Hey everybody.

I've done some searching on the forums but solutions, it seems, are rarely posted. My situation's a little bizaare as well.

My speedometer doesn't work correctly. However, it DOES work perfectly for a period of time. I'm fairly certain whatever the problem is heat related. In the winter, I could drive roughly 90 miles without a problem and then suddenly the speedometer would go dead. It all would go dead. Speedometer won't dictate speed, trip counter doesn't work and neither does the odometer. Cruise control refuses to turn on too (which isn't surprising since it won't turn on unless you're doing 35).

Anyway, now in the summer weather here, I can get about 30 miles before the same things happens. occasionally, if I blast the heat full force I can get it back after about 20 minutes. Then go an additional 15 to 20 miles before it happens again. If i let the car sit for about half an hour, it always comes back and we go through the same scenario.

So, what's the problem here? The cluster, the sensor or the cable?

I had the cluster replaced about 6 months ago when the speedometer wouldn't work at all sometimes. Even when starting up. And now this problem lately.

So... any suggestions? I'm currently thinking it's the speed sensor switch... just not sure why the heat(er) would affect it.

Thanks again guys. You're awesome!

-kirk

The speedometer itself is the problem. I am assuming its analog? You can check to see if the screws holding it to the circuit board are correctly tightened. If not you need to replace the speedometer itself.

I was hoping you weren't going to say that.

Ah well, once more unto the breach, and so on. I'll let you know what I find, though I have no doubt your insights are correct.

Thanks again!

-kirk

I haven't gone back into the cluster yet, but I'd like to understand how this works.

If the speedometer reads zero, would that necessarily cause the cruise control functions to no longer work? I can't even turn the cruise control on (as in, green light under the on button lights up) when this happens. I thought the cruise control ran off a speed sensor, not the reading on the speedometer. I could understand if it ran off a signal from the speedometer to let it know when the car was at 30 mph, but to even have the unit fail to turn on...?

Time to check the electrical schematics.

-kirk

The "speed sensor" in your car is the speedometer. The pick up in the trans is an AC frequency generator. That signal is converted to a digital signal in the speedo then sent to the ASCD[cruise] and the ECM/TCM.

Well, it's been awhile, but I finally got the speedometer switched out with another one (another analog as well as a digital one). Unfortunately, the same thing is happening.

I'm thinking it has to be the pick up in the transmission at this point. Unless there's just something completely screwed up with the electrical system in my car.

I wonder why the ECU isn't reporting it. Maybe it doesn't if the problem is intermittent, or if the problem corrects itself it wipes the error?

-kirk

i've woulda checked the sensor itself 1st since it's the easiest to remove & test...

as discussed in the 240sx FAQs section/near da bottom of dis link

2nd, you can also goto the ECU & goto mode IV... drive w/ the LEDs facin u; u will see the green LED go on when you've reached ~12 mph

3rd, goto a continuity test to see if the wires from the sensor is intact to the cluster: disconnect ECU plug, find terminal #32, put an ohmeter on that pin & body ground, lift rear of car up, spin rear wheel by hand...you'll see a reading /\ if not, u got a break in da wire, yo!

edit: weird that u dont get a trouble code for this sensor; mebbe reset it manually then check again???

Okay guys, closing this one up.

It was INDEED the speed sensor. Finally got out and replaced her (been working on my brother's DeLorean in the meantime). Took the 240 for a wonderful joy ride of 180 miles and not a single hiccup.

So, the lesson? Intermittent speed readings, especially where turning the car off and then on and it works or doesn't work, can easily be caused by a bad speed sensor!

way to go.. but like i said, ECU didn't throw a speed sensor code?

did you have to drain the tranny it replace the sensor? any pictures for the FSM-less? or where can i view the FSM

Check out the speed sensor at the back of the tranny; the harness connector may have a problem.

ricebike wrote:

way to go.. but like i said, ECU didn't throw a speed sensor code?

It's the oddest thing, but no it never did. I'm told that if the speed sensor fails AT ALL it should throw a code that is stored in the ECU. This code SHOULD NOT be cleared if the speed sensor begins working again, correct? It was the first thing I checked before replacing the instrument cluster. But, no speed sensor errors.

That's what led me to replace the cluster... only to find out it wasn't the problem. I still can't figure why the ECU didn't report anything. Maybe that's going too. But so far, no problems. I am happy.

xn7 wrote:did you have to drain the tranny it replace the sensor? any pictures for the FSM-less? or where can i view the FSM

I didn't HAVE to drain the transmission to replace it, no. Would have been less messy, though, and would have saved a rag. But this was far quicker. hehe. I quickly pulled it out and stuck my thumb in the hole with a clean rag (pan underneath to catch anything I missed). I couldn't stop it up, so I had my brother (who was working on his car at the time -- Leatherique is impressive stuff) crawl under and plug it up while I worked the sensor. There's a red plastic gear cap on the end with a clip that needs to be transferred. Then, popped it back in. Didn't loose much fluid, but I "topped her off" anyway, just to be safe. Eyeballed what I lost and replaced it by taking out the shift knob.

I'm not ready to change the transmission fluid anyway, yet. At 138,000 miles, and as far as I know it's never been changed, I'll do it right by changing the filter, oil, driving it for 1000 miles, and then doing it again. I hear all sorts of horror stories about changing the fluid in old transmissions.

I'll get some pictures for you when it finally stops raining out here.

BTW, the FSM is available in pdf from a sticky in this forum. Or you can search for it on google.

rogoman wrote:Check out the speed sensor at the back of the tranny; the harness connector may have a problem.

Check that too, actually. I'm using a speed sensor from Advance Auto which cost $80 (Nissan Dealership wanted $185 for theirs). So, if I didn't have to use it, I didn't want to since I want to do some front end suspension work. Connector was fine and clean. I still have the old sensor. Plan on trying to get into it to see how it works.

Who would want a used speed sensor that has problems in hot temperatures anyway?