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the SI batteries are located inside the cluster itself. they are located directly underneathe the speedo and tach. they are mounted on the SI board. not to hard to find after you split the cluster apart. i know i did it yesturday.
 

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i would check fuses!
 

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i fyou go to a dealer, they have the whole board (batteries cannot be bought seperate) for 200 and some odd bucks. there are a few write ups on how to soder in some rechargeable batteries, ive have one downloaded on my computer, and i could copy and paste if you want
 

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This a quick summary of how to change the batteries for the service interval processor on E30 325's. I don't know how much is different with the 5/7 series, but I have a feeling is basically the same. You can also find an article on this is the November '91 Roundel, but what follows is from my experience.

The processor is on a board mounted in the instrument cluster. The batteries are soldered to the board, and a dealer wants ~$200 because he replaces the whole thing.
To replace the batteries, first remove the instrument cluster from the dash. On the E30, that means first removing the plastic trim below the cluster (at the bottom of the dash just above the steering column) That exposes the bottom screws for a trim piece that wraps around the cluster (couple of screws on the bottom, couple up under the "eyelid") That, in turn, exposes the screws for the cluster itself. There are two (I think) on the bottom, and two on black tabs that stick forward from the top of the cluster. Remove them and pull the cluster forward and out. Take a good look before you unplug the wiring, there are 4 or 5 plugs you have to take out. Once you've seen it, you won't have trouble getting it back correctly, they are color and shape coded pretty well.

Now set the cluster face down on a table (careful not to scratch the plastic face!) On the back, there are several (about 8-10) screws, most around the edge, with one or two in the middle. Take them out, and the whole back will come off, and you will be able to see the board (It's the only board in there, along the bottom and goes most of the way from one end to the other). Remove the board.. to do that, you have to remove two things. One is the engine code plug which plugs in to the board from the opposite side (the outside of the instrument cluster). It is a plastic rectangle about 1"x1"x.5". You have to press a tab to unclip it and pull it out. The other thing holding the board is a plastic piece that is held in by one screw. Take the board out, and you will see the batteries in the middle.

Mine were two NiCad AA cells with something like "Varta 500 RST" on them. These are top of the line NiCads. You can replace them with standard replacements (e.g. Radio Shack AA NiCads), or if you have a good electronics store nearby, order them from Varta. Either one will have the same voltage, so work the same, but the Varta's will have more capacity, so last longer when you pull out the car battery, and perhaps not die as soon either. (The Varta's have 600 mAmp hours capacity vs 450 for the Radio Shack AAs. Both are 1.2V).

A possible source of the nicads is
E.H. Yost & Company
7344 Tetiva Road
Sauk City, Wisconsin 53583
(608) 643-3194

The old batteries are held on by tabs that are welded to the battery on one side, and soldered to the board on the other. Remove the solder with a solder-sucker or solder wick (again, see Radio Shack), straighten the pins and remove the batteries. Be sure to remember the polarity of the batteries for when you reinstall them. The original batteries are marked +/-, but the board isn't.

You have to rig up a way to solder the new batteries to the board. You can take the old mounting tabs off the old batteries, and remount them to the new ones, but that is tough. You can buy batteries with mounting tabs, the Varta's may come with them, I'm not sure. The Radio Shack tabs won't mount directly to the board, I cut them to fit. You could also rig up a wire to go from the tab to the board. Anyway, take a look at it. It's pretty easy to figure something out.

One warning, on my board the tabs were soldered to the board at one point on one end of each battery, and at two on the other end. It turns out that on the end with two connections, one of them is just for strength and has no electrical properties. I just soldered to the one, but if you aren't sure which is which by looking at the board, You can solder a small wire between the two holes and just mount the battery to one. My batteries originally had a small amount of glue holding them to the board. When I replaced them, I replaced it with some silicone gasket material to help keep the batteries from vibrating loose.

All that sounds complicated, but it's really not if you have a little soldering experience. The only things you have to be careful of are to not crack the board while you are playing with it (not a big deal, it is pretty rugged) and when you solder, don't create any bridges (shorts) between leads on the board. That's easy if you just don't use too much solder. It's probably a good idea to charge up the batteries before you install them if you can.
Then just reverse the steps to reassemble the cluster and reinstall it in the dash. Don't forget to put back in the engine code plug! You will have to reset the service interval indicator after you are done, of course.

Good luck, if you have any questions about it, send me a note.
rick

Use CHARGED NiCads ... some boards will NOT charge up a completely dead NiCd battery, and you'll think you need a new board ..
PS: I found this out when the local dealer was starting to change the batteries v. the board .. they told me my instructions didn't work .. so I came down and found out that *some* SIS bards will NOT work unless the NiCd batts. are charged *before* install And *yes* they will now change the batteries, saving their customers mucho $$$ .. over changing an SIS board
 

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I had th same problem with my gauges not working(temp ,tach, econ gauge) i checked fuses 10 and 21 which the fuses were not blown. Then i replaced the SI board batteries following the same directions as stated earlier on the thread not that hard of a job just time consuming. All the gauges work as they should except the econo gauge but these are never that accurate. Took me 1.5 hours. Pelican parts has a decent DIY with pictures check it out. Josh
 
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