that pesky thing called a wiring harness well you could remove the carpeted panel, and use the slack in the harness to pull it out enough so you can lay down on your fully reclined seat, and swap the 2 pins.
Sorry about the miscomunication guys. I remember reading an article where they were soldering/switching pins in the ECU but that must be for something different. I hate mixing up information.

I know the OP has a 91+ car, but for future help of people using a 91+ ecu in a 90 DSM there is this site.
KEYDIVER Chips -1991+ ECU in a 1990 DSM

 

[dsm-onster]

Having a working ISC greatly improves the quality of your idle and the ability to react to changing loads.

Yes, don't go disabling your ISC just because someone managed to get a decent idle w/out it. I still have some drivability issues. I don't have power steering or a/c now. So the issues are minimal. The ISC is factory installed for these two accesories.

 

[Ben]

I also do not have power steering or A/C, and Do have a good, functioning ISC motor. But my idle drops to 500-600 when I turn the headlights on. and I have the idle set about 1000 rpm so it's a huge drop.

 

[dsm-onster]

Grab a multimeter and check the votlage across the battery terminals. Lights off and lights on. There shouldn't be much of a drop. . . It's a stab in the dark, but you perhaps could have an alternator about to go.

My idle drops only slightly with the head lights on.