Top C Posted May 4, 2019 Share Posted May 4, 2019 I have an 87 4.0 automatic MJ and I saw a discussion on the ECU which made me think about a replacement ECU from a later year. I am wondering if a newer ECU would have some changes that would help the engine run better at idle for example and maybe there were some other changes that were done to later models that I would befit from. I'd like to hear of anyone that make an ECU swap and what you experiences were. Thanks in advance Mike Link to comment Share on other sites More sharing options...
87MJTIM Posted May 4, 2019 Share Posted May 4, 2019 You want to get an ECU from a 89/90 XJ. Jeep reprogrammed the ECU to improve HP. Link to comment Share on other sites More sharing options...
Top C Posted May 5, 2019 Author Share Posted May 5, 2019 Thanks for the reply. If I get the ECU from 89/90 you said to reprogram it? not sure how this is done? Link to comment Share on other sites More sharing options...
JMO413 Posted May 5, 2019 Share Posted May 5, 2019 There is no programming. Jeep updated the ECM in 89/90 that is why you must replace the whole thing. The programming is set in them. I did this upgrade, not sure how much it helped I did a couple other things at the same time. I did ECM and injectors at the same time and it was a very noticeable change in idle and throttle response. Cruiser says it's worth while and I pretty much take his word on anything Renix. Here's kinda the Renix godfather around here. If you haven't looked through his website I strongly recommend it. Link to comment Share on other sites More sharing options...
87MJTIM Posted May 5, 2019 Share Posted May 5, 2019 JMO413 is right. You cannot reprogam the ECU. Jeep made internal changes to the ECU that improved HP. Keep in mind, in today's world of smart phones and the internet, our Jeeps are DOS. (showing my age.) Link to comment Share on other sites More sharing options...
cruiser54 Posted May 5, 2019 Share Posted May 5, 2019 Link to comment Share on other sites More sharing options...
Top C Posted May 5, 2019 Author Share Posted May 5, 2019 I was looking on line as I thinking it might better to get a refurbished ECU I see several companies offering last digits 428 & 429. But what I don't see is a question related to the type Transmission doesn't it matter what type transmission you have when comes to ECU? My MJ is an automatic so I think I should be looking for an ECU for an XJ with automatic transmission correct? or that doesn't matter? Thanks again Mike Link to comment Share on other sites More sharing options...
Ωhm Posted May 6, 2019 Share Posted May 6, 2019 Auto ECU (428) or Manual ECU (429) shouldn't matter when it comes to plug and play. Wiring harness should sort out the need for Upshift Lamp (Manual) or not to Upshift Lamp (Auto). Now I have heard that the manual ECU has a more aggressive timing curve used for ignition advance, while others say its the auto ECU that does. Without the proper documentation, no way to prove this one out. My guess is manual. Anyone else care to chime in on this one (Auto vs Manual). Link to comment Share on other sites More sharing options...
omega_rugal Posted May 6, 2019 Share Posted May 6, 2019 if we find the ROM that has the air/fuel maps... Link to comment Share on other sites More sharing options...
cruiser54 Posted May 6, 2019 Share Posted May 6, 2019 Auto ECU has better idle stability. Just grab one from a junkyard. I have no faith the the "refurbished" ones. Link to comment Share on other sites More sharing options...
Top C Posted May 8, 2019 Author Share Posted May 8, 2019 So today I receive my 89 ECU (428) after an agonizing 2 hours of trying to remove the ECU from my 87 MJ I was able to unplug and plug in the new one and it worked. I took it for a quick trip around the block and its shifts perfectly and the idle seems a little better. For those that maybe thinking about doing this a couple of pointers; one if are going to a yard to get an ECU make sure the dash is apart to get at those bolts. Cruiser the bolts holding the ECU on my bracket were not 10mm mine were 3/8 and they were cap bolts and the head was very shallow. This made it very difficult to keep the wrench on the bolt. After removing the bolts the sticker on my ECU says its an 87 model so it was an original. I'm going to replace the bolts with something much easier to install and I'm not going to put the top one in! Also take the lower trim panel off it will it much easier to reach and remove the three bolts holding the ECU to the bracket. Other than having some choice words about where they install the ECU and sore joints from trying get under the dash it wasn't that back to swap it out. Now to see if really see if the swap really pays off. I do have a spare now so that's good thing. Thanks Cruiser for you time and tips on this project. Mike Link to comment Share on other sites More sharing options...
