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 Post subject: LG3 to LN3 swap?
PostPosted: Wed May 17, 2017 11:49 am 
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Joined: Fri Nov 23, 2012 1:46 am
Posts: 10
Location: Hammond, IN
Year and Trim: 1987 LE w/SE Option
1990 SSE
1996 Z-28
The engine in the '87 went and I have a complete running 65,000 mile '90 SSE doner that I can't get titled. Besides engine wiring harness, new mounts and fluids, what else do I need to do the swap? And how does the ecu interact with the speedometer the 87 goes up to 85 and the 90 goes up to 120 mph. I don't want to swap the transmission if I don't have to. I have fsm for both vehicles and a complete exhaust (cat with downpipe to tips for the LN3). what will I have to do electrically?


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 Post subject: Re: LG3 to LN3 swap?
PostPosted: Thu May 18, 2017 9:49 am 
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Joined: Tue Aug 25, 2009 8:32 pm
Posts: 4275
Location: Dearborn, MI
Year and Trim: '93 SSEi
'05 STS-4
'97 Camaro
'97 LeSabre
My personal opinion here...

Pull everything (trans included, the trans got many updates between '87 and '90) from the donor and drop it in the '87. The only harness you have to worry about is the C256 (Dash mid harness, connects the drivers side wire pass through to the computer harness, the dash harness, and the body harness), I think you'll have to re-pin it to match the dash and body harnesses. The only other wiring you might have to do is add a few wires for the TCC and fan relays. If you can pull the entire dash mid harness and drivers side wire harness pass-through from the donor that should make this swap even easier. My recollection is that the passenger side pass-though has everything on it for the LG3 except the TCC and cruise control. When they switched to the LN3 the TCC wiring and internals on the trans changed as well as putting the fan relays on the drivers side harness. Oh, and the LG3 stuff used a different speedo sensor and a module to convert it to 4000ppm. Even more flame on the fire to just swap the dash mid harness out. But where it connects to the dash harness you will have to re-pin it so the gauge cluster works (if you don't want to change that harness out you have to add wires for TCC and fan relays to the drivers side pass through, still re-pin the C256 connector). I don't recommend trying to control the '87 trans with the '88+ PCM, you have to find a way to invert the 3rd pressure switch signal, and depending on the year of the PCM it's going to set a code anyway.

I have wiring schematics for the LeSabre '86, '88, and '89 on hand, I don't have the Bonneville. I don't think much changed from the 86 to 87, but not sure on the rest. It's been a while since I messed with this stuff, so I recommend finding the chassis service manuals for the wiring. Gauges and all that just have to connect all the right wires, and the gauges will read correctly. But so many things were changed in the wire harness pinout those years it's not even funny.

Pulling the dash out to get the harnesses out isn't really that hard. And it's way easier to drop the whole subframe out than it is to pull just the motor.

_________________
I don't know how much boost it takes to blow the tires off. But I know how much I'm going to use!
Image

Runs:
'93 Pontiac Bonneville SSEi - Twincharged, manual, racecar! Build thread
'97 Chevy Camaro - 3800, T5, T-tops, my DD
'05 Cadillac STS - V8, AWD, her DD
'89 Buick LeSabre Limited - winter DD
'84 GMC Sierra Classic - Twin turbo 3800
'97 Buick LeSabre - L67 project
'92 Pontiac Bonneville SSE future project

Doesn't run:
'87 Buick LeSabre T-type - future victim
'67 Buick LeSabre - future victim


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 Post subject: Re: LG3 to LN3 swap?
PostPosted: Thu May 18, 2017 4:35 pm 
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SE Member

Joined: Fri Nov 23, 2012 1:46 am
Posts: 10
Location: Hammond, IN
Year and Trim: 1987 LE w/SE Option
1990 SSE
1996 Z-28
Okay, so engine/trans with harness, dash mid harness, and driver side harness pass through and repin c256?


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 Post subject: Re: LG3 to LN3 swap?
PostPosted: Fri May 19, 2017 10:27 am 
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Joined: Tue Aug 25, 2009 8:32 pm
Posts: 4275
Location: Dearborn, MI
Year and Trim: '93 SSEi
'05 STS-4
'97 Camaro
'97 LeSabre
IF you don't take the dash mid harness and DS passthrough THEN you would have to re-pin the C256 as well as add wires to the existing DS passthrough and change over some connectors under the hood.

C256 is the connector under the dash right at the computer that connects the PS passthrough harness (engine harness) to the dash mid harness (I'm making these names up based on where they are in the car).

If you take the dash mid harness and the DS passthrough my recollection from the Buick manuals is that all you have to do is plug and play - unless you have different gauge clusters. Each gauge cluster has it's own pinout, so the connector between the dash harness and the dash mid harness would need a few pins swapped around.

The body harness should be the same if they have the same trim levels; with the buick the pinout was the same as the base model (added features just added wires to empty slots in the main connectors), but again, you would have to verify with the Bonneville chassis service manuals. I would not trust that the wiring was done exactly the same between the Buick and Pontiac H-body cars.

