PGHRich

I am a new owner of a 1992 Civic LX automatic. I have read all of the threads I could find on fixing speedometer issues and I'm still stuck. Does anyone know how the Vehicle Speed Sensor (VSS) actually works?

My specific issue: When cold the speedometer works fine. After about 15 minutes of driving the speedometer needle starts to drop out, then it settles to about 20 mph, and eventually ends up at zero. Initially I was getting a VSS code with the CEL, so I replaced the VSS (that was before I started reading here). Replacing the VSS didn't fix the problem - I'm sure that doesn't surprise anyone reading this.

Here's what I know:
1. There are no blown fuses
2. The wire bundle is properly tied to the support under the intake manifold
3. There are no shorts to ground on any of the wires at the VSS
4. The wire from the VSS to the speedometer is good (about 0.4 ohms)
5. The cruise control does not work when the speedometer is not right
6. The odometer and trip odometer do not work when the speedometer is not right
7. The VSS connector is clean with a shiny new coat of terminal grease.

I connected an oscilloscope to the signal at the cluster and found that it is normally a 4.6 volt DC signal that drops to zero as the VSS registers. When the car is cold and the speedometer registers properly I read the signal as two pulses in quick succession. As the car heats up the two pulses become one pulse as the needle is jumping. When the signal is just one pulse the speedometer is steady, but reading low. Eventually the pulses go away and the speedometer is dead until the car cools down.

It looks like a sensor that doesn't like the heat to me, but I just replaced it and it didn't solve the problem then so I can't see how replacing it now will solve the problem. (Yes, I installed a new connecting link when I replaced the speed sensor) Are new speed sensors know to fail on installation? If not, I'm wondering about the input to the speed sensor. What spins the connecting link? Could that be bad? Is there something else I have missed in all this?

Thanks for any help
-Rich

EJeff8

Maybe the spider gear connecting to the VSS has broken teeth and you're hitting gaps? This is just a very outside thought.

PGHRich

Maybe, but the way the signal changes as it heats up makes me wonder why. Any idea how I would check for broken teeth? It's pretty tight in there...

EJeff8

No clue sorry haha. I just had a concept, no actual practice.

PGHRich

Update: The battery is three years old and the car was stored for two years before I bought it. I just checked half the cells in the battery and the first two are at 25% on the specific gravity scale. I'm off to buy a new battery to see if that does anything for me. I'll have the guys at advance check the battery and charging system first.

PGHRich

Replace the battery, drove 15 minutes and... you guessed it - the speedometer started twitching. Apparently the battery wasn't my problem. Didn't think it was, but hope springs eternal.

Any other thoughts or things to try?

Former User

How the VSS works:

The ECU, speedo, and cc supply a 5V-10V reference voltage, and the VSS provides a pulsating ground, the rate of which increases with the speed of the transmission.

---------------------------------------

Have you checked for a short to ground when the speedo won't work? Is code 17 still thrown when the problem arises?

PGHRich

Sometimes code 17 is thrown, but it's hit-or-miss. It has probably come up three times in the last 10 trips. I have tried checking for ground with the car cold and hot (just after the speedometer acted up), but the resistance to ground always look good (the resistance is high enough that my meter doesn't register anything but an open circuit), and the wire to the speedo shows good continuity.

Any idea about the double pulse when the system is working? My "scope" is really a slow speed (60 hz sample rate) module that plugs into the computer, so I'm not confident that I understand the whole picture.

Former User

The VSS plug is a common problem, so that's my best guess. Some members get a replacement at a junk yard and splice it in.

redbull-1

Here is a link if you want to read more about a VSS:

http://techauto.awardspace.com/vss.html

PGHRich

Redbull, that's EXACTLY what I was looking for! Thanks for that link. Now off to find a better oscilloscope and improve my diagnostics. I'll post results when I know more.

PGHRich

The oscilloscope showed that the pulses were a perfect square wave for 20 minutes of driving. Then pulses started dropping out intermittently, then pretty quickly every fourth pulse dropped out. After a little more driving it started dropping out two pulses in a row out of every four. Then three out of every four, and finally they were all gone.

I checked the gear shaft, and the gear and pinion are both steel and in excellent shape - clearly not the problem.

The only thing left is the sensor (the brand new sensor) doesn't like the heat. I went to napa and bought the balkamp model and after one half hour drive it was giving proper speed indications. Maybe it's too early to call it a victory, but it's feeling pretty good right now.

To summarize, if the speedometer gets twitchy after about 15 minutes, then reads progressively lower speeds (when you are traveling at a constant speed) then you may have a bum speed sensor that is failing due to heat.