Attempting to add cruise control to 1991 Legacy

[PostalLeggy]

Hey there! I've been slowly but surely fixing up and improving on my 1991 Legacy, and one thing I have been very keen on doing is getting cruise control set up and working. As far as I can tell I have acquired all the necessary pieces and parts but I have had a persistent problem where when I hit the cruise button I can hear my relays click, the light will come on, but it will not stay on when I release the button. 

Parts I have installed include the cable and vacuum components under the hood, the cruise computer (AT) behind the glovebox as well as its two associated relays, the cruse button itself, the 4 wire brake switch, and the cruise lever on the steering wheel. 

The only thing that's not stock and standard is the cruse lever which is part of the 2003 Outback wooden Momo steering wheel that I acquired at the junkyard. Connector was different but all the wire colors were the same so I spliced it in to a factory connector and hooked it up. (Horn works but no idea if maybe this switch has different internals that are upsetting the system?)  

At this point I am sort of at a loss as to what else I could possibly try to do. Is there a sensor that I need to add that I just don't know about? I know that some of the newer Subarus have a speed sensor but I am not sure that applies to the 91. 

I also know that on newer cars a CEL will cause the cruse not to function. I have a constant code 45 related to an atmospheric pressure sensor I guess? Haven't been able to figure out why that code has persisted, it popped up after I did a transmission swap a few weeks ago and I haven't been able to get it to go away. I have read that the sensor in question is actually inside of the ECU, so I picked one of those up today and will be testing it out tomorrow I think. 

 

[PostalLeggy]

Replaced ECU and that got rid of my check engine light, tried the original steering wheel switch for the cruise to see if that made any difference and it didn't. Went through and checked all my connections and didn't see anything amiss. I've even tried multiple cruise computers just to rule that out! 

Edited April 22, 2022 by PostalLeggy

[heartless]

how sure are you about the underhood vacuum components?
the diaphragm inside the one piece is a common failure point. as are hardened, dried out vacuum lines. if the system wont hold a vacuum, it wont work.

you may also need to adjust the cable more. it needs to be equal in tension to the main throttle cable.

[ThosL]

Sounds very complicated.  I have had cruise control on some of the Outbacks I have owned, it worked on the last one but not on my 2002 Forester.  Not a priority for me as it is only useful when all the other cars are going at steady speeds on long trips which I don't do much.   

[PostalLeggy]

5 hours ago, heartless said:

how sure are you about the underhood vacuum components?
the diaphragm inside the one piece is a common failure point. as are hardened, dried out vacuum lines. if the system wont hold a vacuum, it wont work.

you may also need to adjust the cable more. it needs to be equal in tension to the main throttle cable.

I have manually applied vacuum to the diaphragm to confirm that the cable moves when I do so, but beyond that I haven't seen anything under hood work at all. I don't think the system is getting that far to be honest. The most I can get out of it is the clicking of the relays when I hit the button. I also noticed that if I shift it into drive while holding the button down I hear another relay click, so I assume that both of them are working. 

As far as the cable tension I did my best to match it with the throttle cable but I don't know if we've reached a point where that matters, yet. 

Edited April 22, 2022 by PostalLeggy

[PostalLeggy]

One other thought is that I have done the 4WD lockup switch modification, and every time I start the car it throws a TCU code 16 via the "power" light. I'm wondering if cruise control won't work if the car detects any latent transmission faults? Might go quickly undo that mod just to see if that resolves the problem... Still scratching my head over this one. 

[heartless]

10 hours ago, PostalLeggy said:

One other thought is that I have done the 4WD lockup switch modification, and every time I start the car it throws a TCU code 16 via the "power" light. I'm wondering if cruise control won't work if the car detects any latent transmission faults? Might go quickly undo that mod just to see if that resolves the problem... Still scratching my head over this one. 

that is very possible.

 

[el_freddo]

Many have done the Lock up switch mod and not reported cruise control issues. 

I’d be looking into how you’ve hooked up the VSS as this will be required for the cruise to work, in these earlier Legacy’s the sensor is in the back of the instrument cluster, unless you have the turbo SS model where it will be on the gearbox if it’s electronic speedo like our Aussie delivered RS turbos are (the SS is the US version of the RS and early JDM GT, except US SS was only available in sedans I believe). Anyway, side track there. 

Why not hook up an aftermarket cruise control system? Much easier than trying to retro fit a factory setup. I’m hoping we can get to the bottom of this one easily enough! 

What year is the cruise control “ecu” module from? And did it come from an auto or a manual? If from a manual it’ll need a clutch switch bypass - probably just a wire across those pins to say the clutch is always engaged (pedal is at rest position).

Cheers 

Bennie

[PostalLeggy]

10 hours ago, el_freddo said:

Many have done the Lock up switch mod and not reported cruise control issues. 

I’d be looking into how you’ve hooked up the VSS as this will be required for the cruise to work, in these earlier Legacy’s the sensor is in the back of the instrument cluster, unless you have the turbo SS model where it will be on the gearbox if it’s electronic speedo like our Aussie delivered RS turbos are (the SS is the US version of the RS and early JDM GT, except US SS was only available in sedans I believe). Anyway, side track there. 

Why not hook up an aftermarket cruise control system? Much easier than trying to retro fit a factory setup. I’m hoping we can get to the bottom of this one easily enough! 

What year is the cruise control “ecu” module from? And did it come from an auto or a manual? If from a manual it’ll need a clutch switch bypass - probably just a wire across those pins to say the clutch is always engaged (pedal is at rest position).

Cheers 

Bennie

Hey Bennie! If there is a speed sensor on the back of the cluster, I didn't see it or see any connectors for it at all. Is it a part of the cluster itself? My speedo is just a cable going into the transmission above the right hand CV axle. 

The only reason I have went with the OEM cruise control is because I saw that my car had all the connections for it and what little I read indicated that it was 'plug and play'. I have tried cruise computers out of a 91 and a 93. Currently have the computer out of the 91 installed, so the exact year that my car is. 

Edited April 24, 2022 by PostalLeggy

[el_freddo]

@PostalLeggy- there *should* be a reed switch setup built into the instrument cluster inline with the speedo cable. This is how Subaru did it in the EFI L series models. 

If there is no sensor hooked up to the system that would make sense as to why it’s not holding the set speed. 

Cheers 

Bennie