Plan ahead so broken belts and fans don’t ruin your trip

I write often about The Big Seven—seven things most likely to strand you during a road trip in a vintage car (ignition, fuel delivery, cooling, charging, belts, ball joints, and clutch hydraulics). Time and time again, this prioritization has proven to be pretty accurate.

Belts are so crucial that they’re essentially in the list three times. After all, on most vintage cars, one “fan belt” runs both the water pump (cooling system) and the alternator (charging system). If that belt breaks, or won’t stay tight, you quickly find yourself in the breakdown lane because the car begins to overheat. And that is exactly what happened to one of the classic cars in our group as we drove to The Vintage, a BMW event held in Asheville, North Carolina, last month. The temperature gauge in a friend’s 1985 BMW 635CSi began rapidly rising after he switched on the air conditioning. When the needle on the gauge flirted with the red, he pulled off at the next exit.

The root cause wasn’t immediately obvious. The fan belt was still present, though a bit loose, and there was neither the sight nor smell of antifreeze. The 635, like most BMWs of that vintage, has a nifty little belt-tightening mechanism utilizing a toothed nut that meshes with teeth on a slotted belt-tightening track. In this way, you can loosen a locknut with one wrench and move the adjusting nut with a second wrench. The action of the adjusting nut’s teeth meshing with the teeth on the track moves the alternator on the track, tightening the belt. You just tighten the locknut, and you’re done. The problem is that the toothed mechanism degrades. The teeth on the adjusting nut, or on the toothed track, or both, shear off. This often causes someone to hack the mechanism, substituting a regular nut and bolt. That sounds reasonable, but the bolt head sometimes gets caught in the teeth, preventing the very adjustment it was meant to repair.

It took me a while to see that the belt on this car had three problems. First, it was loose enough that it was visually jumping around when the engine ran. Second, it couldn’t be easily tightened because it had the aforementioned kluge of a conventional bolt jammed into the toothed track. Third, when we did try to tighten it, we found that the belt had stretched enough that the alternator was at the end of the slot in the adjustment track. I didn’t know for sure that the looseness and jumpiness was causing the overheating, but it was looser than it should’ve been, so the thing to do was tighten it and see if the hot running problem would go away.

Because the belt had stretched and the alternator had run out of adjustment room, the easy solution would’ve been a new belt. Unfortunately, we weren’t near an auto parts store. So instead, we removed the adjustment track and used a file to elongate the slot in which the alternator slides by about half an inch. This allowed the alternator to be moved further along the track, which in turn allowed the belt to be tightened.

With the belt properly tensioned, we watched the engine and verified that the jumpiness of the belt had been cured. We then drove the car, first on local roads, then carefully on the highway, and sure enough, the overheating problem was gone. It’s extremely likely that the “jumpiness” we saw of the belt was its alternately slipping and grabbing the water pump pulley, and that it was the failure of the water pump to turn consistently that led to the car’s running hot.

As far as why the overheating started when my friend switched on the A/C, I have a theory about that. When the A/C is turned on, it clicks on two electric fans—the blower fan under the dash and the auxiliary fan on the condenser in front of the radiator. Generating this extra current puts an increased electrical load on the alternator, which makes it harder for the engine to turn it, which could’ve caused the belt to start slipping.

That, however, wasn’t the end of my friend’s belt-related problems. That night, as we drove back from dinner, his car’s power steering belt broke. Unfortunately, certain 1980s BMWs such as this one use the power steering pump and its fluid to power the brake booster (the power brakes are hydraulically-actuated instead of using a conventional vacuum booster), so when the power steering belt breaks, you lose both power steering and power brakes. Fortunately, being at a vintage BMW event, someone had a power steering belt in their trunk, and the problem was quickly repaired.

There you have it—not one, but two problems that could’ve been averted simply by replacing both belts before a 2000-mile round trip (or, at a bare minimum, having spares with you).

The Big Seven wasn’t finished with us on this trip though. My 1979 Euro 635CSi was in its sights. About halfway home from Asheville, I opened up the hood at a fuel stop for a routine oil check and was stunned to find that my cooling fan had lost two fan blades and was caught on the lip of the upper radiator tank and no longer turning. Amazingly, this was completely asymptomatic; I’d heard no noise and the car hadn’t been running hot.

The immobilized fan had been spinning on the viscous fan clutch, ruining the bearing and the viscous fluid. There was now a lot of fore-aft play in the fan. I wasn’t sure what exactly had triggered the problem—did a fan blade break, cause the fan to go out of balance, catch the radiator, and free-wheel the clutch, ruining it and the bearing, or was the root cause the wobble in the bearing that caused the fan to catch the radiator and break blades? It didn’t really matter; the point was that I couldn’t continue to drive it like that. At some point, a more catastrophic failure would likely occur, taking the belt or the water pump with it.

I did all that I could do at the moment, which was a parking lot repair to remove the cooling fan. What? Drive without a cooling fan? Are you nuts? Normally, yes, but in my case, this was reasonable because a) The car had a brand-new aluminum radiator that provided excellent cooling, b) The car had a brand-new A/C system including an electric cooling fan in front of the condenser that also provides a fair amount of air flow to the radiator, and c) As long as the car is in motion (e.g., driving on the highway), there’s plenty of air flow through the radiator, even without any cooling fan. The problem comes when you’re stopped in traffic. Last but not least d) I didn’t really have a lot of choice.

Unfortunately, my ’79 635CSi had the original-style fan and clutch that are mounted to the water pump with a bolt through the recessed center, which is only accessible with the radiator removed (in the ’80s, they switched to a different fan that spins onto the water pump with the radiator in place). So out came the radiator, out came the fan, and back in went the radiator. In the meantime, I put out the word that if anyone on my drive home had this old style of fan and clutch, I’d be in their debt, but no one did.

Fortunately, a friend of mine in northern Virginia told me that he had the new-style fan and clutch, and the new-style water pump they mated to, so if I wanted to stop off at his shop and change all three parts, I’d be good to do. Although the car ran cool as a cucumber on the interstate, I figured that the odds of making it all the way home without hitting traffic were essentially zero (anyone who has ever driven through Darien, Connecticut, knows there is traffic there even at two in the morning), so I took him up on his offer. At his shop, he pointed out that the car’s water pump was on its last legs and the plastic expansion tank was cracked. He gave me a used one that he had lying around. A few hours later, I had a new water pump, fan, and clutch, and an un-cracked expansion tank, and for the rest of the drive when I hit traffic, I was thankful that I’d made the right choice.

Although both my friend and I made it home with only a modicum of trouble and delay, here’s the take-away message: Preventative maintenance is the name of the game. It’s one thing to verify that there’s nothing wrong, but it’s quite another to ensure that everything’s right.

Before I drive any of my vintage cars further than Hagerty Plus’ towing distance, I verify the integrity of the cooling system by looking for leaks and sniffing for the sweet smell of antifreeze, checking the belt tension and looking for gouges and cracks, squeezing the hoses to find any that are soft, and rocking the mechanical cooling fan fore and aft to check for play in the water pump bearing. I’d done those things before the trip, combined with the fact that the car was running a little hotter than I liked, and replaced the radiator.

In retrospect, I should’ve replaced the water pump, fan, clutch, thermostat, and expansion tank at the same time, which, with my low use of the car, would probably set the cooling system right for 10–15 years or more. Better to be safe than sorry.

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Rob Siegel has been writing the column The Hack Mechanic™ for BMW CCA Roundel magazine for 30 years. His new book, Just Needs a Recharge: The Hack Mechanic™ Guide to Vintage Air Conditioning, is now available on Amazon. You can order a personally inscribed copy here.