4L60E / 241 Speed Sensor Burnout Help!!

[Midnightburn]

Hi,

I am new to this site, but I am an experienced mechanic, fabricator, and automotive engineer. But I have risen to my level of incompetence. I need some help to figure out what is causing my Speed Sensors to burn out. This is very difficult to figure out and I need someone who is an expert on the subject.

Some details as a foundation: I built a 62 Willys Wagon recently and transfered all components from a 95 Chevy Tahoe. 5.7, 4L60E, 241, and complete wiring harness front to back. The 4L60E is completely rebuilt professionally. Everything has been working perfectly since getting the project on the road about 4 months ago.

A few photos of the vehicle. Willys on top, but all GM underneath.





One day, while driving along at 60, I attempted to pass someone and pushed down on the gas pedal to pick up speed. The transmission up-shifted to 3rd as it should, but after that it never shifted back into 4th. I drove it the rest of the way home in 3rd. The next day, I went out to drive it again and all automatic functions were lost. It would start in 1st, but would not shift to 2nd automatically. I could shift manually to 2nd and it would work fine. However, if I shifted into 3rd or 4th, it would act as if I just shifted it into neutral.

Now, I know this is a well documented symptom of a burned out Speed Sensor. So, I bought a new one. It fixed the problem for about an hour then it burned out again. Of course, going to the auto parts store and buying Speed Sensors at $100 a pop is lame, so I went to the junk yard a few different times and bought a total of 4 so far at a $1 each. I installed a second one and it lasted about an hour much the same as the first. So, I checked all fuses, all connections, did ohms tests, etc. Everything checks out fine. No identified shorts and no opens.

In an attempt to eliminate components from the equation, I have replaced the PCM (with one from the junk yard with the same service number on it), I replaced the VSS Buffer, and 4 Speed Sensors. None of these components have resulted in success. After replacing the Speed Sensors each time, it works perfectly for about an hour as I mentioned before.

Here are a few details, as I understand them, that may help to understand the connections between these components. The vehicle speed sensor (VSS) sends alternating current (AC) to the VSS Buffer or DRAC (the little white box) at 40 pulses per driveshaft revolution. The VSS Buffer then divides and conditions the signal for the electric speedometer, ECM, cruise control, and anti-lock brakes. I don't currently have the speedometer signal connected to a gauge, so I can’t verify if that is working or not. I removed the anti-lock brake system, so it is completely out of the equation. And my cruise control has never worked, but is fully connected. Speedometer and cruise control failure is often included in reports from people experiencing the shifting problems detailed above. Most people also indicate the anti-lock brake light comes on erratically.

I have had the system scanned twice at 2 different places. Both times, it showed no Speed Sensor signal. No other codes were indicated.

At this point, I really have no idea what the problem could be other than maybe a short in some wiring, but I really don't think that's it. Not ruling it out though. There has to be something that is sending too much voltage to the Speed Sensor, causing it to burn up.

HELP!!! I need a true expert to answer this question.

If you can please respond here or give me a call anytime I would be grateful. I have read hundreds of forum posts on the subject, but none ever answer the question and in fact most are filled with comments like “Have you checked the air in your tires lately?” I need a real expert that is trained with GM systems.

Kevin – 208 863-1718

[Midnightburn]

I may be thinking wrong about the way the Speed Sensor works. I assumed that it receives a signal from the VSS Buffer, but that's not correct is it? In fact, isn't it true that the Speed Sensor is nothing more than an electro-magnet that sends out a very small Alternating Current (AC) that is generated from the reluctor ring spinning in front of the magnet? Is this true? If it is true, then it seems to me that there is NO reason that a Speed Sensor should go bad other than just breaking and/or getting old. However, if stray voltage is feeding into the Speed Sensor, then doesn't it make sense that it would cause it to burn out?

Here is something I discovered this morning. When I turn the key on, my brake lights are faintly lit, which they should not be. And out of sheer luck, I decided to unplug the cruise control module under the hood. The lights went out. I double checked it by plugging it back in a few times to verify and it consistently lit up the brake lights. It's very dim and hard to notice, but it's there.

So, somehow a backfeed is occuring through the cruise control unit and back to the brake light switch. I also noticed that the cruise control unit to PCM wire has the same low voltage. I am not sure if that is normal though. I assume the cruise control unit sends a signal back to the PCM via this wire, but I don't know. And I don't know if it should be hot when the vehicle is just sitting. There is no stray voltage in the wire heading back to the VSS buffer.

I don't know if this helps. And I don't know the relationship between this issue and burning out Speed Sensors. But maybe some of you experts can make sense of it all.

