ENGINE SWAP GUIDE - ELECTRICAL                          HT4100 to Olds

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Gauges & Warning Lights

New MPG Display

Cruise Control

Torque Converter Lockup

Climate Control

Speedometer Conversion (Digital to Analog)

HT4100 Circuits and Bulkhead Connections

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Nailed It!


As mentioned previously, one of the challenges of a 4100 swap-out is that some of the systems (Cruise, Warning Lights, Climate Control, etc.) are integrated into the ECM. Armed with the factory service lit and troubleshooting diagrams, as well as some real-world experience on these cars, I've put together some easy-to-follow solutions for each. Personally, I like to re-use as much factory wiring as I can, use solder and shrink tubing, and attempt to make the modifications look like it rolled off the assembly line that way. But, if you prefer a more 'quick and dirty' approach, there are some options for that as well.

Assuming you're converting to a carb, DO remove the fuse and relay for the fuel pump circuit which are marked on the accessory fuse block under the driver side of the dash. DO remove the two 3 amp fuel injector fuses. The in-tank pump operates at around 14PSI and will flood your carb. A mechanical pump will pull through it fine.

The original HT4100 made use of an engine harness utilizing "two" firewall connectors. One is the 6-sided “environmental bulkhead” connector on the center of the firewall and contains mostly low current, signal level I/O to sensors and actuators. The other is a square-looking center bulkhead connector (immediately adjacent to the enviro conn) which contains the more conventional circuits, like the alternator feedback/warning light, AC compressor, starter solenoid wire, etc. Really it's two halves that share a center mounting screw; one half for the engine, the other half for engine bay components. For engine removal, leave all the harnesses in place and just disconnect these two connectors.

Once the engine is out, whether on hook or on stand, go ahead and disconnect all of these engine connections, being careful not to break their clips. Retain the harness for your new engine. Resist the urge to cut any wires at this time.

I highly recommend adding oil pressure and temp gauges so you can monitor your new engine, but retain the warning lights to grab your attention if needed. Decent quality gauges are expensive, and rarely look good screwed under the dash. Should you want something stock-looking, functional, and with features you can't get in the aftermarket, I've developed a digital display unit designed specifically for 1982-85 Eldos and Sevilles.

  • Oil Pressure Light:

    The oil pressure switch on the 4100 has three terminals. One grounds out the light in the instrument cluster, the other two close a circuit that powers the fuel pump as long as oil pressure is present. We only care about the light ckt, so clip the Tan wire (Ckt 31 / 'BZ' on the square bulkhead connector) at the pressure switch and re-route this to your new oil pressure switch. On the Olds, this is typically at the top front of the engine just above and behind the waterpump. A switch from a carb'd Toro or Riv (without gauge package) will work fine and are cheap. If also running a pressure gauge, use a brass tee and pipe nipple to share this port. The other two unused wires can be clipped off anywhere it's convenient, preferably closer to the firewall as they still carry power, unless you're planning to reuse them for other purposes (like a gauge?). I like to stagger the cuts and tape ends to prevent shorting, and hide my cuts in the protective convoluted tubing for a cleaner look.

  • No Charge / Batt Indicator:

You really don't need to do anything different here. This warning light is used as part of the alternator excitation circuit, which is pretty standard for most OEM charging systems. If you plug in the 2-pin flat-blade connector to your new alternator exactly like the original was (or just bolt the 4100's alternator in), this light will work correctly and your battery will charge properly. However, you may need to extend this wiring if your new alt/bracket package is out of reach.  Same goes for the +12v charge wire from the alt to the batt. Play it safe here...that batt wire is unfused.

  • Service Soon / Service Now Indicators:

    If you've decided to remove the ECM entirely, it's a non-issue. If you're keeping the ECM (and there are some good reasons to, see below), simply remove the dash bulbs or cut the respective wires at the ECM and you're good (diagrams below).

    For instructions on removing the speedometer, click here.

  • Coolant Temperature Warning Light:

Even with an aftermarket temp gauge, there are good reasons for keeping your Coolant Temp light, especially with an analog dash where it'll grab your attention. Below are several options.

  • The 4100's ECU not only uses the engine block mounted temperature sensor to adjust fuel mixture and to delay turning on the Climate Control's fan until coolant is warm, it also drives the Coolant Temp light which will come on at 269F. I seem to recall mine would even flash to get my attention.

You could keep the 4100's temp sensor and wiring and screw it into an open coolant port; the ECU doesn't care what engine is sitting under that sensor, so long as it's connected. But it's important that temp sensors are in flowing coolant, which rules out the rear heater port location on an Olds intake. Plus 269F is a little hot for my liking. Be sure the 4100's harness ground eyelets get connected to the engine.

