Miata Mailing List: August 1996, Message #52
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From: Tim Lightfoot
Several folks on the list have commented on a mysterious dash rattle that shows up between 2000 -2400 RPM. After having the same really annoying rattle for the past year, and trying all the usual cures ( banging on the dash, stuffing foam rubber between all the electrical connections, etc.) I set aside three days and decided to completely disassemble the dash to find and cure the rattle. Actually, I found three surprising sources and cured the rattles: 1) The most annoying rattle was a slight humm (or vibration) that occurred consistently at 2100 RPM or when the car was revved through the 1800-2500 RPM range. After completely tearing apart and taking out the dash, I found that this rattle was caused by the almost complete disintegration of the rubber grommet that surrounds the outermost hot water pipe that runs to the heater through the firewall. The grommet was destroyed on the inside, therefore giving the appearance when you are under the hood that it is still functional. However, revving the car to 2100 rpm revealed that the water pipe vibrated happily at that frequency and set up quite a din on the firewall. This grommet is only $2.65 at the local Mazda dealer and is easily replaced from within the engine compartment (I.e. you don't need to tear apart the dash). 2) A secondary rattle was coming from a surprising source: the air bag diagnostic module and back-up battery. After going through the dash thoroughly and being generally impressed with Mazda's efforts to reduce squeaks and rattles, I was quite shocked at the mounting brackets and construction of the air bag components. The air bag diagnostic module and backup battery on my '92 Miata are mounted on two metal brackets that are bolted straight to the firewall framework. There are no rubber isolation washers or other vibration isolators, so the vibration of the car is passed straight to the diagnostic module and back-up battery. This usually wouldn't be a big deal if these components were mounted securely. However, these components (which are housed in bright blue plastic housings), are basically slipped onto 3-prongs of the metal support brackets. That's it: no mounting screws, locks, etc. The back-up battery is even suspended on only one side. Since the metal brackets slide on 3 prongs into the plastic case and since the brackets are mounted to the firewall all of the vibration is transmitted into the plastic cases. I found that both the diagnostic module and back-up battery cases had broken from around the bracket prongs and were each held on by only one prong in each case. Metal rubbing plastic is not a pretty sound or sight. I was able to fix both cases with the application of liberal amounts of industrial strength epoxy, plus I built a couple of simple brackets to help secure the components to the main mounting brackets. I also used some small rubber washers on the two mounting points for the metal brackets on the firewall to help cut-down on the transmitted vibration. The only problem with this fix is that you have to take out ( at a minimum) the steering column. Also be sure to disconnect the air bag correctly before you start playing with the back-up battery. The Miata enthusiast's manual was my guidebook. 3) A smaller rattle that I had localized to the instrument panel turned out to be interesting. On the back of the instrument panel facing (the big black outside trim), there are four prongs that hold on some rubber sheets that help reduce squeaks from the steering column (I think). On these four (plastic) prongs, are metal "clinger" washers; the kind that have four teeth that don't come off. Anyhow, in disassembling the dashboard, I found on of these washers (with it's prong clenched in it's little teeth) loose in the hollow part of the dashboard trim. It only took three days to find them, but finally, a quiet car again!