I really hope so.

[Midnightburn]

No luck. I put a new speed sensor in and unplugged the cruise control module. The speed sensor lasted about 15 minutes and burned out. Good thing I bought 5 of them today for $3.00. Although, they are dwindling fast.

I have heard that the throttle position sensor can cause this issue, but I don't see the connection in the schematic. It doesn't look like they are related in any way.

Help!

[Jimmiee]

No luck. I put a new speed sensor in and unplugged the cruise control module. The speed sensor lasted about 15 minutes and burned out. Good thing I bought 5 of them today for $3.00. Although, they are dwindling fast.

I have heard that the throttle position sensor can cause this issue, but I don't see the connection in the schematic. It doesn't look like they are related in any way.

Help!

You are correct in your assumption. The sensor is a signal generator and produces it's own signal which is an A/C sine wave on a very low A/C voltage signal. The signal is analog which needs to be converted to digital. This happens at the buffer or DRAC as it's called.
I have never heard of a sensor burning out? When you say it's burned out what do you mean? Usually the only thing that happens to these sensors is they go open from vibrations. There are no moving parts and no implied voltages. Maybe you have a short to voltage which is seeking a ground through the sensor or something like that.

[Midnightburn]

Thanks for the response. When I say the speed sensor is burning out, I mean that it works for 15 minutes then it goes bad. There are no physical signs on any of the sensors that indicates damage. And the weird thing is that I get the same 1100 ohm reading through a good one as I do a bad one. This is turning into something resembling fear factor!!

When you say "Maybe you have a short to voltage which is seeking a ground through the sensor or something like that." What do you mean? Can you give me an example?

I do assume that there must be stray voltage backfeeding to the sensor. But from where? I am totally lost at this point.

[Jimmiee]

Thanks for the response. When I say the speed sensor is burning out, I mean that it works for 15 minutes then it goes bad. There are no physical signs on any of the sensors that indicates damage. And the weird thing is that I get the same 1100 ohm reading through a good one as I do a bad one. This is turning into something resembling fear factor!!

When you say "Maybe you have a short to voltage which is seeking a ground through the sensor or something like that." What do you mean? Can you give me an example?

I do assume that there must be stray voltage backfeeding to the sensor. But from where? I am totally lost at this point.

The reason I brought up voltage is because you said they were "burning". They can not burn without voltage. <G>

The way the sensor works is as the reluctor wheel passes the magnet in the sensor it interrupts the magnetic field and causes it to collapse. When this happens the sensor produces an A/C signal.

To check the sensor you need to set your meter to A/C volts. Hook the meter leads to both sensor terminals and spin the wheels. You will see the voltage increase. Now turn the dial to Hz. Hertz is cycles per second. Now you see this number increase as you spin the wheels.

[azdrtdog]

do you have two vss units in that tran/tcase setup is the one in the tail of the trans case also doing this or just the one in the tcase i have seen the trans not used and just the one on the case feeding

[Midnightburn]

Solved!

For a second time, I loosened each ground wire on the engine block (2 on the intake and 1 on the back of the head) and cleaned the surfaces then retightened.

Then before doing anything, I decided to put in an old speed sensor (one that had previously "gone bad") to see if it would work and it DID!.

So, I put in another "bad one" and it shifted fine again.

A quick recap of what I think has been going on:

Prior to today, each time I would put in a new speed sensor it would shift fine and the scanner would read correctly. Then the transmission would quit shifting and the scanner would indicate that there was no VSS signal. I did this a number of times (4700 times or something like that) and each time that I would install a new sensor it would shift fine for about 15 minutes.

This of course told me that I was burning out speed sensors! Wouldn't it tell you the same thing?

What this tells me is that one of the ground wires didn't have a perfect connection, but it was enough to allow everything to work... Until it got hot!! Then things would change. And I suspect that the reason a new VSS would work each time is that they had a different density because they were cold (or room temperature), which would result in just enough of a ground to allow it to work. HOWEVER, even this was not consistent. Many times after it would quit working, I would let it sit overnight and try it the next morning. EVERY SINGLE TIME it would NOT work and I had to put in a new VSS to get it to shift. It's been like SAW IV around here!!!!

I am still not quite sure if the synopsis is correct, but everything is working correctly now. For now!! I will not be satisfied that this is totally resolved until I get to drive it for a few days.

This has to be one of the strangest issues I have ever experienced. And I have been building 4x4's for over 20 years.

[Jimmiee]

Glad you fixed it. Ground problems are the number ONE cause of sensor failure and trouble codes!