Speedometer and Odometer Failure
 

Hello everyone. This forum has been a huge help for me over the years, but I'm stuck on the problem my 89 240 is currently giving me.

About a month ago my speedometer stopped working. Along with it, the odometer and the cruise control.

Based on the other threads regarding speedometer issues, I've checked two things so far:

1. The speed sensor at the rear of the car. All wires look really nice. As far as I can tell they are still flexible and aren't cracked or broken. the sensor seems secure and so do the connections. Also, based on the on board diagnostics, the sensor is sending messages to front of the car.

2. Once the sensor seemed to be working, I pulled out the instrument panel to check the speedometer chip board. I was surprised. Everything looks brand new. No swollen components, no shotty solder.

I'm completely stumped. Maybe I'm missing something.

Please help.

Thanks

I think I'd be verifiyng that hte sensor pulses are making it to the speedometer... jack up the back of the car, put it on stands, put it in neutral, pull the instrument panel, and find the speedometer signal into the back of the speedo, which is the green-white wire into the 3-pin connector (the black wire is ground, the blue wire is switched power), put a digital volt meter across this and have someone spin the rear wheel and you should see voltage pulses. if your meter has a "Hz" frequency setting, this should read proportional to the speed the wheel is spun at.

if you see that, then the odds are the speedo itself has gone bad. the cruise control gets a cleaned up version of this same speed signal from the instrument panel.

This is really helpful! Thanks Pierce!

I'll get started on this Saturday when I can get some help. I'll let you know what I come up with.

I may be getting ahead of myself but, if the speedometer itself is broken, where's the best place to pick one up? Do they vary much by model year the way the circuit boards do? Can I just replace the meter and keep the current circuit boards?

Also, do I understand you right? If the sensor is talking to the speedometer but the meter is broken, then fixing the meter will fix the odometer and the cruise?

Sorry for what may be elementary questions, this is new territory for me.

the odometer is driven by the speedometer circuit, as is the cruise control.

I can't help you with which year 240 speedos interchange. I know a little bit more than I should about 1991/1992 740/940 speedos (ARRRRRRRGH).

I believe that any non-abs car that an electronic speedometer should interchange with yours. I believe in '91 they used different speedometers depending on whether the car had ABS or not. '92 and later has the ABS speedometer unit and will not fit yours.

I would try to get a speedometer off of a 90 or earlier.

Your problem is the little plastic gear in the speedometer itself, it has lost a tooth or two--240 are notorious for that. IPD sells the gears as well as Dave Barton who has done extensive research on the topic. Just Google the name, you will find all you need.

I thought that only broke the odometer, the speedo and cruise control would still work fine ?

the OP said all three were broken. to me thats an electrical problem on the speedo board.

Agree with Pierce on this one. Since both the speedo and odo aren't working, then its either not getting signals from the rear speed sensor or there is something wrong with the speedo circuit board. I have seen occasions where there are bad (cracked) solder joints on the speedo circuit board that cause these sort of symptoms.

First of all, sorry it took me so long to get this test done.

Here's an update.

While driving over the last couple weeks an interesting thing started to happen. Every once in a while (very randomly) the speedometer will jump just for a second. It doesn't seem to be reading the actual speed but it does move around. Except one time, it did read the speed. I could tell because I was excelerating onto the highway. It was slowly moving up for about 5

Today, a friend and I jacked up the car and ran the test Pierce mentioned above. Unfortunately, I don't have a volt meter with Hz. So I'm not sure how this affects the test.

When we touched the red/black (green/white-- at least I'm assuming. Since there wasn't a green/white on the three prong harness) and the black we saw 12v. This didn't increase when the wheels were spun. We tried the test again and the second time we didn't read anything, not even 12v.

While we had the harness out, we went ahead and pushed the prongs tighter together just in case they were getting a wobbly connection.

Still nothing. The speedmeter hasn't budged.

Does that confirm that the speedometer itself is broken? If so, what do I make of the intermittent action?

