Cruise Control

Cruise Control                                                           HT4100 Conversion

This page is primarily intended to assist with restoring cruise control after an engine swap on 1982-85 Eldo (and Seville) HT4100 Cadillacs.
However, it should also prove useful for CC troubleshooting on all '79-85 Eldos. Info on Riv/Toro CC systems is at the bottom of the page.



All HT4100 Eldorados and Sevilles have their cruise control operated by the ECM. Now, if a Service Soon/Now light is triggered, cruise will no longer work. Even with the ECM in place and powered, removing the 4.1 won't make it happy. Though there are a few caveats, the trick to restoring cruise is to pick up a compatible control module used on the V6 / Diesel / '77-80 cars.

Starting in roughly 1977, Cadillac transitioned to fully electronic cruise control systems, away from the electro-mechanical Cruisemaster transducer system that was introduced 10 years earlier. Using an electronic module and a photo-transistor speed sensor from the speedometer, maintenance and wear-out were eliminated, and features like Accel/Decel, memory, and tap-up/tap-down were added.

The '79 Eldo cruise system [K30] is the last year for this original style setup, and uses a 3-position dash switch and unique cruise module. In the middle “Cruise” position, the system behaved like the old style CC setups. Hit the set button on the turn stalk and cruise would be set. When you flipped the switch up to “Auto”, it then gave you “memory” functions, like one-touch resume. This system was found on all 77-79 Cads, making it a '1-year-only' design on the downsized Eldo.

In 1980, the Eldo and Seville went DEFI with the 368, while the other Cads were still carb'd. According to my '80 Electrical Manual, all Cads, whether DEFI or carb'd, still used a standalone cruise module (different p/n). CA-Spec 80's had an Olds 350 which also used a module, as did the diesels. DEFI cars did use the speed sensor as an ECM input, but cruise was still independent.

In 1981, all the Cads picked up the 368 V864 arrangement, and the cruise functions were moved to the ECM where they would remain through 1985. Non-DEFI Diesel and V6 cars still had a module.


How it works:

This cruise module takes a pulse input from a photo-diode on the back of the speedometer and actuates a pair of solenoids (power & vent) on the cruise vacuum servo under the hood. One applies vac to accel, the other vents to decel. The nice thing is that the 4100 Eldo already has the speed sensor on the back of the speedometer cluster, and a servo with the solenoids, you just need to add a controller. The 'gotcha', is that 1979 & 80 turn signal stalks have cruise switches that are normally-closed. 1981-85 use the normally-open type. This means you either need to find a controller that that'll play nice with your switches, or swap the stalk, which is a fairly quick job if you find the right color match.

The dash control switch is compatible 1980-'85, the only trick is rewiring these inputs from the ECM to the new module; wiring did vary by year.

On non-DEFI cars (diesel, V6, Olds 350 with analog injection) with an analog speedometer, the photo-diode was wired right to the cruise module. But if you ordered a digital speedometer, a buffer/amplifer module was added to condition the signal so it could drive both the speedometer and the cruise module. This buffer/amp is also present on '80-85 DEFI cars to condition the signal for the ECM, regardless of speedometer type. 

An '82-85 Eldo with an analog speedometer will no longer need the buffer amp since the ECM doesn't need to know speed, but there's no harm in keeping and using it. If you have a digital speedometer, then I do recommend keeping it as part of the circuit.

Not all buffer/amps are the same. Since there's no real reason to seek one out, the below table is more for curiosity's sake than anything else.

1980 w/368 DEFI (no digital speedo that year) - (Used with NC set/accel switches) 25030820
1981 w/368 & Digital Speedometer or Diesel & Digital Speedometer 25031080
1981 w/368 & Analog Speedometer 25007215
1982-85 w/HT4100 & Digital Speedometer or Diesel & Digital Speedometer or V6 25007285
1982-85 w/HT4100 & Analog Speedometer 25007260

Buffer/Amp on Eldorado w/Digital Speedo (82-85)
Located next to ALDL Connector

Buffer/Amp on Toronado Digital Speedometer (1985)

Note, there was actually a little-known service bulletin released to address erratic cruise control on diesels with digital speedometers.
Click Here to see the wiring modification


Where to track down this cruise module?

The below usage is pulled from the Cad Parts Catalogs. The preferred option is the 1981+ Diesel/V6 Module; I've used it in the past and can guarantee it works. But realistically, any of these should do the trick as long as you've got a compatible stalk. At least one other source indicates the red module was used in '80 with the 2-position switch, so it can be made to work, and appears to be much easier to find. Wiring Diagrams are in the next section.

On the Eldo, the cruise module is visible/accessed by removing the headlight switch, then unscrewing two 7mm screws directly from below. It'll then snake out the dash opening.

25030620: 1977-79 C,D,E,K; all engines (red module)
Requires 1979/1980 turn signal stalk
25031299: 1980 C,D,E,K; all engines (blue-gray module)
Requires 1979/1980 turn signal stalk.
25031303: 1981 C,E,K; non-DEFI cars (yellow module)
All modules 1981-up work with '82-85 HT4100 factory stalk.
1546747: 82-85 C,D,E,K; non-DEFI cars (yellow or white module)
       Personally confirmed an '83 Eldo diesel is white.
Example of Eldorado '79-Only "3-position" Cruise Switch: p/n 1614813
1980-1985 Eldos & Sevilles use the same "2-position" dash switch: p/n 1617627



Don't let the wiring intimidate you, the goal is to modify your existing '82-85 4100 wiring to match that of the particular module you happen to want to use. This means re-directing the speed signal to your module rather than the ECM, and re-directing the power and vent solenoid wires from the ECM, to your module. The dash switch wiring will also need to be modified depending which one you're got. I've summarized the differences below. Also, each diagram is clickable for a larger version which can be printed and marked up as you see fit. Please note: Check wire colors and terminal locations carefully, as they did change over time.

