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The HT4100 Swap Guide for the Eldorado


The Touring Coupe is on the road and has successfully completed several hundred miles (update: thousands of miles) of interstate and in town travel without a single hiccup! View the glamour shots, interior pic’s and under hood shots here!  


This page is designed to help those folks who are looking for more power & reliability in their 82-85 Eldorado’s. It's divided into 4 sections: The Basics (parts required and where to find them), The THM 325-4L, 4100 Removal, and 350/403 Installation (with tips).
-The Cruise Control & 4th gear lockup sections are also very helpful

If you've ever driven a HT4100 powered FWD Cadillac then you know how smooth these engines can be, you also know how underpowered they are. I'm planning to swap out a frozen 4100 from my Touring Coupe, and drop in an Olds 350 with port fuel injection. For those who are interested in a similar swap, this may be of some help..... 

Here Are The Basics:

 The HT4100 engine / drivetrain and parts are the same for all 1982-85 Cadillac Eldorado’s and Sevilles. The '80 and '81 Eldo’s and Sevilles use a unique digital fuel injected 368. Pre-82's used a THM 325 without a lockup convertor.

If you're looking to add FI, this may be of some help. The 1975-1979 Seville’s (and '79 Eldo) all use an analog electronic port fuel injection on an Olds 350 block. The FI system is made up of a fuel rail, 8 injectors, a modified intake manifold and dry throttle body. There's also a unique distributor that contains a couple magnetic switches, and of course the computer and temp sensors that screw into the intake. These parts can be swapped right on to any Olds 350/403 in place of the regular intake and carb if you want to keep some form of Fuel Injection.

View a technical article about the FI     (scans copyright of Joe Rakes)



The Seville Fuel Rail and Injectors

  Cleaning the injectors with an Apple II!

If you swap out the 4100, you lose the digital readout for MPG/Average. I'm planning on leaving the original computer (ECU) connected since it is also part of the climate control system. I'm just removing the un-needed sensors and wiring under the hood that go to the original ECU. More about wiring at the bottom of the page.

Going with the Q-Jet? Better pull the fuel pump's fuse and relay. The engine mounted mechanical pump will pull enough fuel.

Since the '79 Eldo is the same body/frame as the 82-85, and since it originally came with a 350 Olds, the 350 will drop in if you have the necessary parts. Here's what I can tell you:


If you can find any one of these cars below with the 350, you have everything you need. Otherwise, any Olds 350 block plus any 307 car below has everything you need.

(Excluding diesels)                     


 Riv: 231/350

 Toro: 350

 Eldo: 350FI


 Riv: 231/307/350

 Toro: 307/350

 Eldo: 368DFI


 Riv: 231/252/307

 Toro: 252/307

 Eldo: 368 V864


Riv: 231/252/307 

 Toro: 252/307  

 Eldo: 4100


 Riv: 231/252/307

 Toro: 252/307

 Eldo: 4100


 Riv: 252/307  

 Toro: 252/307

 Eldo: 4100


 Riv: 252/307

 Toro: 307

 Eldo: 4100


The reason you need the 307/350 car is that to make the 350 fit, you need the related front wheel drive parts that only the above cars have.

So far, the FWD 307/350 parts you need are:

Oil Pan
Oil Pickup/Pump
Oil filter mount
Flywheel (it's specific to FWD if you're using a RWD configured engine)
Front engine mounting brackets (watch out, read below)
Diff support bracket
307/350 Passenger output shaft/bracket
Exhaust manifolds

With all of this, the 'new' 350 should mate right up to the THM325 4 speed transmission found in all 82-85 Eldorado’s. The other thing to consider is that the 4100's ECU controlled the overdrive. Diesels controlled the lockup convertor with a temp switch and a vacuum switch. Locate the necessary parts from a diesel car and all is well. Without a TPS and other inputs, the ECU will not activate the TCC. A toggle switch can be added, but a latching relay system is much better. See bottom of page for details.

Regarding the climate control, it turns out that my climate system works perfectly with both 4100 temp sensors removed. Pressing defrost turns the fan on, then I'm able to select other buttons. Temperature is still regulated exactly as before. Be warned, idiot lights will no longer work properly so gauges are in order (and are good insurance too). If you want to mount the 4100 sensors, I'm told the threads are not metric after all. They are not simple open/close switches like the 350's. To play it safe, and since the factory warning lamps are impossible to see in daylight, pick up a couple gauges as well. 