  • If you don't like the above solution, take a page from the Riv/Toro/Diesel/V6 cars. Mount a coolant temp switch on the engine (like a TS43T - 1/2" NPT), and wire this directly to your Coolant Temp light. When the switch sees an over-temp situation, it completes a ground to the engine block. The other side of the dash light already has switched 12V on it, and you can even re-use one of the (now spare) engine harness wires to connect to your switch. So which dash circuit is the Coolant Temp light? Depends.....

If you have a digital speedometer, then the Coolant Temp lamp is in the left “Info Center” panel. You would connect to the dark green wire at position 17 of the left side IC connector and it should light up.

If you have an analog cluster, then the light is in the top right of the speedo face. Grounding/connecting to the dark green wire at position 12 of the connector behind the right side of the cluster should light this up. These clusters omit the bulb in the left info center.

In either case, simply trace this wire back to a convenient place to cut and splice to your new temp switch under the dash. The diagrams below can assist also see the Docs Page for additional diagrams. If you want to make use of the old sensor wire to re-route to your new switch, it's yellow wire ckt 410, which goes through 'D' in the environmental connector; clip it under the dash and wire it up.

Image from the Speedometer Conversion Page.
Positions 12(R) and 17(L) are circled. Ignore rest of the markups.

Dashboard backshell showing the Left and Right
Connectors (as well as digital speedometer pigtail)

And as Columbo says, just one more thing....If you'd like your Coolant Temp light to come on as a bulb check when cranking, the way it originally did, run a wire from position 35 of the ignition switch (this should be an empty terminal) to the side of the bulb you've connected the new temp switch to. When you turn the key, it grounds out 35 and your bulb should light.

Use Ckt 35 for your bulb check. Should be empty on HT4100 vehicles,
image above from diesel/V6 cars. Don't tap into an existing ground terminal
or you'll backfeed your warning lights.

Pigtail & 194 bulb from Fuel Data backlight

  • I hesitate to even mention this...but if you'd rather not deal with any existing wiring, scavenge a bulb/pigtail from a spare Fuel Data backlight, provide one side with switched 12V, and run the other side to your new engine temp switch. It'll twist and lock right into the back of your speedometer housing where the old Coolant Temp lightbulb was, and is plastic-bodied so it's insulated from the speedo mylar circuit traces.

Stop Engine – Temperature:

What the heck is this?! This light is driven by a metallic switch that threads into the block and closes at 320F! It also activates a warning chime, but lets face it, if this is going off, the 4100 is already a molten puddle of aluminum. You may have noticed that if the keys are in the ignition and the driver door is opened, this light illuminates. That's because they share the same warning chime. If you want to maintain this functionality, find a location on the new engine block to screw the metal temp switch into and retain that circuit. Otherwise, you can leave it disconnected and pretend you're driving a Riviera.


Climate Control:
For parts interchangeability and the unnecessary "nuts and bolts", see the Tech Section

The Climate Control Head (panel with controls and display) primarily does the heavy lifting in the system. It works with the blower module on the heater box to control blower speed, it works with the AC programmer under the passenger side dash to modify temp and direct air to where it needs to go, and it receives some key data from the 4100's engine ECM. (Technically, it receives all data on the data stream so it can show diagnostics when asked to.)

There are two pieces of data that are important for the climate control: 1. Outside Temperature. 2. Coolant Temperature.

Outside temp is obvious, but why coolant temp? It's because the CC Head needs to know if the engine is warmed up or not before it switches on the blower when Heat is called for, so it's not blowing cold air on your feet.

So, for a functioning Climate Control, you have 3 options:

Option Impact
1. Leave ECM in place & powered, as well as moving the 4100's coolant temp sensor over to the new engine.

Pro: All systems continue to work as designed (Outside Temp, Blower for Heat, Coolant Temp light).
Con: You may not have an available coolant sensor port on your intake, especially if using an aftermarket gauge.

2. Leave ECM in place and powered, but don't bother with moving the coolant temp sensor/wiring.
     Note: This is the preferred option, IMO.
Pro: Outside Temp works.
Neutral: Blower will work, but you have to tap the Defrost button first as an override.
Con: Coolant Temp warning light no longer functions.
See above for solutions.
3. Remove and/or Disconnect the ECM.

Pro: CC Head realizes there's no data stream, defaults to a situation where blower actually works just fine. A call for heat results in instant blower operation.
Con: Outside temp button no longer works.
Con: Coolant Temp warning light no longer functions.
See above for solutions.