1979: As mentioned prior, this uses a red module with 7-wires, plus the 3 to the speed sensor. It's intended for a 3-position dash switch. There's no reason you couldn't swap the 3-position switch in and follow the factory 79 wiring diagram. (also see '80 diesel)
1980 Diesel / Olds 350 CA: This module is also 7-wires, plus the 3 for the speed sensor, but it uses the 2-position dash switch all HT4100 cars have. Aside from the switch, the wiring appears identical to '79, which makes me suspect one could substitute the more common red module and use the '80 wiring diagram.
1980 DEFI: Same as the 80 Diesel/350 except a buffer/amp is involved; also note the speed sensor connection to the ECM. For a 4100 conversion, the '80 DEFI isn't very useful.
1981 Diesel / V6: Note that modules from '81+ use 8-wires plus the 3 for the speed sensor. This is a good candidate for a 4100 conversion.
1982-85 Diesel/V6 w/Digital Speedo: If you have a digital speedometer, use this one, just reference the appropriate 4100 drawing below as your bulkhead connector may be different.
1982-85 Diesel/V6 w/Analog Speedo: If you have an analog speedometer, use this one, just reference the appropriate 4100 drawing below as your bulkhead connector may be different. No need to use the buffer/amp on your conversion if you don't want to.


These are the HT4100 'on-car' diagrams you'll use to interface to your existing wiring. Please note that the bulkhead/interconnect terminal positions on the V6/Diesel diagrams above are mostly different than the HT4100 locations below.
1982-85 HT4100 w/Digital Speedo: Re-route circuit 961 from the ECM to your module speed input; leave the buffer/amp in place and don't mess with ckt 994.
1982-85 HT4100 w/Analog Speedo: Re-route circuit 961 from the ECM to your module speed input. You could also drop the buffer/amp entirely and wire the speed sensor directly to your new cruise module.
1982-85 HT4100 Servo Solenoids: Here's where to pick up the circuits for the power and vent solenoids; clip the wires under the dash and re-route to your cruise module. This image is of the orange ECM connector showing ckts 402 & 403 for the two solenoids. You can also grab these ckts on the inside of the firewall bulkhead connector if it's easier (images below).
This diagram is taken from the 1983 Cadillac Service Manual: Eldo/Seville w/DEFI (4100). Circuits 402 and 403 activate the servo solenoids, and the return sides of the two solenoids are tied together and return as circuit 919 to the ECM. You'll want to disconnect 919 from the ECM (clip the wire) and ground it as shown in the non-DEFI diagrams above.
This diagram is taken from the loose-leaf 1984 Cadillac Chassis Wiring diagrams for the 4100 Eldo/Seville and shows a different style bulkhead connector with a different ground/return arrangement (ckt 86). Also note the connector style for the solenoids. If you have this arrangement, you'll want to make sure ckts 86 and 151 are grounded.
And for good measure, an overview of the whole 4100 cruise system, dated Jan 1983.


Incompatible Cruise Systems:

You might think a Riv or Toro would be a good donor, but unfortunately, no. Only the Cads had a fully electronic cruise in the early 80's.

Cruise Systems used on the Toros and Rivs. These are NOT compatible.

  • 1979/80 Toro: On/Off dash switch and old style 1-button stalk. Mechanical system.
  • 1981-83 Toro: New turn signal stalk with on/off & resume integrated. Still a mechanical system.
  • 1984/85 Cruisemaster III style. Now electronic, but would need entire system...turn signal stalk, module, servo/bracket, vac reservoir, and harness ends to make up a harness if integrated on vehicle. Not recommended.
  • 1979/80 Riv: No dash control. 1-button stalk. Mechanical system.
  • 1981: Updated stalk like Eldo (w/resume), but no dash control. Mechanical System.
  • 1982/83: Cruisemaster style stalk with integral wiper controls.
  • 1984/85: Cruisemaster III style. Now electronic, but would need entire system...turn signal stalk, module, servo/bracket, vac reservoir, and harness ends to make up a harness if integrated on vehicle. Not recommended.

  • Note 1: “Custom Cruise III” debuted in '84 on many GM cars and these have the black, barrel-shaped, 'corporate' turn signal stalks with on/off/resume and integrated wiper controls. This system is electronic, but use a special servo that has a feedback signal. You'll see these everywhere as they have 5 pins and a wire harness attached. There's a possibility you could use one of these servos and ignore the feedback circuit, but I personally haven't tried it. The CC3 module itself is totally different than the Cad modules.
  • Note 2: While the post-85 RWD Cads continued to use a dash-mounted switch and a turn signal stalk that looks like the 80-85 cars, one might think these would be a good donor for a module. However, the underhood servo looks like a CC3 style, and the Cruisemaster manual suggests the stalk switches are 'normally open', not 'normally closed'. Just the opposite of what you need. Until I can get some actual hardware in front of me, don't count on those cars as donors.

Additional References:
1. 1968-1982 Cruisemaster - See Chapter 6.
2. 1984 AC Delco Custom Cruise III PDF.

Return to Main Page     Corrections? Additions? Contact Webmaster Cory Heisterkamp 2022