The Details:

For success, I highly recommend picking up a parts car and swap engines directly. It will save a lot of time and trouble. If that's not possible, here are the parts to locate individually.

* A deep sump oil pan is necessary to clear the half shafts. They're available on any 307/350 in the Toro's/Riv's and '79 Eldos. California spec Eldos boasted the 350 as well.

* An oil filter mount from one of the above motors is needed as well to clear the frame. There are some with oil cooler line ports, which can be used with the 4100's radiator, and some without. It's your choice if you want the cooler.

* When getting the oil pan, grab the pump & pickup as well, you need it to fit the new pan.

* If the engine you're putting in is from a RWD setup, you may need a smaller flywheel to fit the FWD trannies and starters. See below about how to get one.

* On the driver's side of the engine, there' s a small L-shaped bracket bolting the differential to the engine block. You'll need this from the donor car (the 4100's is slightly different) See pictures of installation.

* The front of the 4100 is supported by a pair of L's that form a U-shape. They bolt to the sides of the engine block and mount at the front cross member. (see photo). These don't fit the Olds motors. What's needed is an '82 or later U shaped, one piece bracket, from an Olds or Riv. Earlier Olds' brackets (2 piece) will fit the engine block, but the mounts won't line up with the holes in the frame's cross member. If you can't get an '82 or later one-piece U, then the earlier setup will work if you drill the cross member.

* Obviously FWD manifolds are needed

* And finally, the passenger side output shaft has a bracket that mounts to the side of the engine block to support it. This bracket is slightly different for the 4100's so you'll need an Olds bracket to make it work. It's easier to just grab an Olds output shaft with bracket attached then to mess with undoing the bracket. (see picture under installation)


The 325-4L

Another thing to consider is the amount of torque the 325-4L can handle. It turns out that I'm swapping in an Olds 403 since the 350 I was planning on going with was sold to someone else. For me, a shift kit will be necessary and a full throttle shift would probably be the demise of the tranny. An earlier THM 325 w/out OD would be more robust and should be available in 79-81 E cars.

To activate the torque convertor you may want to grab the equipment off of a diesel car that was used in place of the computers on the 4100 models or see below for details.

Shift Linkage- The 4100 used a series of linkages from the column to the tranny. The Olds' used a cable. You can either drop the column and transfer the parts from the Olds to the Cad so the cable will connect, or modify the Cad's linkage mounting point that originally attached to the 4100's exhaust manifold. Or, locate an Eldo with a 350 diesel and use that shift bracket.


Step 1: Remove the usual interferences- fan and shroud, vacuum lines and hoses, power steering lines, electrical wiring, overflow tank, cruise servo and pollution equipment near the passenger side firewall. Pretty much everything that looks like it will be a problem. Also set the A/C compressor on the inner fender without disturbing the lines.

Step 2: To pull out the transmission and engine as a whole, unbolt the front engine mounts from the cross member. See the diagram below- remove the 90Nm bolts. Also, undo the driver side exhaust flange. The passenger side flange is difficult to get to. I found it easier to unbolt the whole manifold. There are also oil cooler lines to disconnect, and transmission cooler lines to disconnect. See below...

The 4100 ready to be pulled.

This pict is supposed to show the unbolted exhaust manifold. Also in this area are trans cooler lines. Disconnect them from the trans and remove an Air tube attached to the back of the block. It runs down to the cat and makes things a hassle.

After fighting the transmission mounts, the 4100 started to give. More about that below!

The arrow points to a pair of oil cooler lines, disconnect them at the radiator as well as block. Set aside for later if necessary.

The starter wires were attached to a bracket on the lower engine block passenger side (attached to output shaft). This is accessable once the engine is partially out. Just pull the output shaft out of the diff.

Finally success! The HT4100 sits in the driveway.

Notice the driver side exhaust pipe that interferes with engine removal, it decided to rest on the transmission electrical connector before being 'persuaded'. Top arrow points to where the Air tube was located. Left arrow shows exhaust manifold still attached to pipe, pushed out of the way.

Even though the inner bolt holes will mount to our 325-4L, the flywheel is too large for the starter to even fit. Get yourself the proper flywheel or call National Automotive Lines @ 1-800-428-4300. Ask for part FW633. $25.95 gets you a flywheel that works-

The John Candy mobile! He drove one just like this in that movie where he and the family vacations seaside. This '77 98 provided the 403. This yard also provided the FWD parts we'd need: oil pan, pump/pickup, filter mount and exhaust manifolds. We still have some PS hose issues to deal with.

