F-Body Europe

Dude, that cluster worked great. I need to get the white faces for it though, it just doesn't look right with the black now. I took it out on the Bahn and everything seems the same. Thanks.

RT1

Glad to hear it. Now I know for future reference that a 150 speedo will work in a 110 speedos spot in a fourth gen. Now enjoy your 95,000mi car.

Not quite !

If you pay atention you'll see that the tacho is 25 percent slow ! At low rpm's you hardly see it but at higher rpm's you will. Very logical since your missing 2 cylinders, hence the 25 percent difference:)
Not a biggy though.

Been there done that back in '96.

I was talking with someone about the 200mph spedo. And from what I gathered it's completly plug and play. I just can't figure out how. I know when you change tire size you need to reprogram for the difference so the spedo will be accurate. So my here is my question.

1. False Data
2. If it's not false data how does it work.

I kinda messed with my mind when I was trying to figure it out. Wondering if someone could break it down for me.

The PCM simply sends a signal to the cluster that controls the speedometer.
This signal has a certain 'strength' that is converted by the speedometer to a certain speed.
The V6 and V8 speedometer are way different in scales, YET it DOES show the correct speed no matter which one you use !
The PCM value is simply converted to the correct speed apparently using a universal conversion method.

As long as ANY cluster uses that same method it doesn't matter how the scale is set .. 110 mph, 155 mph or even 200 mph ..

Btw. the actual speed is read in the transmission using the VSS (vehicle speed sensor), this value has upto 4000 pulses per mile. The more pulses the more speed.
The PCM uses the programmed gearing and tiresizes and then converts this to speed (mph).

Golly Sergio... Had to ruin it for me didn't you. I was so happy. Just Joking. I hadn't even noticest yet about the difference on the tach.

Randy... You think I put that thing in with 95K, it's back under warranty. HEHEHE

RT1

Thats funny, Ill bet if you took it in for service, they wouldent even notice. They would probley look at the odometer and be on there merry way.

Sergio, just a question, I have been pondering why I haven't noticed that diff in the tach yet. I am not a mechanic, but wouldn't 2K RPMs on a V8 be the same as 2K RPMs on a V6? I know that the power isn't the same, but they both would be turning at the same 2K, right.

RT1

I think it has to do with the voltage signal that the gauge is getting. There is not a manual tach, so it is just a digital signal.. Kinda like the aftermarket shiftlights need a signal booster to work on our cars... Sergio?

The signal is indeed digital and comes from a dedicated RPM sensor unit that simply checks the coils firing for each cilinder.
The V8 cluster tacho expects a certain amount of pulses for a rpm.
The V6 works the same but when you look at the comparison:
8:6 = 2 cyl difference out of 8 = 25% difference

Let me explain this.
Let's assume for a second that the V8 cluster sees for instance 8 pulses (each cilinder spark) as 1 rpm.
It thus takes the V8 8 pulses for 1 revolution (just hypothetical).
It however takes 6 pulses for the v6 to do 1 revolution, yet the cluster will only register 1 rpm when it has received 8 pulses so before it registers 1 rpm from the v6 you have already done 10 pulses !
This causes the cluster tach to be 25% slow (25% off, showing less rpm than the actual rpm's)

At lower rpm you will hardly see it, but at higher rpm's the 1/4 difference shows quite a bit ! 4000 rpm on the tacho is actually 5000 rpm !

People who have an automatic never had that 'rpm/speed/gear' interaction/comparison and simply don't have a good reference as you cannot tell actual rpm by sound.

Just put it in 1st and see to what rpm you can take that badboy, i bet it stops right at 4500-4600 rpm

Hope this explains, It took me about 1 week to think of this theory after reading the GM workshop manual ignition section over and over again

Wow. Thanks guys for putting me straight. It took me a couple of read throughs to get that, but that was just the level I needed it broken down to. I thought it would have been in the computer thingie, not the gauge cluster. Thanks again.

RT1