Prom programming

[Corypheaus]

New to the forum, so if I post this in the wrong spot I apologize.

Just acquired a 1990 blazer (no driveline. Was stock 350 700R4) I already have an sm465 4spd from a previous build, I was told I would need to update the prom to match the 4 spd MT. I cannot find anything about this except for a previous thread on here.

This is my computer
Is there someone in here that can help with this?

Thanks in advance

 

[Bent77]

Works best to zip the vss and tcc out of that.
The tools to do this are few and far between anymore

The easiest way for you would be to call Howell Eng and tell them specifically what you have, and have them zip a chip for you

 

[dyeager535]

Been a very long time, but as I recall you can jump the p/n switch so it thinks it's always in gear.

Not optimum, (generally minor, but extensive) differences in the tune between auto and manual, but back when OBD1 was still commonly messed with, it was pretty normal for folks to just jumper the P/N switch if the only change was to a manual. You could always try it and see what you think.

At this stage if just keeping things stock, you *might* be able to find someone selling a manual trans PROM. Have you checked rockauto? At one point they were selling them.

 

[scouthead]

Yeah. This is true. I have an 1988 with sm465 and there are some differences. Prom was the biggest one I can remember. I had tried to go the TBICHIPS.COM route at one point, and it didn’t go well- trying to mail order a custom chip from someone that doesn’t have your vehicle in hand for REAL tuning seems really dumb now looking back! Wonder if Brian is still around and if he is still burning chips or if he has a stock chip he would sell you?

 

[scouthead]

I think the Ignition control module (and possibly the MAP) was different at the time of release- back in the day I was told not to run the AUTO version on a manual trans, or the other way around…
Since then I’ve had some repairs and gone down a rabbit hole and found some info online that Around 1995 gm changed ALL of the ICM to match, and anything you buy today “should” be correct???

 

[scouthead]

Aaaaaaand there’s a guy out in Santa Ana Ca that used to have a Chevy only wrecking yard. It’s called “Chevy Truck and Foreign” on 5th street. If they are still there and you can google their phone number…
He used to have a stack of computers sitting on the shelf in the back of the office.

 

[Corypheaus]

Yeah. This is true. I have an 1988 with sm465 and there are some differences. Prom was the biggest one I can remember. I had tried to go the TBICHIPS.COM route at one point, and it didn’t go well- trying to mail order a custom chip from someone that doesn’t have your vehicle in hand for REAL tuning seems really dumb now looking back! Wonder if Brian is still around and if he is still burning chips or if he has a stock chip he would sell you?

I did manage to find a 1990 k1500 4spd manual chip for it, hopefully it works

 

[scouthead]

I did manage to find a 1990 k1500 4spd manual chip for it, hopefully it works

Huh. Dunno! If it were me I would give it a shot though!

 

[dyeager535]

I did manage to find a 1990 k1500 4spd manual chip for it, hopefully it works

Same ECM? That's all I'd be worried about. As long as it came from a 1227747 ECM (or whatever the service replacements are, people love to throw ECM's at problems lol) I don't see why it would matter.

 

[joshuak]

I think the Ignition control module (and possibly the MAP) was different at the time of release- back in the day I was told not to run the AUTO version on a manual trans, or the other way around…
Since then I’ve had some repairs and gone down a rabbit hole and found some info online that Around 1995 gm changed ALL of the ICM to match, and anything you buy today “should” be correct???

Happen to remember why?

I have a project that went from a 700r4 to nv4500 and seemed to work fine.

 

[dyeager535]

Happen to remember why?

I have a project that went from a 700r4 to nv4500 and seemed to work fine.

Interesting post on the ICM's: https://www.gmt400.com/threads/whic...d-acdelco-d1943a-or-d1984a.55395/post-1370622

This thread made me recall hearing something about some differences in the past. The above post confirms it.

Interestingly, my TPI setup will cut out at around 3000RPM if I'm hard on the gas, even though I suspect the ICM is not the issue. Something to keep in mind though as I work through it. I think this is a complete Chinese replacement distributor.

As I recall MAP sensor differences were because of turbo applications.

 

[Corypheaus]

Same ECM? That's all I'd be worried about. As long as it came from a 1227747 ECM (or whatever the service replacements are, people love to throw ECM's at problems lol) I don't see why it would matter.

I was told that it would pop and backfire every time I let off the throttle no matter what speed or rpm

 

[dyeager535]

I was told that it would pop and backfire every time I let off the throttle no matter what speed or rpm

Well you'd need to know what ECM its out of. Apparently (according to Rockauto) that vintage could have had the same ECM, but also potentially another one not native to the square bodies. If the ECM number is different, I wouldn't expect the chip to work properly. Usually GM changed the programming (even if the chip physically fits) because the circuits used were different. Those are tied to physical differences (easy comparison, but not applicable here to my knowledge, is the air inlet temp sensor used on car TBI but not trucks) so the programming is different to account for that sort of thing.

FWIW, I see the same ignition module from a 1989 K5, 1990 K1500, 1995 K3500 (w/454), and 1995 K1500 when paired to a 350. Pretty sure those are the same across the board for TBI trucks.

 

[Corypheaus]

It came off a 1990 K1500 ecm is a 1227747

 

[dyeager535]

Should be no problem running it.

Trans, gearing, AC, etc all effect programming, but it should work regardless.

 

[HankScorpio]

What motor are you putting in with the SM465? T-case?

I swapped a 1990 Blazer from the 700R4 to a SM465/NP205 with the stock 350TBI. You will get a check engine light without a VSS. Assuming you are using a SM465/NP205 you need to send the tail housing to Offroad Design and have it machined to accept a tone ring and VSS. Once you do that the TBI will get a speed signal and the check engine light will go away. Just ignoring the light is an option, it will run, but it will never be quite right. Fuel economy will suffer and it will likely be running in some base programming or limp mode type of thing. To get full power and good drivability the TBI wants that VSS signal.

I drove mine with the VSS NP205 for a year and then ditched TBI completely for a 454 with Edelbrock ProFlo4.

If you have the stock NP241 passenger drop with the VSS (1990 and 1991 only) you could adapt it to the SM465 but that will likely require an output shaft change on the trans or an input change in the T-case and a different adapter. I think you have to mimic the factory SM465/NP208 setup to run a NP241 on a SM465.