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My 1993 945 with Rex/Regina and a Yazaki cluster has had a number of irritating electric problems over the years. This one showed up several months ago, and was finally fixed last weekend. I've driven the car about 40 miles and the code has not come back, so I'm satisfied it's taken care of since before it would return after just a mile or two.
OBD would set a 3-1-1 code, "speedometer signal absent or faulty". The speedometer itself has always behaved normally, so my first concern was not the typical crumbling wires at the sensor on the real axle. The speedometer is connected to the main printed circuit by a small flexible leg of the circuit which terminates in a small black plug that fits into the rear of the speedo unit. if your cluster is unmolested, there will be a tamper-evident white plastic cover over it. 2 issues here: the plug is extremely delicate, and it is hard to tell whether or not the connections are tight inside it. Second, the design is extraordinarily poor: having a portion of the printed circuit (which is delicate and prone to broken traces even on a good day) be asked to work in a flexible manner is inviting problems.
My workaround was a jumper wire from a point inside the speedo unit to the spade terminal on the back of the cluster where the wire to the ECU attaches. There is a convenient access point inside the speedo, where there's a hard wire jumper on the inside near the edge, with plenty of room to get a soldering iron on it--ideal attachment point. I have fixed broken traces in this cluster before, using conductive paint (buy from hobby supply places, or use one of those defroster grid repair kits), but did not think that was a viable option given the flexing this is subject to. Only permanent modification to the cluster was drilling a hole in the plastic body to run the wire thru--and that's less sketchy than I had originally feared, as there's actually plenty of room to drill without hitting anything (as long as you do it with the speedo unit removed from the cluster body).
I would have preferred to use a smaller gauge wire for this, the one I had was larger and stiffer than needed. But this was the least difficult choice of the wire I had on hand.
These photos show a 1993-1994 cluster. 1991-1992 clusters used in the late 740 should be similar. No idea if the 1995 and later cluster has this issue.
Hope this helps someone!
John
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1989 245 245K / 1993 945 131K
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