cruiser54 Posted May 9, 2019 Share Posted May 9, 2019 12 hours ago, TOP said: So today I receive my 89 ECU (428) after an agonizing 2 hours of trying to remove the ECU from my 87 MJ I was able to unplug and plug in the new one and it worked. I took it for a quick trip around the block and its shifts perfectly and the idle seems a little better. For those that maybe thinking about doing this a couple of pointers; one if are going to a yard to get an ECU make sure the dash is apart to get at those bolts. Cruiser the bolts holding the ECU on my bracket were not 10mm mine were 3/8 and they were cap bolts and the head was very shallow. This made it very difficult to keep the wrench on the bolt. After removing the bolts the sticker on my ECU says its an 87 model so it was an original. I'm going to replace the bolts with something much easier to install and I'm not going to put the top one in! Also take the lower trim panel off it will it much easier to reach and remove the three bolts holding the ECU to the bracket. Other than having some choice words about where they install the ECU and sore joints from trying get under the dash it wasn't that back to swap it out. Now to see if really see if the swap really pays off. I do have a spare now so that's good thing. Thanks Cruiser for you time and tips on this project. Mike It's much easier to remove bracket and all with the 10mm headed bolts. enjoy that ECU!! Link to comment Share on other sites More sharing options...
Hobobob3040 Posted September 11, 2021 Share Posted September 11, 2021 I recently got my hands on a 86 mj and was wonder if I could do the ecu swap out with an 89/90 ecu? Link to comment Share on other sites More sharing options...
cruiser54 Posted September 11, 2021 Share Posted September 11, 2021 Tell us more about your Jeep. engine size etc. Link to comment Share on other sites More sharing options...
omega_rugal Posted September 12, 2021 Share Posted September 12, 2021 On 9/11/2021 at 5:29 AM, Hobobob3040 said: I recently got my hands on a 86 mj and was wonder if I could do the ecu swap out with an 89/90 ecu? 86 didnt get the 4.0 yet... Link to comment Share on other sites More sharing options...
cruiser54 Posted September 12, 2021 Share Posted September 12, 2021 Hey, either they respond or they don't. Link to comment Share on other sites More sharing options...
Smokeyyank Posted January 10, 2022 Share Posted January 10, 2022 Quick question, I snagged a ecu from a 89. It's a 428. Can I swap it in my 88? I have a 5 speed. Link to comment Share on other sites More sharing options...
87MJTIM Posted January 10, 2022 Share Posted January 10, 2022 Yes, you can plug it in. The 428 (auto) has a slight programing diff for shifting. Since you control the shifting, it is not an issue. Link to comment Share on other sites More sharing options...
Smokeyyank Posted January 10, 2022 Share Posted January 10, 2022 5 minutes ago, 87MJTIM said: Yes, you can plug it in. The 428 (auto) has a slight programing diff for shifting. Since you control the shifting, it is not an issue. Great, thanks. Link to comment Share on other sites More sharing options...
Smokeyyank Posted January 12, 2022 Share Posted January 12, 2022 Just to follow up on this. I swapped the ECU yesterday and have been poking around in the MJ. Here are my thoughts. -Start up idle is improved. -Overall it has more power through the power band. Just a lot smoother. -Low end torque is improved For the 1ish hour it took me to do it not a bad little upgrade. Link to comment Share on other sites More sharing options...
cruiser54 Posted January 12, 2022 Share Posted January 12, 2022 You guys need to keep grabbing those ECUs whenever you can and keep your old one as a spare. Smokeyyank's experience is typical. The trick to replacing them is pulling the ECU toward you on its bracket and removing the 3 nuts to get it off. Link to comment Share on other sites More sharing options...
CMFW Posted February 3, 2023 Share Posted February 3, 2023 Hi. I have a 1990 MJ. is it worth putting in say a 91 or 92 ECU? thanks. Link to comment Share on other sites More sharing options...
cruiser54 Posted February 3, 2023 Share Posted February 3, 2023 Different fuel injection system after 1990. Yours is the best Renix ECU. Link to comment Share on other sites More sharing options...
CMFW Posted February 9, 2023 Share Posted February 9, 2023 On 2/3/2023 at 1:16 AM, cruiser54 said: Different fuel injection system after 1990. Yours is the best Renix ECU. Thanks friend. I appreciate you. Link to comment Share on other sites More sharing options...
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