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My 81 Caddy v8-6-4 works intermittently

2K views 19 replies 7 participants last post by  drmenard 
#1 ·
Hi All,
This is my first post on here. I have a 81 Sedan DeVille with just under 50k miles. I've attached a picture.

I've gotten the v8-6-4 to operate by grounding the wire which goes in the transmission, it's a black wire. The sensor in the transmission wasn't engaging it, but I was able to get it to work by grounding directly. It only comes on after warming up and reading about 30mph.

Lately however, this is not always working. It seems like it works more when it's warm out or when going up a hill, something reconnects. I've checked and tested my ground connection. Any ideas where to check?

I've ordered the manuals online.
No engine codes when running the diagnostic. It works manually by trying the fuel buttons in diagnostic mode.

Once I did get vehicle speed sensor, but after resetting it hasn't come back. Cruise also doesn't work, it lights up but doesn't run the throttle, perhaps related?

Thank you for your time! Any ideas are appreciated!
-ric81caddy
 
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#5 ·
Hi All thanks for the replies. ��

I just received my factory manuals, and like you guys mentioned, the temp needs to be up before system operates. The system operates manually perfectly fine in diagnostic mode, I again checked and no fault codes. With this information it has to be a sensor wiring.

I can't do too much at the moment as my right hand is in a cast due to torn ligaments in my wrist, but I did manage to find the sensor, front of engine near alternator. I wiggled the connector a little bit. It's definitely the original, it still has blue engine paint over spray on it. The wires look like they could use some attention soon.

I started up the engine cold, started right up, I'll try it today and see if it worked.
Most likely I need to replace the sensor and connector, but that will have to wait until I get both arms back.



I love my car and I enjoy learning how everything works. Thanks for the help!
 
#9 ·
I drove the car today and the v8-6-4 was cutting out again. I came home and read the way to read sensors. Unfortunately I let the car cool down, I started it up and I was getting temperature readings of only 157 degrees. I also had vehicle speed sensor failure. I'm wondering if it's causing the issue?

I will drive the car tomorrow and make sure I get temp up to 185.
I may have a loose connection for vss. Car runs great otherwise.

I'm also thinking keeping the third gear signal grounded may be causing the vss fault? Perhaps a logic error, that the third gear switch is engaged while just taking off? Just thinking :)
 
#10 ·
"vehicle speed sensor failure" would certainly interfer with 8/6/4. If I remember correctly, the speed sensor is in the speedometer (or am I mixing things up with my 1984 Cutlass?). It might simply be the VSS connector that's not properly seated. It's crowded in the area where the connector is.
 
#11 ·
Hi all, some more updates. I drove my car about 30 miles today, it only went into v8-6-4 mode intermittently a few times, mainly stayed in 8. I checked for trouble codes, none found.

I'm getting about 12-13 mpg combined. I think that's a little low. Car is driving fine.
Checked diagnostics, after driving for about 15 miles, the coolant temp is only reading 160F! The manifold temp sensor, recently replaced, was reading 168F. Diagnostics did indicate the systems were operating in closed loop.

Think this is due to a bad sensor or sticking thermostat?
I'm thinking the engine isn't warming up or thinking it's not warming up.

I didn't get any vss failure today.

Thoughts?
Thanks!
-Rick
 
#13 ·
Hi All,
I got my car back with thermostat, temp sensor, and new O2 sensor installed. The car idles much better now when warm! The car seems to be running well. Oh and guess what, it's now operating v864 in a consistent manner. Around town once hitting about 30 mph on level or downhill, the car switches to v-4. On the freeway it will go to v6 then v4 modes when it can. Overall she tries to be as thrifty as she can by going down to the lower cylinder modes. I can see why people didn't like it when it came out, the switching is kinda crude, and v6 mode vibrates some. They should have made it a "feature" that you can enable when you want more economy. When my wrist is back to normal I'll add an under-dash "eco" mode to enable on longer freeway driving.

How much mpg improvement would using this more help? Probably 1-2mpg? I didn't buy this car to save gas, but just curious. ��

I'll need to check vss still, I got another code. Everything works except cruise.

Thanks for the help! I love my car.
-Ric81caddy
 
#14 ·
Our 1981 RWD car would get the same or 1 mpg better with the modulated displacement than the 1980 Eldo (same engine without the system). The RWD was heavier than the Eldo, but had taller gears. Real world, I wouldn't say too much difference. It was mostly a gimmick that they tried to appease the EPA because their ratings are based on theoretical rather than actual mileage. Unfortunately the MPG requirements went up again in 1982 and there was no way they could get such a big engine approved without paying big $$$ in fines. Funny how it always seems to work out that way...

