Symptoms:
Can not engage cruise control
Mild dip in revs when stopping and engaging N, occasionaly stalls if they go low enough
Failsafe engine mode sometimes


I think because I can't engage CC I'd be looking at vac leaks first. Would that be a good start?
I have already checked the breathers.

Thanks

 

Should be nice to know your details such as engine capacity and year of the car.
You might be right, I had the same symptoms when the indy forgot to connect a vacuum hose after replacing the valley hoses (1998 AJ26 4L).

There is a bunch of hoses next to the throttle body (US passenger side of the TB), follow each of them and you should be sure.
Mine was the one going to the TB, hard to see, even harder to reach.

 

AJ26 engines have a vacuum actuator, so if it's a 98-99 it could be a vacuum line.

 

Sorry, didn't realise the details weren't in my info

it's a 98 3.2, AJ26 engine

It has been ok for a very long time, until the last week or so when these problem began to appear.
Although cruise doesn't engage, the light on the button does illuminate until it decides to enter failsafe mode.

I think there might be a split in a vac line or maybe cam covers so I'm going to check all that tomorrow. To me it seems like there's no vacuum, which means cruise can't engage, then after a bit more driving it gets bad enough for enging management to detect the problem and activate failsafe. I suppose there's other stuff it can't do because of the lack of vacuum but I haven't noticed anything obvious apart from no cruise and dipping revs coming to idle/.

I haven't read codes yet so maybe there's something logged there that will help.

 

It is the other way around; the CC engages, up until power is needed, then it causes the failsafe mode due to lack of vacuum.
Vacuum problem for sure.

 

+1 what Eric said.
On my 98' XJR (US also) there's a dual vacuum reservoir next to the washer fluid fill area with plastic vacuum tubes going to it. Lift the D/S bulkhead cover where the brake master cylinder is and there should be 3 vacuum solenoids with adapters. Or check your cruise vacuum reservoir on the throttle body as well.

 

I just did some more checks - the two vac tanks were under vacuum, so could have supplied the vacuum needed to the TB when called for. All the vac pipes were ok, no splits.

Could the solenoids be the problem?

I did notice when I press + to activate, it does not fail immediately and appears to take some time before going into failsafe; could be minutes later.

One thing I haven't tried is to turn the cruise button off and go for a drive - this should tell me if it is cruise going wrong and causing failsafe, or otherwise.

I know it could also be the brake switch, but I doubt it. If that was the only cause, while driving cruise should engage immediately, which it doesn't. I would expect a faulty brake switch to cause failsafe only when braking (I have indeed had that problem already, and fixed it).
But this is different - I'm not braking when failsafe happens, and cruise just does not want to engage. It does appear to pick up ever so slightly when I press + but I could be imagining it.

I'll go out and see what happens with the cruise button off; I'm expecting not to see failsafe mode. If that is the case, I suppose it could only be the solenoids or reservoir on the TB.

 

Stalling at idle sounds like dirty idle air valve, my eclipse had similar issues when the IAC valve got gunked up. Turning on AC while at idle would make it much worse and occasionally stall. Believe I read around here jags use a similar system, so it's worth looking at the throttle body and cleaning that part out, if someone else can confirm.

 

The idle problem has been elimiated, probably a dirty breather I cleaned out.

The cruise problem continues. I did a test today, to work out if the solenoids are at fault. I removed all the electrical connections and set out on my 10 mile drive to work. Only when I attempted to engage did the problem occur, but it took a minute or so for failsafe mode. With the cruise button on, if I do not attempt to engage, nothing goes wrong. So it's only when I try to use cruise control.

I did notice that on the way back I could engage cruise ok as long as I was on a steady flat, with very little fluctuation in revs required to maintain speed. As soon as I got to an incline, speed dropped and revs increased. Oddly though, it didn't register a fault.
I might be wrong but I think this is because the stable vacuum in the throttle body reservoir (or if that's not it, a stepper motor perhaps?) holds the speed when engaged, but changes can't be made because there's no solenoid control. I also think there is very little electronic throttle adjustment on this engine compared to the later version, which is why any slight deviation in speed/revs fails to be catered for by anything other than the vacuum lines.

So, my next test will be to remove the vacuum going to the throttle body (but maintain it to the rest of the stuff that needs it) and repeat my tests I made today. That will tell me whether a vacuum really is required to the throttle to maintain a steady speed, and where to go next for troubleshooting.

I have also checked the wiring going to the solenoids - it's fine, as are the solenoids themselves, which is why now I can really only think the throttle body is the cause.

Anything I have assumed or incorrectly 'thought' above is welcome to be negated if anyone knows more about this vac system, thanks.

 

Shortly after my last message I rigged up a trio of LEDs to monitor the states of the solenoids (still disconnected - these were wired directly into the loom connectors).

I noticed a pattern of when they were active or not, but in particular for number 3. This one appears to switch according to whether the cruise button is on or off and in P or N, and when the button is on in D or R, whether braking or not. I noticed that after braking sometimes it flickered a bit and when I jabbed the brake pedal or nudged the gubbins behind it, it stayed on.
Therefore, I have to change my thoughts and work on the brake switch being faulty. I'll be checking that later today when I can get under and remove it.


About the ability to engage cruise with all the solenoids out... on the way in this morning I noticed I could only do that when the brake activated correctly, so I do now think the brake switch is dodgy. It's a bit annoying, not too difficult to change as I can work by touch in awkward positions, but it's the cost of the things even used, and then there's no knowing how long it will last. I might just see if I can get the same microswitch if they're still made.

 

I admire your analytic approach of things, wish more ppl. could do so.

Good to read you are up to something, and learning an other lesson how interconnected that brake switch is to things in our cars.

 

Thankyou, I thought I was starting to assume too much so I wanted to check each part of the system individually.


Today, on the way back home I saw some definitive cause and effect going on. The LEDs I had rigged up were still connected and the light for solenoid 3 (that I mentioned changed according to which gear and braking or not) was now in the incorrect state all the time. Unlike before when it flickered a bit and I could put it right by tapping the pedal/switch mechanism, this time that did not work.

When the cruise button is on and D or R are engaged and not braking, the light would normally be on if the switch is working properly. However, with all this in place, I could only get the light to be on as expected by lifting the brake pedal fully up. From it being untouched to this position, the only movement was whatever the pivot joint allowed as fre emovement - hardy any at all but it was enough to get that light on.

Remembering how the switch works, I know that at rest (ie not touching the pedal) the pedal is pushed back up and this pushes against the switch mechanism and clicks the switches in. When the brake is pressed, the switches are released.

I removed and inspected the switch but couldn't see anything visibly wrong. When I refitted it I pulled out the auto adjustment thing that clicks round (I hope someone knows what I mean, difficult to explain) to accommodate differences in physical situations from car to car. Each click is probably a degree or two, dunno but something really small anyway.

The subsequent test drive then showed the switch operating correctly - not flickering, not having to lift the pedal, it was properly on or off when it should be.
In the morning will be the real test, now I have reconnected the solenoids, but I think (hope) I'll be updating to say it was the switch, while not faulty, but needing adjustment rather than replacement.

 

Yes indeed it was the switch.
I had discounted that initially because when it previously went wrong it was the microswitch itself, but this time it appears it just needed a bit of adjustment.

I should have checked it first but I wouldn't have hit on the adjustment thing immediately. It's worth knowing for the future anyway.