Ben dropping off the 4100 (arrow). We thought the engine would be at home with the other 4100's, theres another visible one right there!

Here's the engine freefalling as Ben pulls his truck forward. Rest well HT4100!

And after more jockying, the 403 on an engine stand, ready for a cleanup. Talk about a busy weekend-

One of the most difficult parts of removing the 4.1/325 as a package was the way the rear of the tranny was mounted. Note the U-shaped brackets on the diagram (bolt D goes through one, and arrow 3 points to the other). These will not permit the transmission to be pulled out forwards even with bolt D and the opposite one removed. There's no easy way to remove bolts A, B and C on the driver's side, so we pulled out bolt D and placed a floor jack underneath that corner of the trans. On the passenger side, the 3 bolts holding the U-bracket on were accessible so we removed them. Now it was necessary to jack up the rear corner of the transmission so the U-bracket would clear the bracket and bushing it mated to (held on with A,B,C). This was all done while the engine was being lifted and pulled forward to get the best angle. After clearing the rear brackets as the engine was lifted, it became apparent there was still an Air tube connected to the block that led to the cat. The right exhaust pipe/flange is also a bit of a problem so a pry bar helps keep it from interfering as the engine is hoisted out.

The E,K Series pict above also shows the pair of L brackets used to hold the front of the engine. Discard these in favor of the single Olds U bracket that will fit the stock rubber mounts. Or, if you're unlucky and only have the pre-82 Olds L brackets new holes will need to be drilled in the cross member.


Some things to think about....

Keep the factory ECU connected, it's used for other info for onboard operations.
Using 307 or 350 brackets allows the use of the stock AC system in place of the log style compressors found on replacement engines.
Will the cruise control work when finished? Probably not, so consider the affiliated cable cruise system from the donor car, or a factory electronic system used with the Cad's that's discussed below.
The 4100 uses an oil cooler. If you use a filter mount that doesn't provide for cooling lines, plug the threads at the radiator.
A shift kit isn't a bad idea when upgrading-
You'll lose the fuel economy data, but that's about all.
Consider installing temp and pressure gauges since the stock idiot lights can't be seen in daylight anyway!
Going the Carb route? Remove the FI pump's fuse and relay and the mechanical Olds pump will pull enough fuel from the tank.

Putting it back together:

Ben cleaning up trans fluid during shift kit installation.

 Important pieces: The FWD Oil Pickup and Pump

Important Pieces: The diff support brackets: The required Olds bracket on the left and the old 4100 bracket on the right.

This you'll need, the rakish Olds bracket to bolt to the Olds block. (Thanks Jeff for the pict and Bruce for the part)

The deep sump oil pan ready for installation.

An Olds oil filter mount without cooler lines. Needed for frame clearance.

On the left, the post '81 Olds U-bracket engine support. On the right, pre '82 Olds front engine support brackets. Either can be used, but pre '82s require cross member hole drilling for the rubber mounts to mate up.


   The 403 with FI trim, ready for installation

From here, it's a simple matter of reinstalling the engine. It's difficult to reinstall as a complete trans-engine package, and is easier if you remove the rear left and right transmission support brackets. By placing a jack under where the trans will sit, the whole package will level out when the rear makes contact with it and the front engine mounts will drop right into place in the cross member. Then it's a matter of supporting the rear of the engine with the cherry picker, removing the jack, and installing the rear trans brackets. Install the proper nuts/bolts through the brackets and the whole thing is in.

Now it should be easy.... connect the flywheel to the torque convertor and install starter. The 4100 use to have an oil pressure switch at the oil filter housing and the solenoid wire runs with them. Go ahead and connect the solenoid wire up to the starter. Two of the other three wires actually close the fuel pump circuit. Play it safe and cut them off at the firewall, connect the third oil pressure light wire to the 350's oil pressure switch.

You can also install the Olds y-pipe and exhaust manifolds. The Olds block connects to the heater core in two places. The intake feeds it and the return is at the pump, meaning that the old return on the 4100's radiator needs to be plugged.

The 307/350/403 brackets should accommodate the AC compressor, Olds power steering pump (metric fitting version), and the alternator. If you nabbed your replacement engine from an E-body then there should already be a bracket on the carb that will accept the throttle cable and trans shifting cable.

The Olds distributor can be fed from the same B+ wire as used by the 4100, the other wires are not needed now. If you've got the carb, connect the supply and return lines and make sure the in tank pump is disconnected.


