Auto Service World
Feature   September 1, 2001   by Rick Cogbill a.k.a. Slim Shambles

Mercedes Mayhem

(Thanks to John Cornett-Ching of Summerland Auto-Tech, Summerland BC, for this month's solution. If you've got a sticky solution you'd like to share with our readers, contact Slim at r_cogbill@telus.n...


(Thanks to John Cornett-Ching of Summerland Auto-Tech, Summerland BC, for this month’s solution. If you’ve got a sticky solution you’d like to share with our readers, contact Slim at r_cogbill@telus.net)

Baron Von Frederick didn’t mind paying for repairs, as long as his 1986 Mercedes 420SEL performed to its German-engineered specifications. But the $2400 he had spent at the local dealership last fall on his heating and air conditioning system had failed to make it work properly.

“Und now zey vant $2500 more to retrofit zee air conditioner!” The Baron pulled on his white handlebar mustache agitatedly. “I think eets time for another opinion, ja?”

“No promises,” I replied as I took the keys, “but leave it with us and we’ll see what we can do.”

I pulled the car into the shop and opened the hood. The crew gathered around as I turned the A/C to high. “Looks like it’s working,” called out Tooner, as he watched the compressor engage, and the low side hose began to cool down. “What’s Freddy complaining about anyway…” All of a sudden the compressor cut out and stayed out.

I turned the controls off and then back on again. Once more the compressor began to work and then suddenly cut out, never to go again unless I started from scratch.

“A minute and 48 seconds,” announced Beanie proudly, brandishing at his new German-made digital watch. “I timed it.” Sure enough, the system would work for one minute and 48 seconds every time, and then quit.

“Doesn’t sound like a low charge problem to me,” I said getting out of the car. Tooner bypassed the compressor cutout switch to keep the compressor running.

“Nope,” said Tooner. “With air in the ducting at 36F, I’d say it’s cooling just nicely. No need for a retrofit here; he’s got a simple control problem.”

I agreed, but a closer look into the way Mercedes-Benz decided to control their heating/cooling systems was enough to convince us that “simple” was a word unknown to German engineers.

I sent The Bean off to surf service bulletins, hoping something would turn up.

I was having a coffee when Beanie came in with a printout in his hand. “Don’t know if this will help,” he said, “but this article mentions a two-minute timeout on the compressor controls. Two minutes is awful close to one minute and 48 seconds, if you ask me.”

I read the article over. Certain models had a speed sensor mounted on the back of the A/C compressor. If the dash control module didn’t see a compressor rpm signal within 30% of the engine rpm after two minutes, it would shut off the compressor to avoid damage.

“So if the fan belt slips, the A/C would stop working,” theorized Beanie.

“Exactly,” I said, “or if the compressor was slowing down due to excessive pressure. But that’s not the case here.”

I went through Von Frederick’s previous repair bills that he had left on the front seat. “Aha,” I announced, holding up one for the work done last fall. “The Mercedes dealer replaced this dash module in the last repair. You’d better take a closer look.”

Tooner removed the dash control unit, and I called up the parts department at Mercedes-Benz. “Look for a part number,” advised Heinz. “Tell me what it is and I’ll trace it back.”

“126-830-08-85,” said Tooner when I asked. “Is that the right one?”

I consulted Von Frederick’s invoices. “It says here he was billed for a 126-830-09-85.” I called Heinz back.

“Hmm,” he said. “That number you have is for the 88-92 models, the ones with the speed sensor on the compressor.”

“But this is an ’86 model,” I said. “There is no sensor.”

Heinz consulted his computer again. “Then you need 126-830-05-85. I’ll order one up.” He sounded sheepish. “I guess we’re buying this one?”

“Ja,” I said, and hung up. We plugged in the new module when it arrived, and everything was wunderbar.

“So let me get this straight,” said Beanie during coffee break. “The dash module was looking for a speed signal that the compressor couldn’t generate.”

“Right,” I replied. “The ’86 models didn’t use speed sensors. So when the module couldn’t get a speed signal, it would shut off the compressor.”

Tooner dunked another donut in his coffee. “So what did Baron Von Frederick think of his repair bill? We spent a lot of hours on that one.”

“No problem,” I said. “But his next stop is Mercedes-Benz, to have a few words with them over his previous repairs.”

“Oh?” said Beanie. “Will that be in English or German?”

“Don’t matter,” grunted Tooner. “Nobody’s gonna need a translator when he gets through.”

About The Writer

Rick Cogbill is a freelance writer living in the Okanagan valley of Southern British Columbia. A licensed technician with over 25 years in the automotive repair industry, including ten years as a shop owner, Cogbill creates his comic scenarios with Slim, Basil, Tooner, and The Bean out of actual case histories. “What you have just read is true,” drawls Slim Shambles. “Only the names have been changed to protect my hide!”


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