_________________
I don't know how much boost it takes to blow the tires off. But I know how much I'm going to use!
Image

Runs:
'93 Pontiac Bonneville SSEi - Twincharged, manual, racecar! Build thread
'97 Chevy Camaro - 3800, T5, T-tops, my DD
'05 Cadillac STS - V8, AWD, her DD
'89 Buick LeSabre Limited - winter DD
'84 GMC Sierra Classic - Twin turbo 3800
'97 Buick LeSabre - L67 project
'92 Pontiac Bonneville SSE future project

Doesn't run:
'87 Buick LeSabre T-type - future victim
'67 Buick LeSabre - future victim


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 Post subject: Re: LG3 to LN3 swap?
PostPosted: Fri May 19, 2017 9:42 pm 
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SE Member

Joined: Fri Nov 23, 2012 1:46 am
Posts: 10
Location: Hammond, IN
Year and Trim: 1987 LE w/SE Option
1990 SSE
1996 Z-28
They do not have the same clusters, the 90 has the dic and a compass and the 87 doesn't, I want to keep the 87 gauges. So add some connectors under the hood and repin the c256?


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 Post subject: Re: LG3 to LN3 swap?
PostPosted: Mon May 22, 2017 3:52 pm 
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Certified Bonneville Nut
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Joined: Tue Aug 25, 2009 8:32 pm
Posts: 4275
Location: Dearborn, MI
Year and Trim: '93 SSEi
'05 STS-4
'97 Camaro
'97 LeSabre
To keep your cluster re-pin the connector between the dash harness and the mid harness, not the C256. I really recommend getting the CSMs for both cars to make sure you get it right, and verify that the wiring pinout is the same between any other harnesses that connect to the parts from the donor.

_________________
I don't know how much boost it takes to blow the tires off. But I know how much I'm going to use!
Image

Runs:
'93 Pontiac Bonneville SSEi - Twincharged, manual, racecar! Build thread
'97 Chevy Camaro - 3800, T5, T-tops, my DD
'05 Cadillac STS - V8, AWD, her DD
'89 Buick LeSabre Limited - winter DD
'84 GMC Sierra Classic - Twin turbo 3800
'97 Buick LeSabre - L67 project
'92 Pontiac Bonneville SSE future project

Doesn't run:
'87 Buick LeSabre T-type - future victim
'67 Buick LeSabre - future victim


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 Post subject: Re: LG3 to LN3 swap?
PostPosted: Tue Jun 27, 2017 8:08 pm 
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SE Member
SE Member

Joined: Fri Nov 23, 2012 1:46 am
Posts: 10
Location: Hammond, IN
Year and Trim: 1987 LE w/SE Option
1990 SSE
1996 Z-28
Okay I'm still A little new to all this wiring/electrical stuff. I will get the csm' s for the 87 and the 90 they aren't the same trim level. To keep the cluster repin the dash and mid harness; take all engine bay related wiring from donor and rewire up to dash/mid harness is what I'm understanding from all of this. Are factory service manual the same as chassis service manuals? Both factory service manuals have what appears to be full wiring schematics in them. Thanks for all the help and advice.


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 Post subject: Re: LG3 to LN3 swap?
PostPosted: Wed Jun 28, 2017 10:23 am 
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Certified Bonneville Nut
Certified Bonneville Nut
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Joined: Tue Aug 25, 2009 8:32 pm
Posts: 4275
Location: Dearborn, MI
Year and Trim: '93 SSEi
'05 STS-4
'97 Camaro
'97 LeSabre
Yes Chassis Service Manual is the same thing as Factory Service Manual.

Once you have those in hand you can verify that all trim level differences (HVAC controls, gauge cluster, fuel pump and security, etc) are accounted for and connected/re-pinned as needed.

_________________
I don't know how much boost it takes to blow the tires off. But I know how much I'm going to use!
Image

Runs:
'93 Pontiac Bonneville SSEi - Twincharged, manual, racecar! Build thread
'97 Chevy Camaro - 3800, T5, T-tops, my DD
'05 Cadillac STS - V8, AWD, her DD
'89 Buick LeSabre Limited - winter DD
'84 GMC Sierra Classic - Twin turbo 3800
'97 Buick LeSabre - L67 project
'92 Pontiac Bonneville SSE future project

Doesn't run:
'87 Buick LeSabre T-type - future victim
'67 Buick LeSabre - future victim


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 Profile  
 
 Post subject: Re: LG3 to LN3 swap?
PostPosted: Thu Jun 29, 2017 12:42 pm 
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SE Member
SE Member

Joined: Fri Nov 23, 2012 1:46 am
Posts: 10
Location: Hammond, IN
Year and Trim: 1987 LE w/SE Option
1990 SSE
1996 Z-28
Awesome! I do have them for both cars.


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