Enjoy it! Glad it worked out :thumbsup:
 
#17 ·
So I've been running my Caddy for a while, running well. I averaged 16.5 mpg combined city / highway driving in Los Angeles with the v8-6-4 working. I'm planning to add a switch so I can control when to use the system. I totally see why it wasn't popular when new. The switching and v-6 modes are downright silly, especially for a Cadillac owner. The engine is nice and smooth in all 8, but since I got the v864 working it likes to be in 4 cylinder as much as it can. When I'm up at 70 it's the silly v6 mode a lot of the time.

I plan on driving this car 60% of the time, my other car is a 1984 el Camino. I'd like to get opinions to just drive the car in v8 mode, or keep using v864 from a reliabilty perspective? Is using the v8-6-4 placing significant additional wear on the engine? My car has 48k miles.

Happy Saturday and thank you!
Ric81caddy
 
#18 ·
I might be ever so slightly opinionated on this subject.

I used to tell family members to buy RWD 8/6/4 cars because they were an exceptional value when a few years old. Consequentially I've worked on many over the years.

Lots of talk by people who have not a clue and just repeat stuff ruined the reputation of the Cadillac modulated displacement powerplant. It continues today when geek types chant the "not enough computer processing power back then" mantra.

Unadulterated horse dung.

If you were there, try to remember 1981 was the first year of Generous Motor's electronic engine manglement systems. Generally speaking they were a huge step backwards in reliability over a well tuned carbonator/HEI engine. Cadillac used a very different system than the rest of GM. Their DEFI eventually proved more reliable than at the onset but the engines themselves were pitiful by that time.

Gorilla mechanics made things even worse. Yes, they were called mechanics back then instead of tekk-nicians. Plenty of throttle body injector wires managed to get pinched in the air cleaner to TBI adapter and short out one injector drive. Of course that never happened until a few days after replacing a dirty air filter and it would leave the car dead somewhere on the road.

Owners who ran the gas tank down to almost empty frequently would land up on the side of the road with a burned up fuel pump.

Pretty much all the complaints about the 8/6/4 were electronic management related complaints except for the vibrating V6 mode and occasional engine ping heard when re-lighting unused cylinders.

The 8/6/4 was kind of a gimmick but it did squeeze a little more economy out for drivers who did a lot of highway driving. The national speed limit back then was 55 MPH and most people knew that they would not get away with cruising much over 60 MPH. Today's higher cruising speeds makes 20 MPG on the highway very difficult with these older cars. Same goes for older 425 powered Cadillac.

The main advantage of modern modulated displacement engines (I have a 2015 Tahoe) is advancement of intake and exhaust plumbing. A little more compuker power can't hurt but if that were true then all it would take to make an old Cadillac 368 play as well as a modern engine would be a modern ECM. Nobody will ever make that come true because there's more to it than that.

BTW, in the interest of accuracy, GIGO applies to claimed fuel economy. A dash readout may not be anywhere as accurate as the old fashioned fill it up and calculate it yourself method.

If I owned a modulated displacement 1981 model I would simply put a dash mounted S.P.S.T. switch in series with the 3RD gear normally open sensor wire on the side of the TH-400.

Happy motoring!
 
#19 ·
Hi All,
I'm replying back to this old thread. I ended up getting a small switchbox/switch which grounds the v-8-6-4 so I can enable/disable v-8-6-4 when I want. My 3rd gear switch never worked, but grounding the terminal does work. Keep in mind, my car only goes in v-8-6-4 after reaching 25-30 mph.

As far as mileage, it's about 1.5-3 mpg improvement with driving with it on on combined city/freeway. This is taking the pump gallons used and dividing into the mileage after trip meter reset. The factory MPG isn't too far off if I use the average MPG on the sentinel.

My other car is a 1984 El Camino and after driving it, driving the V-8-6-4 Caddy is "interesting". Once I get used to it, I switch it back on, but sometimes it's a pain.
I think the two biggest issues is that the V-8-6-4 system relies on the driver to put their foot into the gas more. It can be a little unnerving at first, you start going up a hill and slowly give it more gas, the system will try to keep it in V4 and V6 as much as possible. Once v8 mode is reached all power and smoothness returns. The second issue is that the v-6 vibrates due to un-even firing. Perhaps if they turned off the lockup torque converter it might have helped things... oh well.

I plan on mostly running my car in v8. I turn on v-8-6-4 if I'm going on a long trip or to show someone how it works. The power (ahem, all 140hp) in full time v-8 follows the driving experience of a Cadillac, so that's the mode I intend to use most of the time. :)
 
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