My compliments to you for the marvelous job you are doing to educate the public. Please, help me solve a problem that has perplexed the mechanics who handle my car.
I drive a Volvo S 80, manual, 1999 model. It has served me very well until recently, when it developed a problem; it started losing power when in motion. Once the problem starts, even if I change gears from fourth to first, it does not respond.
The first time this happened, I managed to reach a petrol station and a mechanic there suggested that the car needed servicing.
When I told him that I had serviced it only the day before, he opened the bonnet and played about with the gear levers, and soon the car started moving as though nothing had gone wrong.
He suggested I check the dif oil, which was full. I took it to my mechanic, who took it for 10-kilometre test drive with me, but he found nothing wrong.
After about a month of smooth running, the same problem occurred again when I was on Thika Road . I stopped, opened the bonnet and played about with the gear levers like the mechanic at the petrol station had done, and once again the car started
moving as though nothing had gone wrong. My mechanic has ruled out sensors, pressure and clutch plates as the sources of the problem.
I recall last year in one of your masterpiece contributions, you talked about a linkage (or was it leakage?) problem you ounce experienced after a visit to a garage. Could this be the problem with my car?
If it is, how can it be addressed in a Volvo S 80 without closing my bank account?
Patrick M. Kuuya
Yes, siree, yours does sound exactly like a linkage (not leakage) problem.
In theory, a manual transmission is very simple. You have an input shaft and an output shaft, and the gears are mounted on these shafts. Changing gears simply involves swapping the meshed (interlocking) cogs on the two shafts in such ratios as to create
low torque-multiplying gears (1 and 2 mostly), intermediate gears (3) and high- speed gears (4,5). Top gear is when there is no gear reduction, that is, the input shaft and output shaft rotate at the same speed.
Sometimes there is an overdrive.
In old vehicles, the manual transmission was called the sliding mesh gearbox, because the gears “slid” along these shafts as you engaged them using the gear lever.
Then came the constant-mesh gearbox, which is slightly different. The gears are constantly engaged to each other (hence the name), but unlike the sliding mesh gearbox, they rotate freely on the shafts.
To “lock” them onto these shafts required something called a dog clutch (I would need diagrams to explain these things properly but bear with me for now), which was located between these gears (cogs).
What happens is that the dog clutch acts as an intermediate device and locks the freely rotating gears onto the shafts. The advantage of the constant mesh gearbox is that gear changes were a lot easier to execute – with a sliding mesh gearbox double- declutching is a must, like it or not, while with the constant mesh, one could swap gears without the double-clutch.
Modern manual transmissions use an advanced form of the constant mesh gearbox, called the synchromesh. The dog clutch has now evolved into something called a synchroniser unit, and this is where I think
the problem lies. Let me explain.
The dog clutch is a positive clutch, that is, it has “teeth” that engage into grooves on the sides of the gears. It is a very strong piece of kit, which is why it is still in use in tractors and some hardened rally
vehicles. The synchroniser unit is a dog clutch with extra conical surfaces both on the dog and on the gears ahead of the grooves. There is a baulking ring and a blocking ring (confusing terms, I know), which
are used to match the speed of the gears and the speed of the shafts as well as prevent engagement if the speeds are unmatched.
As you move the synchroniser to engage the gear and lock it on to its clutch, these conical surfaces progressively get into contact, matching their speeds, until the point where they are rotating at the same
speed, and that is when the dog teeth engage with the side-grooves of the cog, and you are now in gear. The speeds are synchronised, hence the name “synchromesh”
Now, the linkage: I think your linkage has lost its tenacity and this is what is happening. As you move the selector (gear lever) from one position to another, the synchroniser unit also moves (as it is supposed
to), but play within the links and/or joints means
that the synchroniser does not fully engage the gear. The conical surfaces are in contact, but the unit has not moved far enough to lock the dog teeth onto the cog itself, so there is what we call “slip”. Slip is a speed differential between two surfaces that are
in contact. Either that, or the car is simply not getting into gear at all.
Ignoring the technical soup I just offered above, let’s talk about the linkage. It is a system of rods and joints that connect the selector (gear lever) to the gearbox itself, which is sometimes not where you think it is. Given the number of joints involved, and in
some cases the materials, warping, breaking or disengagement of components is not a rare problem. The Mazda I had had soft plastic linkage cross members with metallic selector rods, so you can imagine how easy it was to twist things out of shape. If the
linkage either loses its geometry or gets any play, your arm movement on the gear lever will not be accurately translated into corresponding movement inside the gearbox. That is why with a worn linkage, you may hear drivers complaining of “not finding a
gear” or a particular gear not engaging; or in some extreme cases the vehicle won’t even go into neutral.
Luckily for you, the linkage is a (relatively) simple mechanism that can be fixed in seconds at almost no cost. It might cost just a little bit more if you need to replace bent or broken parts, but again, nothing to complain about. It is possible that one of the
synchroniser units is failing but I find this unlikely: it typically affects one gear only, not the entire box.
Late last year, I disposed of my Toyota AE 100 and imported a Nissan AD. It seemed that I traded reliability and peace for problems. I am a hustler, working part-time and farming on the side. I wanted a car I could use to transport light cargo (read
vegetables and milk), go to the office with, park in church on Sundays alongside fancier vehicles and not get flagged down by the police at every road block. I settled for the Nissan AD Van Y12 instead of a Probox.
