Auto Service World
Feature   December 1, 2003   by Rick Cogbill a.k.a. Slim Shambles

Used Computer ‘Prom’-blems

"Gee, I don't know," said Beanie. "No offense to you old-timers, but I think today's technicians know as much as you guys did, maybe more.""Balderdash!" growled the usually mild-mannered Basil, slammi...


“Gee, I don’t know,” said Beanie. “No offense to you old-timers, but I think today’s technicians know as much as you guys did, maybe more.”

“Balderdash!” growled the usually mild-mannered Basil, slamming down his coffee cup on the table. “I remember things you’ve never learned, like how to adjust a set of dual-points or reprogram timing curves on a distributor.”

“Exactly,” I agreed. “Beanie, you still don’t get it when I try to show you how to adjust idle mixtures on a carb.”

Beanie shrugged. “You’re right,” he said. “The point is, who cares? Cars are different now. With OBDII and beyond, it’s a whole new world.”

Tooner tossed in two cents worth of opinion he couldn’t afford to spend. “The kid’s right. Reminiscing about the old days is one thing, but keeping up with Detroit is another. Some days I wonder what those engineers are smoking.”

“Fine!” I pushed a work order and some keys in Beanie’s direction. “If you know so much, then grab your scanners and flow charts and fix this ’88 Celebrity.”

“What’s the symptoms?”

“Speedo cuts out; Check Engine light comes on; and then it stalls,” said Basil smugly. “Us ‘old geezers’ already know what it is, but we’re going to let you figure it out for yourselves.”

Beanie and Tooner took on the challenge, and began their investigation. Basil poured himself another cup of coffee. “I wonder how long until they discover that the ECM has a loose internal connection on the circuit board?”

“Don’t know,” I said, reaching for another donut. “We sure had a lot of those X-body cars with the same problem back then, didn’t we? Just give the computer a little tap, and they’d fail every time, with the exact same symptoms.”

To their credit, Beanie and Tooner managed to figure out the problem, but not before making a lot of checks and reading a lot of flow charts. I think banging on the computer was a last ditch effort by a frustrated Tooner, but in this case it paid off.

“Okay,” said Beanie breathlessly. “We’ve ordered up a used ECM. With over 371,000 km, we don’t want to spend a ton of money on the car.”

“Great,” I replied. “But what took you so long?”

“Hey, back off,” growled Tooner. “We found some other problems that complicated things.”

“Yeah,” chimed in The Bean. “We had to patch a hole in the air intake hose between the Mass Air Flow (MAF) sensor and the throttle body…although it didn’t seem to make any difference on how it ran.”

“And what about that weird data we got from the MAF sensor itself?” broke in Tooner. “It read the same whether it was plugged in or not.”

Basil and I looked at each other. “Er, boys, you might want to know…”

Beanie held up his hands. “Hold it right there, Pops. This is our baby, and once the new ECM gets here, this car will run like a top. You’ll see.”

Well, the computer arrived, but all was not well in Techy-land. Once it was installed, the vehicle actually ran worse, and was now throwing MAF sensor codes. After a while, the two younger members of our crew came crawling for advice.

“Hmm, let me guess,” said Basil. “The used ECM came with its own Prom, and you didn’t bother to change it with the original from the vehicle.”

Beanie glanced at Tooner. “Er, no. We thought that changing it would flex the board unnecessarily, so we just left it in.”

“There’s your problem,” I said. “Besides a lot of bad ECMs in those days, GM also had trouble with their early MAF sensors, so they came out with a kit to change the vehicle over to a Speed Density fuel control system. They disconnected the MAF sensor relay to bypass the MAF sensor, but left the assembly hooked up so that the internal Manifold Air Temperature (MAT) sensor would still work. Then they installed a Manifold Absolute Pressure (MAP) sensor into the system.”

“Right,” added Basil. “The Prom was also updated, and I suspect this car has had the upgrade. The used ECM you got probably has an old Prom, so you need to swap them over.”

Once they did, the car ran great, considering its age. “And you guys do all right for a couple of old timers,” admitted Beanie. “I can see how keeping a bunch of memories in your head might actually be useful.” He paused. “Hey, is that why your hair starts to fall out when you get old, to make room for all that knowledge?”

“No respect,” said Basil, shaking his head. “Just no respect.”

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!”

(Thanks to Dan Leslie of Summerland Auto Tech in Summerland, BC, for this month’s technical solution)


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