RAV4 speedometer sometimes stops at 0, sometimes (less often) works perfectly

Question

I have a 1998 Toyota RAV4 (FWD, automatic transmission). The speedometer sometimes works, sometimes doesn't, seemingly journey by journey - one time, it stuck at 0 at the start of a journey then began working perfectly after about 5 minutes on the road, but other times it either works from start to finish of a journey, or doesn't work at all.

My question is similar to Intermittently fluctuating/non-functioning speedometer in Volvo 940. Possible causes?, but I've never had the needle "wobble around", and my problem seems to be per journey rather than speed related (and obviously it's a different vehicle).

A garage has checked it over, fixed some rusty wiring, sorted out some other electrical issues with the headlights and changed some fuses, and after this it works more often than it did, but it still often fails.

I'm interested in what could cause this kind of "one day it works, one day it doesn't" behaviour, seemingly not affecting anything but the speedometer needle.

I don't think it could be weather related (I'm in West Africa and it's pretty much hot and dry every day, rare tropical rains and no apparent link between rain and the days it does/doesn't work), though I've seen an article suggesting that in some older RAV4s with sunroofs (which mine does have), rain water can leak in and cause problems like this by corroding a connector block (but their problem has different symptoms to mine).

I've read some sources suggesting it might be linked to the transmission - in my case, the automatic transmission seems to work fine regardless of whether the speedometer is having an off day or not. The only thing it seems to affect is the speedometer dial itself.

Answer 1

There are three things which come to mind with your issue:

• The wiring which is going to the speedo sensor on the transmission is somehow messed up. This could be a break in the wiring, or it could be corrosion in the socket where it connects to the wiring harness. You can do a continuity check between the wiring harness at the ECU (or TCU if so equipped) to see if the wiring is bad. This seems less likely.
• Your speed sensor in the transmission which sends the signal back to the ECU is bad or going bad, causing an intermittent fault. I don't have a test for you on this one, though I'd bet there is one. A grounding issue for the sensor would fall in here as well.
• You either have a grounding issue at the speedometer (in the dash) which is intermittent. I doubt this is the case because you'd be seeing it in the other gauges as well. You could however, be looking at an issue where the speedo itself is going bad ... I'm doubting this as well.