Note: Should you choose this option, do not connect the engine bay ground eyelets detailed below.

Please Note: The ECM gets its sensor ground through the environmental connector (cavity "N" & 'V' ckt 450) to a ring terminal that grounds to the engine. I know you'd like to dispense with that harness entirely, but you need to ground this ring terminal to make the ECM happy for options 1 & 2 above. If you don't, the data stream to the CC head will be down and it'll act like option 3: no outside temp, no delayed blower, no functioning coolant temp light (if you kept the original sensor).

There are two ground eyelets, each with a pair of wires crimped in. Get them to a reliable ground stud and all will be well.


Circuits & Such

Colors and terminal locations provided below for the circuits worth keeping, and those you can dispense with. You may want to retain a few of these for future circuits (like gauges). If cutting off, stagger your cuts to avoid shorts, and leave a little length so you can hide these wires in the convoluted tubing. Shrink tube or elec tape on wire ends is a good idea, too.

It's possible colors may vary year-to-year, but I'd expect connector positions to be reliable. Info below from the 1984 Diagrams.

On the Environmental Connector: No Longer Needed

M – (ppl): O2 Sensor

Q – (tan): Manifold Air Temp

P - (gray): TPS

S – (drk blue): Test Point

F (blk), G (Ppl/wht), H (tan), B (wht) - HEI Connector (for ECM timing)

C – (pnk): ISC

L (wht), O (lt grn), J (red), K (lt blue) - Injectors

A (grn/yel): Canister Purge Valve

R (lt blue): Fuel Pump Prime Connector

On the Environmental Connector: KEEP …. if retaining the ECM. If not keeping the ECM, you can also omit the following, using none of the Enviro Conn circuits.

I (lt grn): Outside Temp Sensor

U (blk): Outside Temp Sensor Return

N (tan): Ground Eyelet (ground to engine)

D (yel): Coolant Temp Sensor

T (blk): Colant Temp Sensor Return

V (blk): Ground Eyelet (ground to engine)

On the Center Bulkhead Connector Half: No Longer Needed (see image & caveat below)

Note circuit numbers are given; see following chart to convert to terminal position

333 (tan): Brake Low-Vacuum Switch

120 (lt blue): Oil Pressure switch signal for Fuel Pump

435 (brn): EGR

436 (brn) AIR Valve

426 (dk blue): ISC

425 (lt blue): ISC

429 (blk/pnk): AIR Valve

639 (pnk/blk): Common for AIR valves, Canister Purge, EGR


On the Center Bulkhead Connector Half: KEEP (see image & caveat below)

420 (ppl): Transmission Connector Ckt “A”

422 (tan/blk): Transmission Connector Ckt “D”

204 (dk blue): AC Clutch

402 (grn): Cruise Servo Power Solenoid (ground return is local to block)

403 (dk blue/wht): Cruise Servo Vac Solenoid

86 (brn): Cruise Servo Vac Solenoid Return

6 (ppl): Starter Solenoid

438 (dk grn/wht): Transmission Connector Ckt “B”

3 (pnk): HEI 12V+

37 (lt grn/blk): Engine Metal Temp Switch (for warning light)

450 (blk/wht): 2 wires- one to ground eyelet which should be grounded to engine. Other to ISC: can clip this one out if desired.

31 (tan): Oil Pressure Warning Light

25 (brn): Alternator light (required for alternator to function)


The center bulkhead connector terminals are arranged in an X-Y format
with numbers down and letters across.

This image is from the 84 Electrical Guide and matches how my '85 is built.
It does NOT appear to match the '83 shop manual, which shows a different
style connector. Either '83 (and '82?) is a different style, or the manual is
in error. Click Here for that diagram.

Below are the terminal positions relative to circuit numbers.
These should be correct, but double-check on your car before hacking away.

333: 6H
420: 5H

120: 4H
422: 6G
204: 5G
435: 6F
436: 5F
426: 4F
402: 5E
6: 4E
438: 5D
3: 4D
37: 6C
403: 5C
86: 4C
425: 6B
450: 4B
429: 7A
31: 6A
639: 5A
25: 4A

1984/1985 Center Bulkhead Connector - Emphasis on Lower Half

Note: If you decided not to retain the ECM, I also recommend NOT connecting any of the environmental connector grounds (no enviro ckts at all, actually). If you do, your Climate Control will wait for an “engine warm” signal that's never coming, and require you to hit 'Defrost' first as an override for the blower motor to work. Thereafter, it should operate correctly until the next key cycle. If you don't connect the grounds, it knows there's an issue, and will work without waiting for ECM input. Of course without an ECM, outside temp won't function.

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