Backlight module controls defogger, sends radio signal

Hey there, time traveller!
This article was published 17/01/2003 (8351 days ago), so information in it may no longer be current.

QUESTION — I have a 1997 Buick Park Avenue Ultra, with 153,000 km. Recently the rear window defogger ceased to function even though the switch on/off light continued to work.

The manual tells me that the same fuse protects the circuit for exterior mirrors heating, rear window de-fogger and radio antenna. This suggests that all are on the same circuit but the radio reception continued to be excellent.

My Buick dealer took about three hours to correct the problem which he described as a “faulty connection” under the dash that would eventually have also cut off the radio antenna — please explain this. Does three hours sound reasonable?

The next morning it worked fine. The morning after that it failed to work. The radio reception was still normal and the switch on/off light still worked. My dealer then discovered that the “real problem” was a failed module that had “nearly melted”. What is this module and why would it nearly melt.

ANSWER — A description of how the rear defogger works on your Buick may help explain many things. When you push the rear defroster button on the A/C control panel, a voltage is changed in the AC programmer (computer) located on the heater housing. The programmer then sends a data communication signal to the car’s body computer, which then controls the operation of a relay which powers the Heated Backlight module. The Backlight module then provides power to heat the rear window grid. It sounds complicated and it is! One bad connection anywhere in the wiring between these components and the defogger doesn’t work, so it could possibly take three hours to check every connection.

The Backlight module not only controls the defogger operation, but also amplifies the FM radio antenna signal and sends it on to the radio. The FM antenna is actually the same lines on the window as the defogger grid. The two antenna lines above the defogger grid are for a separate AM antenna. When the rear defogger is turned on, there is a lot of electrical current flowing through the Backlight module — about the same as it takes to operate the headlamps. High resistance in the module could create a lot of heat, causing it to melt. Eventually, the FM radio reception would also be affected. It amazes me how complicated some systems on vehicles have become!

QUESTION — I own a 1968 Ford Mustang Coupe 289 CID 2 barrel carb. I have a cracked manifold on the driver side. This is a summer only car. How important is it that this item is fixed upon getting it on the road this spring. It seems to be a rather costly repair just to keep the noise down and the horsepower normal. What could happen trying to get the old rusted on manifold off? Could this lead to more repair bills?

ANSWER — On a vintage vehicle like your Mustang, a cracked exhaust manifold will have no immediate effect on driveability, however a cracked manifold can allow hot exhaust gases to leak onto the spark plug wires. The wire will eventually char, causing an ignition misfire.

Exhaust leaks in the engine compartment could allow poisonous carbon monoxide to enter the passenger compartment. This may not be as much a problem with the open window motoring of summer but repairing the leak should still be a priority.

Finally, a leaky exhaust sounds terrible, so if you don’t want your car to sound like a clunker, fix the problem.

The exhaust manifold may appear rusty, but often they can be unbolted from the engine in a few minutes. Before starting, get a replacement manifold from an auto salvage yard or a new one from one of the numerous antique Mustang parts suppliers. If a bolt is difficult to turn, let penetrating oil soak in for a day or more and try again. Broken bolts can often be removed from the cylinder head after the manifold is off. If the bolt cannot be removed, (unlikely) then the cylinder head needs to be replaced.

Muffler shops replace manifolds all the time and are experienced in dealing with any difficulties that arise. It is possible to do it at home, but the aggravation may not be worth it. I would still supply the replacement manifold, because shopping around for one can get you significant savings.

Jim Kerr is an experienced mechanic, instructor of automotive technology and freelance journalist. You can e-mail questions to Jim at the address below.

kerr.jim@sk.sympatico.ca