Speedometer cuts out while driving

[laxcoach]

I have an 84 and when I am about 30 minutes into a drive my Speedometer jumps up and down a few times then cuts out all together for about 20 minutes before it jumps a few time again and comes on for a short while. Then cuts out agin.

At same time as speedometer cutting out the Odometer stops working.

any ideas? I had a friend suggest the speedometer sensor
in transmission but I am unfamiliar with what it looks like. I have a Chilton's book but it only "mentions" sensor and where it plugs in but does not give a picture or advise if replaceable. Is this easy to get at or do I need to have entire transmission dropped
and opened to get at? any other suggestions as to what this might be?

 

[Magmotor]

If it's an automatic your looking at #43 on the pic. I can only suggest pulling the cluster and checking the j-connectors, cleaning, if possible re-solder and checking the connection at the sensor speed end. Maybe someone with an '84 can jump in and help out. Good luck with it...ave

 

[jim coulter]

I have an 84, but I've never experienced that before. Whether or not it could be the speedometer signal generator, I couldn't answer, but my manual does say how to replace it.(if it only told you how to check to see if it was good or not). 1)disc. battery neg. term. 2)raise vehicle and secure with jacks 3)the speedometer signal generator is located on the left rear end of the transmission, just in front of the catalytic converter. WARNING: make sure cat has cooled down, they get very hot. 4)disconnect electrical connector from signal generator 5)remove hold down bolt and clamp from signal generator. 6)remove the signal generator 7)installation is the reverse. Wish I could find out how to check it, I'll keep looking and post if I find anything.

 

[berky2500]

Mine had a similar problem although it only worked above about 35 mph and then it was always off by about 20-30 mph. It ended up being the speed sensor. The speed sensor is a permanent magnet mounted on a shaft, the shaft rotates on a gear connected to the output shaft of the tranny. Pulses are output from the coils in the sensor to the cluster which has a circuit calibrated to the frequency of the pulses. I tried measuring the output of the sensor, but since the amplitude of the pulses are the same, and it is the frequency of the pulses that indicates speed, a voltmeter does no good. If you have an oscilloscope you might be able to check the signal.

I would go through the check of the cluster and check the wiring and solder joints, I had that problem also though not with the speed circuit. Then I ended up changing the speed sensor. I bought a new one from Discount Chevy Parts Online for about $69. The sensor has a oring around the outside that makes it very tight to get out, you have to rock it back and forth in little bits to get it out. Be prepared for a bath if you haven't dropped the pan, there is a good bit of trans fluid that comes out that big hole.

It was much harder to get the new one in, because the oring was much tighter. It took a lot of grunting and pushing and finally making a make-shift press out of a 2x4, large socket and pry bar to push it back in.

You can check to make sure it works by leaving it plugged in and connecting the shaft to a drill. Especially if it has two speeds, then you could compare the old to the new, and fast to slow.

Enough rambling, hopefully you have enough to decide which way to go.

 

[laxcoach]

Mine had a similar problem although it only worked above about 35 mph and then it was always off by about 20-30 mph. It ended up being the speed sensor. The speed sensor is a permanent magnet mounted on a shaft, the shaft rotates on a gear connected to the output shaft of the tranny. Pulses are output from the coils in the sensor to the cluster which has a circuit calibrated to the frequency of the pulses. I tried measuring the output of the sensor, but since the amplitude of the pulses are the same, and it is the frequency of the pulses that indicates speed, a voltmeter does no good. If you have an oscilloscope you might be able to check the signal.

I would go through the check of the cluster and check the wiring and solder joints, I had that problem also though not with the speed circuit. Then I ended up changing the speed sensor. I bought a new one from Discount Chevy Parts Online for about $69. The sensor has a oring around the outside that makes it very tight to get out, you have to rock it back and forth in little bits to get it out. Be prepared for a bath if you haven't dropped the pan, there is a good bit of trans fluid that comes out that big hole.

It was much harder to get the new one in, because the oring was much tighter. It took a lot of grunting and pushing and finally making a make-shift press out of a 2x4, large socket and pry bar to push it back in.

You can check to make sure it works by leaving it plugged in and connecting the shaft to a drill. Especially if it has two speeds, then you could compare the old to the new, and fast to slow.

Enough rambling, hopefully you have enough to decide which way to go.

 

[laxcoach]

sorry, I am new at this and having trouble replying.

Berky,

did you have to drop everything to get to drive shaft to check if it worked?

The Make shift press: Did you make a hole in middle the use
crow bar to pry on?

Thaks for info as I think this may be problem. I did have
cluster reworked.

laxcoach

 

[berky2500]

No didn't have to drop anything. The sensor is right in the tailshaft of the tranny, one bolt and a c-shaped metal clip to hold it in. It is keyed to the hole so the new one only goes in one way. The hardest part is getting the car up high enough to give you room to move around. I had the car up on four jacks, that was before I built by 12" full car ramps.

The real problem is you can only really get one arm up there, and when you are trying to pry it into place, three would be helpful.

I put the piece of 2x4 against the left side of the tunnel, then a socket over the sensor, then pry between the two. I think that I turned the 2x4 sideways so that I had a tight fit to start. Go slow and push evenly. I was only moving the pry bar a little bit at a time.

 

[laxcoach]

Thanks,

When I finish Carpet, Which I am down to doors then I will
go after this.