67ls1camaro

Anyone had a problem like this?
I've got Classic Instruments Speedtacular speedo with an LS1/T56 in my 67 Camaro. Complete custom harness from Speartech. When backing or when coming to a stop, the speedometer needle jumps wildly and reaches full scale before dropping to zero. I think I've got all CI's sensitivity DIP switches set correctly and I even added a noise filter to the power. The problem persists. If I shut the engine off and coast to a stop with the IGN on, there is no problem. I know it sounds like engine noise, but I thought the filter would fix it. BTW, I ran separate ignition power for the speedo and the tach.

OldSchoolFormula

I have the same exact problem using an automatic and Stewart Warner gauges. Noone has given me an answer on what it might be. Mine won't settle back to zero though, it continues bouncing wildly between 120-1160 MPH or so until the car moves at a speed greater than 10MPH. I also can have the speedo act normal upon shutoff of the ignition.

bczee

iTrader: (12)

I have a friend with Dakota Digital in his 57' Chevy.. He has problems with the speedo also.. 1) reading 90+ when doing 30, 2) Speed jumping around in different speeds, 3) not going to zeor at stops, etc. From what other people else said when he talked to other owner of 57 chevy's with the same dash.. almost everyone has the same problems.. He is trying to work with Dakota now to resolve.

67RSCamaroVette

iTrader: (63)

i had that problem, it was a bad ground at the back of the engine head. I'd check the grounds for sure.

OldSchoolFormula

I've tried moving grounds, adding extra grounds, running seperate grounds... and the same for power. But hey, if the grounds work for soemone else that would be great. Give it a shot.

dhutton

iTrader: (1)

Sounds like it might be noise or bad pulse shape on the speedo connection. Two things come to mind that I might try:

1. Place a large resistor (47kohms) to ground on the speedo connection.
2. Place a small capacitor (not sure what value but try 100 pF to start) to ground.

These suggestions might also make it a little worse but that would give an idea of what the problem is.

Try to find someone who knows about electronics to help you with this.

Best thing would be to find someone with an oscilloscope who could take a look at the speedo signal and power lines for you. I'm in Dallas and if you are nearby could take a look at it.

Don

67ls1camaro

Here are the things I'm going to try:

1. Set another car battery on the passenger floor. Supply power to the
speedo using this battery. I kind of expect to see the same problem since it goes away without the engine running and coasting to a stop with the ignition on.

2. Maybe run a separate wire from the transmission (T56) vehicle speed sensor so the signal doesn't go through the computer. Anyone know if the computer changes this signal?

3. Take the car to a local fuel injection shop. He says he's got some kind of scan tool (apparently not the normal just read the codes tool) that we can connect while driving.

BTW, I've got good grounds.

Any other ideas?

G-Body

iTrader: (4)

The computer does change the signal. The transmission puts out a 40 pulse per revolution sine wave AC signal. The output from the computer is a 4000 pulse per mile square wave DC signal. Some speedometers are will work on either signal, while others will only work on one of the signals. From what I have read it looks like more speedometers work on the 4000ppm output from the computer.

67ls1camaro

So really it seems the best way to find out what's happening is an oscilloscope. I think there are scopes for laptops. Has anyone used one of these? I'm a little surprised the guage companies haven't addressed this. Maybe I'll send an email.

OldSchoolFormula

So far it seems as though only of us who stray from Autometer seem to have problems. I wonder if Autometers are less sensitive to a variety of signals that these speedo's can pick up, or if the ECM sends out much faster pulserates for accurate speedometer readings at low speeds. Does your speedoemeter correct itself at a certain speed, say 10MPH?

OldSchoolFormula

Try anything?

67ls1camaro

Haven't tried anything yet.

Yes, the speedo starts tracking at speeds above 10 or 20 mph. The problem appears to be only at low speeds with the engine running. Or when backing
(at low speeds).

OldSchoolFormula

Makes me really wonder if the computer sends out really rapid pulse counts at low speed for accuracy/smoothness for the stock gauge. I wonder why it seems that the autometers don't pick up on it.

OldSchoolFormula

Anything? I get nervous everytime a cop gets behind me because I have no idea how fast I'm really going..

Rodder

iTrader: (2)

http://www.stewartwarner.com/Catalog/espeed5.html

Looks like Stewart Warner makes two models of speedo, and neither works with the stock GM PCM output. One works with a sine wave, and the other works with a minimum of 16,000ppm square wave. If you have the sine wave model, you probably need to tee into the VSS sensor on the trans. If you the square wave model, you might be able to tweak the speedo calibration in the PCM to get enough ppm to stabilize the SW gauge. Tweaking the PCM that way may have a bunch of side effect--I'd me more comfortable with a sine wave unit tapping off the sine wave, or using a converter box like Dakota Digital's SGI-5. I haven't use an SGI-5 and don't know how well it works, but it sounds like it might be the solution.

OldSchoolFormula

Bought an SGI-5, and it didn't seem to improve anything. If I tap into the VSS sensor, the transmission doesn't shift.

As for being compatible, http://www.sw-performance.com/products/CTGElecSpeedos these guys all are supposed to be. They've revamped their site so I can't find the instructions on it anymore, but they accept both square and sine wave and 4000PPM. At least they are supposed to. The speedo works correctly with the engine not running but the car coasting and again, perfectly at speeds above 10MPH. Definately odd. I'm about to just go ahead and pick up a magnetic pickup kit for the driveshaft and be done with it. I just wish I knew why the PCM was sending out so many pulses, or why the speedo is couting pulses that it shouldn't be. I don't think the PCM believes its doing 140-160MPH sitting tere idling because the coolant fans will kick on, and they shouldn't if the car was going that fast.

Whatta PITA.

hotrod51

iTrader: (1)

I have had this problem with different speedometers, but mostly with Classic Gauges. I talked with the tech at CG and was advised to power the speedo from the accessory side of the ignition switch and run a separate ground for the speedo. It sounds crazy, but I did like he said and no more jumping. I usually have to run a pulse translator on CG but not on other brands. Dakota Digital makes them, even if you order it from Classic. I have the best luck with Autometer and VDO. Hope this helps.