Tech Tips: Summer 1993

Overdrive Faults
Ken Gillanders
Temple City. CA

Fully 90% of overdrive problems that are not electrical faults are traceable to either the gear oil level being too low, or an improper adjustment of the solenoid, so we’ll skip these and concentrates on the others.

An overdrive that fails to engage can sometime be traced to dirt between the check ball and seat In the pressure regulator valve, or sometimes to a badly worn pump. Both are quite rare, though possible, so don’t overlook them. Worn or broken rings on the operating pistons or the accumulator piston can make for a lazy-shifting or slipping unit.

However, the real winner is a partially or completely blocked bypass port in the operating valve. The valve, which is activated by the solenoid through the operating shaft and lever, has a small hole bored in it (about .018″) which is very easily blocked by dirt, etc. The results of blockage can be really fun to find. First, partial or intermittent blocking results in slow engagement, but most noticeable is that there is no compression-assisted slowdown. The unit appears to hang between overdrive and direct drive and feels like it is not in gear. Basically, what is happening is that the partially blocked valve will not allow the oil to return to the sump fast enough, and the pressure build-up above the valve tries to engage the over drive while the unit’s springs try to engage directly, and there we hang between the two. The fun begins when the bypass port becomes completely blocked. Above a certain speed, the pressure above the valve becomes high enough to engage overdrive, no matter what you do or where the switch is.

Fortunately, the valve blockage is easy to repair. With the transmission tunnel removed, the access to the valve is on the right side of the top of the overdrive unit and is under a 7/16″ plug. Under the plug is a spring and check ball assembly which can be removed with a small magnet. After it is cleaned, it can simply be set back in place, hole-in end facing up, followed by the ball and spring assembly. It is very straight forward and easy to remedy once you find it.

SU Carta Dashpot Oil
Jim Taylor
Kansas City Austin-Healey Club

The purpose of the SU damper is to retard the rapid upward movement of the piston on the rapid throttle opening associated with acceleration. This delay in piston movement causes a momentary decrease in pressure at the throat, thus achieving a momentary increase in richness much the same as with an accelerator pump. The weight of the oil in the damper determines the amount of dampening. A lot of race mechanics use automatic transmission fluid for some misguided reason obscure to me. I think it is too light for normal use. SU publications recommend 20 weight, which I would follow. (Moss sells the correct SU dashpot oil: #220-225 is S6.95 for a 125ml. bottle -Ed.)

Do not overfill the damper. The proper procedure is to fill “below the top of the hollow piston rod”, not “below the top of the chamber neck”. Overfilling just spills over into the suction chamber and makes a mess. One easy check is to remove the damper and then re-insert it. If you feel resistance before you reach the threads on the cap, you’ve enough oil.

Wire Wheel Care
Ron Phillips. Austin-Healey
Association, La Habra Heights, CA

The second most frequently asked question about our cars must be “how do I take care of my wire wheels?” “Should the car be off the ground to hammer the knock-off, or should the wheel be on the ground?” Well, that’s two questions, but the problem remains…Whether you call it a knockoff, a knock-on, a nut or a locknut. BMC felt inclined to issue a Technical Service Bulletin on the matter. In short, you should always hammer the nuts on or off with the wheel off the ground and free to rotate. While they don’t say so, many people believe this will decrease the force transmitted into the spokes from each blow. If the wheel is not free to turn, then the spokes take the full brunt of each hammer blow.

The rest of the information is definitive! If you do as BMC says, your wheels should last much longer than if you don’t. How many of you have had to hacksaw off a wire wheel, as It had become rusted to the hub? A few of you have. So inspect, grease, and Check your wheels at least once a year, preferably before the rainy season. As an added precaution, why not use an anti-seize lubricant that comes in a large can with a brush in the lid. One can should last a lifetime and isn’t all that expensive at a local auto parts store.

One final word, don’t forget to use RTV silicone inside the wheel hub on top of the spoke heads – to seal out water and prevent grease or anti-seize lubricant from being spun out onto your spokes and wheels. Several of us have used this technique for years and it works great!

What can you do if either the wheel hub or the center of the wire wheel is too worn? It costs at least $25 a wheel to dip (removes rust and paint), true (tighten each spoke uniformly, replacing several that won’t turn), and repaint (who wants rusty wheels?). By getting new wheels, you might get by with a worn hub a bit longer. A somewhat temporary trick is to use a couple of pieces of shim stock from .001 to .003 inches thick, spaced around the hub to tighten up the splines. It is even possible to dress up the splines with a small file. But, these fixes just prolong the inevitable: sooner or later the rear hubs will have to be replaced if they have been run with loose wheels.

It makes good sense to take care of your wheels and hubs. II you don’t, you will cripple your car… As a postscript, an aluminum can will do in a pinch to get you home if you spin a wheel. Use it for shim stock and the contents to soothe your pending expenditure.

(The shims mentioned are for very temporary use only. Putting an old wheel on new splined hubs, (or a new wheel on old splined hubs, for that matter), will act to no advantage, since the old component will wear out the new component!-Ed.)