Diagnosing a speedometer failure on a 1999 Mercury Cougar

(Update) Bottom line: the VSS signal is too low for the instrument cluster. It needs at least 7.2V on the VSS signal wire at the instrument cluster for the speedometer to function. When the instrument panel is unplugged, the VSS puts out enough voltage. But when instrument panel is plugged in, it loads down the VSS signal voltage. Read on for the whole story. 


Original post
I just bought (Sept 2014) a 1999 Mercury Cougar that had a non-functioning speedometer. From what the previous owner told me, the speedometer stopped working maybe 6-8 months ago. It turns out that the last oil change sticker on the window was within about a thousand miles of the current odometer reading. The sticker was dated January 2011. Somebody has a poor estimation of time. The digital readout on the left side of the instrument cluster would sometimes read 1 or 2 MPH when set to average speed. But the speedometer needle was stuck at 0 MPH.

The first thing I did was check the OBD-II reading on the vehicle speed sensor. There was no code for a VSS problem. I can plug-in my bluetooth OBD-II and have the speed read out on my phone's Dash Command app. So this tells me the problem was not with my VSS.

Next, I checked the connections to the instrument cluster. When I unplugged the instrument cluster and plugged it back in and turned the key to on, the needles went to full and back to zero. The speedometer gauge itself seemed to be fine. So the problem was between the VSS and the gauge. The VSS to PCM was good because I could get data out of the OBD-II port.

The VSS signal goes to pin 9 on the white connector. It's a white/blue wire on my car. To test for signal there, I jacked up one front wheel, put it in neutral, and connected my volt meter between pin 9 and ground (frame). I rotated the tire by hand and saw the wire getting 12 volt pulses. That told me that the signal was getting to the instrument cluster.

The problem appears to be between the plug and the gauge. It sounds like I need a new instrument cluster. Ford part number 10849 for $338.46. For now, I'll just use my phone's app to tell me my speed.

While I was in there I decided to replace a mini instrument panel bulb that failed. The tiny bulbs are number 2721. For some reason, they aren't listed in the parts catalogues at auto parts stores.

I found an instrument cluster on ebay from a junkyard for $28 and ordered it. Unfortunately, its speedometer doesn't work either. It's not the gauge itself. It's somewhere in the electronics on the board. Does every 1999 Cougar speedometer fail?

Update 28 August 2015
I decided to try one more instrument panel, so I ordered one online from a junkyard. I was hoping, but getting ready for disappointment. The speedometer didn't work on it either. Sigh. I figured it was somewhat unlikely that all three had failed. So I did some more looking online and found a helpful post about the instrument panel loading down the VSS signal. Now I can't find that post to link to it.

From my calculations, my VSS signal at pin 9 on the white plug has an output impedance of 8.7 k ohms. That's too high to drive the speedometer. In case you're curious and want to check my methods and math, I rotated the tire until the VSS had a high signal of 11.34V without the instrument panel attached. I then connected a 3.95 k ohm resistor from the white plug's pin 9 to ground. That loaded the VSS signal down to 3.53 V.

I was getting about 4.9 V when the instrument panel was connected. Apparently the PCM is happy with a lower VSS signal voltage. That's why the OBD-II app would show the speed, but the speedometer wouldn't.

So I took a variable resistor and connected it in series between 12 V and the instrument panel pin 9. I'd scratch it across the 12 V terminal to get a bad square wave signal and see if the speedometer moved. I turned the knob until I found where the cutoff was. The voltage on pin 9 had to be above 7.1 V on my panel or it wouldn't move the speedometer. I then measured the variable resistor and it was 3.9 k ohms.

The speedometer operates by frequency, and the voltage of that signal needs to be above about 7.2 V for it to work. 

Now I need to figure out if a new VSS will have a low enough output impedance to drive the circuit. If not, then I need to install a signal amplifier. But first, I need to use my 3.95 k ohm resistor directly on the output of the VSS to measure the output impedance and make sure the problem is not somewhere else in the circuit.

I'll update this blog on the results. Feel free to leave a comment if you found this helpful.

Update 5 September 2015

I put a new vehicle speed sensor on it. Done. The speedometer works now.