Mountain State News, vol. 17 no 2
Mountain State Chapter
From: M Barry Ritchey
Subject: Speedo Fix for '90 325iX
To: 325 Owners with bad speedometer/odometers:
Several months ago my '90 325iX started to have speedometer/odometer problems in the mornings. The speedo would stay at 0 and the odometer would not advance. The problem worsened with the onset of Fall and cooler mornings. The speedo/odo would usually wake-up after about 5 to 10 minutes/miles of driving...and stay working until the next morning or cold start. Here's a long winded account of my quest to fix the speedo/odo.
My first attempt at a fix was to crawl under the car and check the connector at the rear differential. Maybe it was loose or the wiring was frayed/damaged. Everything appeared fine. I checked for bad solder joints on the instrument circuit board. I dreaded pulling the instrument cluster.
Since I didn't have a ETM, there was no easy way of tracing the wiring into the instrument panel. I thought I'd start at the source and tap into the two leads (parallel connection) at the differential connector. I soldered some female connectors to some 14 gauge wire and then soldered wire to my standoff connections. I ran the tap wiring through the drain slots in the spare-tire well into the trunk, out the trunk, and then into the interior by way of carefully closing the rear door (I have a 4-dr) on the wiring. I could now have a DMM located at a convenient location (the console) to monitor voltage at the differential connector. A regular DMM wasn't telling me much after a few days so I borrowed a Fluke scopemeter (portable LCD scope). When the speedo worked, there was a nice pseudo-squarewave (0-8v). When the speedo sleeped, there was just mV noise. I now suspected the speedo or instrument board. I borrowed a Fluke scopemeter. I disconnected the cable at the differential and just measured the voltage across the two leads. It was always 8 volts DC (actually 7.9X volts). There was no significant AC component. After 4 days of cold morning starts and rock steady voltage at the connector, it was time to measure across the transmitter leads coming out of the differential. The Scopemeter, displaying resistance vs. time, showed a nice high resistance spike occurring about every 2.5 ms at around 30 mph when the car was warm. The switch was normally closed (~0 ohms) and was momentarily opening up into the megaohm range. For 4 mornings the meter would just read 0 ohms for the first few miles of driving and then suddenly show the opening spikes of a working sensor. The transmitter was bad!!
To make this too long of a story shorter, I ordered the differential transmitter (and o-ring) from my local dealer in Albuquerque for under $35, installed it, and the speedometer and odometer are working fine for two weeks now.
Lesson learned: check the easy and obvious first. You don't need to have a oscilloscope to diagnose this problem. You can slowly roll the car and look for resistance staying low and then momentarily going high to spot a good transmitter. A bad transmitter (in my case) stays closed (low resistance) when you roll the car (or spin a wheel with the car on jacks)
Here are the part numbers for my '90 325iX.:
62-16-8-357-020 Transmitter ($37.28 list)