Also, we're not reading any ohms when we touch the leads of the sensor.

Should that read anything?

Did you have your voltmeter set to Hz as Pierce stated in his post? I doubt it as you posted your results in volts. The sending unit in the differential sends pulses. You need to use a multimeter that can sense these pulses. Some of the nicer multimeters will actually measure the frequency of the pulses - thus the Hz setting. If you don't have such a meter, you can try to measure AC volts. If you get 0V on the AC settings, then I suspect the problem is the sensor in the differential or the wiring between the differential and the cluster. If you get a non-zero reading when you measure the AC volts, then you are getting pulses and the problem is most likely in the cluster.

I have that same symptom, usually running the heat for a while will cause the speedometer to work, my odometers awaiting a new gear from me.

I would suggest locating a decent speedometer at a junkyard and slapping it in, be sure to change the odometer counter though if you can, its not easy selling a car with 300,000+ on the odo.

Fixed it!
 

Hey everyone!

Thanks so so much for your help. At the end of the day, this fix required a new speed sensor. I went ahead and bought one from the dealer to avoid buying a faulty part. In the process, I found out that cars with ABS and Non-ABS require different sensors. This was interesting to me.

I replaced the sensor and everything's been working great!

I think the most interesting thing about this repair is that the on board diagnostic system gave me the wrong information. The diagnostic test indicated that the speed sensor was sending and receiving information. Because of this, I assumed there were other issues.

Anywho, I changed the sensor and all is well.

Thanks again for all your input.

I have a question about 240 (244 in this case) odometers. Just replaced the gears on my 1989 244 and the odometer is working (i.e., moving and adding "miles") but the counters on the total mileage and the trip mileage are going at different rythms!!! I thought they both moved driven by the back 15 tooth gear wheel and therefore they should be showing the same increments....as an example, for each 5 "miles" increment on the trip counter, I get about 7-8 "miles" increment on the total mileage counter. It's as if one was counting miles and the other km!! Any ideas about how this could be happening?

Thanks!!!

Xavier

I thought that was normal for the trip gauge to go faster?


btw, Speedometer randomly "pulsated" on me after a 70mph drive, still need to get behind the cluster and sort some stuff up.

the trip gauge has a 0.1 mile digit, but otherwise, they should move at the same speed.

Thanks for the response...the crazy thing is that they don't move at the same speed (and I mean the mile digits, of course I don't look t the 1/10th mile digit on a white background)...the total counter goes way too fast (about a 30-40% faster)....

Xavier

do look at the 1/10th gauge on the trip meter, see if its starting/stopping, it should be moving smoothly from digit to digit as you travel at a steady speed...

the 1/10th digit (on white background) of the partial trip counter should obviously go 10 times faster than the 1 mile digit of both the total and the partial trip counters. But the other digits should accrue miles at the same speed...in my case (why me?) the TOTAL counter 1 mile digit goes 30-40 % faster than the 1 mile digit on the trip counter...!!!

Xavier

Thanks,

Yes, I looked at that....it's totally smooth...and, by the way, accurate (I've checked it on trips of which I know well the distances....). I can't figure this one out!

Xavier

Today I reset the trip counter (pushing all the way several times) and went on a test.....the trip counter accrued 5.3 miles (which more less corresponds to the distance I drove) yet the total counter accrued 8 "miles"......it's mind boggling I'd say. I emailed Dave Barton and he can't figure out what's going on either. On another forum somebody suggested that the trip counter might be skipping somehow but (a) I checked carefully and the 1/10th digit (on white background) it's accruing units smoothly and without hesitance or stopping, and (b) the trip counter is the one that is accurate!

Any ideas? Any help will be appreciated....ultimately, if I can't do anything about it I'll take that the total counter is indeed measuring km!!! (how that happened without changing the tooth wheels on the counters themselves seems to be one more of these mysteries 240s bring from time to time......).

Xavier