For the first three months the vehicle was okay. Then it started overheating. Its fuel consumption increased significantly and it just picks up speed even when the accelerator is not pressed. I took it for check-up and the first thing my mechanic did was
remove the thermostat. That did not solve the problem. I got another mechanic, who advised me to do all sorts of things, from cleaning the radiator to changing it. We did blow the radiator but that didn’t help cool things.
I didn’t change the radiator because after I googling a bit, I realised that quite a number of people around the world had complaints of overheating with the car’s sister, the Nissan Wingroad, and some suggested changing the vehicle’s throttle assembly unit.
Eventually, the vehicle was hooked to a computer diagnostic device and they told me the problem was the throttle assembly control. I paid a good amount to get a second-hand one but it has not solved my problem, which only seems to have got worse. I
cannot do more than 30 kilometres before I see the red overheating warning light.
Sorry to use you as an example but some readers ought to see this. I took a considerable amount of flak last week for insinuating that the Wingroad is a feeble, facsimile of a car and one would be better off in its rivals from other parts of Japan. Following
your email, I needn’t say more. The Nissan AD is a Wingroad in a less fancy outfit, so there.
You are not here to be made an example of, I know, so let’s get down to business. I’m amused by the way mechanics solve problems. A problem with cooling? Remove the thermostat. Next thing will be: problem with the suspension? Remove the springs
and shocks. One other thing: why did they all rush to cure the heating problem but ignore the increased consumption and self-throttling?
You say the vehicle still heats up. What happened to the consumption and the acceleration? Check your radiator hoses, water pump and coolant. Also check the electrical system, particularly the wires and fuses. When you changed your throttle assembly, did
you install the correct replacement? Check your throttle position sensor, it could be the wonky one here.
Please let me know about the Toyota Crown Royal Saloon with regard to fuel consumption, stability on the road, availability of spare parts and cost, servicing, comfortability and the better option between 2.5 and 3cc.
In addition, how does cruise control works?
I’m going to skip right past the first part of your question because: a) I’m sure I have discussed this before and in case I haven’t, someone around you who either drives a Crown or hopes to drive a Crown will; and b) this is information readily available on
the Internet. So I will be explicit on the second part of your question: how cruise control works.
First up is what it does: the cruise control system regulates the speed of the vehicle without the driver’s input. If, say, the driver wants to maintain a constant speed of 100km/h, there are two ways of going about it, depending on the exact programming of
a) S/he will accelerate to 100km/h then at that point engage the cruise control. The car will maintain that speed from that point.
b) While driving at whatever speed (more than or less than 100), the driver will set the cruise control (typically manifest as a secondary pointer in the background of the speedometer) to 100km/h, at which point s/he releases all pedals. The car will take it up
from there and either slow down or speed up to 100km/h and hold that speed.
Disengaging the system can be done manually via the system controls, or by depressing any of the two or three pedals. Again, depending on the vehicle, releasing the pedals might either cause the system to take over again, or the system might have to be
This is how it does it: with cars that use accelerator cables, there are two cables at the throttle valve: one to the accelerator pedal and the other connected either to a solenoid or a vacuum-powered servo assembly. These typically receive signals from the
speedometer cable/gear in the transmission or from a wheel sensor to “know” when, and by how much, to power a vehicle by opening the throttle. In the case of a fly-by-wire system where the throttle is electronically controlled, the engine management computer (ECU) governs the throttle opening to maintain speed.
I’m not a huge fan of cruise control. Keeping one’s feet off the pedals means that it takes marginally longer to get to the brake pedal in case of an emergency. If you choose to keep your foot hovering over the brake pedal, it will get tiresome really fast.
Cruise control is also yet another avenue — besides convoluted infotainment systems —through which a driver loses attention from the very act of driving. It provides one lower level of engagement, giving an allowance for distraction.
Lastly, cruise control can be hard on fuel. While in Cape Town, South Africa, I once had the opportunity to test drive the then new Jaguar XJ saloon with a 3.0 litre turbodiesel engine. I set the cruise control at 120km/h (the prevailing speed limit on one of
those roads) and watched it strut its stuff.
This is the thing. The cruise control will hold the car at 120km/h, and hold it everywhere. That includes through corners and going uphill. Barrelling into a corner in a Sh16 million car at 120km/h on my very first outing with absolutely no influence on the
power delivery is not something I want to repeat any time soon, unless I am driving a sports car built to do just that. Neither is going up a 15 per cent slope at 120km/h; the car downshifted twice, the revs flared, turbo boost came in one huge chunk, torque
was dumped through the rear axle and I looked like a proper sociopath as I rocketed past people who were struggling to ascend the same slope at lower speeds. The consumption suffered as a result, which was ironical, given that the whole idea behind a
diesel Jag is that it is meant to burn a lot less fuel compared with petrol variants.
All this is not factoring in my massive OCD. I like counting revs, which is why all the cars I’ve had came with manual transmissions. In an automatic car with cruise control activated, you will see the tach needle dance up and down as the transmission hunts
desperately for the right ratio to fit in the power and torque bands as the car struggles to maintain its preset speed.