More Electrical


I've eliminated the original injector wiring, pulled the respective 3 amp injector fuses, and removed the ISC wiring. Pull the fuel pump relay and fuse if running a carb. I tied back the TPS connector incase it may be needed later. The weather pack connector to the 4100 distributor can be eliminated as well. I've removed both stock temp sensors and used an on/off style temp switch from a donor car, with a new wire, running to a lamp that's in place of the old Coolant Temp light. Ah, O2 wire can be cut, as well as the air diverter valve connections. That leaves two connectors for the cruise modulator, a connector for the A/C compressor, the B+ to the dist, the connector for the alternator and a couple miscellaneous connections, such as the canister purge and outside temp sensor at the front of the car.

The ECU also used the coolant temp sensor to judge if it needed to activate the Coolant Temp light. You're better off using the simple idiot light temp switch that the Olds blocks came with (near front of intake) and wiring the terminal to a lamp. When the engine overheats, current flows from + from the ignition switch to the idiot light and to the temp switch which grounds itself to the block. I've installed such lamps in place of the originals in the INFO CENTER. If you want to get fancy, the ignition switch has a pair of ground wires. When the key is turned to START one of these wires provides ground. Run a wire from this ground source to the coolant sensor side of the idiot light. Now when the key is turned to START, the lamp will light showing if it's functional. Or buy a gauge.

For oil pressure it's easier. The oil pressure switch on the 4100's is mounted at the oil filter housing. It has three terminals, one tan wire that runs to the oil pressure indicator, and two blue wires that when connected, activate the fuel pump. Follow these blue wires to the firewall and snip. Reroute the tan wire to the front top of the Olds block where the oil pressure sensor is located. Put on a connector and presto! Turning the key to RUN should show the oil indicator lit.

The Cruise Control

 The factory ECU handled the cruise control operations but will not allow the cruise to function if a 'Service Now' code is set. Since the distributor is totally different, a Service Now code will be set, usually along with others stemming from missing sensors and wiring. Here's how to handle it- Find a diesel or non 4100 car (Buick 4.1 V6 will work too) and grab the Cruise Control Module. It will connect to all the cruise switches and circuits. All it takes is some very basic plug swapping and a few splices.

The bright yellow Cruise Module

Click here to see the cruise diagrams and what to do!


The Overdrive Lockup

Follow these steps and you can't go wrong!

B,D, and A correspond to the connections on the transmission of a 4100 car.

Cut the grn/wht and tan/wht wires right at the red connector, don't leave any wire there.

Here's how it works:
When the first LED goes out, you know you're out of first gear. This is merely for convenience. When your rpm's look good and you're at highway speeds of 55 or better, press and release the momentary pushbutton. The relay will lock up energizing the TCC and the second LED will light. Tapping the brake pedal will disengage the TCC. Piece of cake!

The Shift Linkage Bracket 

 You may be wondering how to mount your shift linkage bracket to the new Olds engine. Toronado’s and Riviera’s used a cable from the steering column to shift the trans, there's no way to connect such a cable to the Cad's column at the firewall, without disassembling the whole column and mixing parts. My recommendation to you is to find a shift bracket from an Olds equipped Cad. If you get your hands on such a bracket, you're on easy street. If not, it's possible to fabricate such a bracket. See below-

The bracket on the left is used with the Olds equipped Cads. The rubber grommet at the right of the picture is where the linkage pivots. This is actually what the bracket would look like if you were underneath it. At the top of this picture you can see the leading edge of a hole that runs east/west in this pic’s. A stud runs through there and into one of the exhaust manifold bolt holes. This effectively holds the bracket to the side of the engine. At the bottom left of the picture where you can see the end of the backwards L, there's a hole that runs north/south in the pict’s. A bolt goes through here and fastens to the rear driver's side of the engine at the edge of the head. Yes, there is a hole there. It all makes sense when you see it in person. The right picture shows the 4100's linkage. The grommet is the pivot point, and both holes fastened to the side of the engine at the old exhaust manifold.

Hopefully you've got it taken care of from here.

Good Luck!

Thanks to Bruce Roe for helpful 403 tips, he's reengineered his '79 Eldo to accept a small block Olds and THM 425.


Go ahead, if you dare!

This Swap Guide Was Originally posted on by Cory Heisterkamp. This guide was the basis for our own swap on my buddies 84 Eldo. I have seen this guide posted on many sites and since it is main reference for this type of swap I have decided to post it on my own site along with my own guide in dealing with the 368cid (little bro) swap since radar58 has been down for several months now.

-Big Block