Hello everyone, I am new here and posting for the first time. I have a 1971 chevy c10 with a 5.3 ls1 motor and 4l60e transmission combo. I bought the truck from the a guy who did the conversion. I have a cablex installed but cannot get it to work properly. The speedometer is all over the place. It will sometimes peg out when I put the truck in reverse. Sometimes it will show that I am doing 25-35 mph when only going 5 mph. Even when I am going a constant speed of 50 sometimes it may show I am doing 60 or 80. There is no consistency. I also notice that when I turn on my headlamps or turn signals the speedometer will peg out or fluctuate. The cablex passes the "field test", 16 mph for twenty seconds, which supposedly means the box and cable are functioning properly. By the way this is the newer 30 gear unit. The ground is directly to negative terminal on the battery. The power is from the fuse block by the drivers footwell. The signal is from the pcm using a psi harness. I live in meridian mississippi and not many people are familiar with this box/ls motor conversion. I have called abbott and they claim it's a signal issue and to get the signal directly from the vss. Although I am reluctant to do this since I have read that people have had issues with shifting after splicing into this wire. Psi the wiring harness company also says not to splice into the vss. I know my power supply is a constant 12-13 volts. The battery itself is grounded only to the engine block and not the chassis/frame itself. Could this be an issue? Any tips or suggestions would be greatly appreciated. Also is there anybody near here that is familiar or has installed multiple boxes with success? I would be willing to pay someone to get it working.

 

Wouldn't hurt to ground the engine to the frame. Also don't hook it directly to the VSS, you can run it off the PCM signal. Make sure its set correctly at 4000ppm.and that the tune is right.

 

I agree with ls1nova71, don't splice the VSS.

Does the Cable-X have a "high" and "low" sensitivity input or selector? I've seen issues similar to that when there is a high sensitivity input used with our (at least Gen III) PCM's to an aftermarket speedometer conversion box (aftermarket gauges, digital dash, etc.). The symptoms I've seen were similar to yours but also included displaying a reading while the vehicle was stationary, speedometer following the engine RPM when sitting still and vehicle speed increasing or dropping at random times.

Welcome and good luck.

 

The engine block is grounded to the frame. Would additional grounds from the battery to the frame and body be helpful?

 

Can't say if it will help, but certainly won't hurt. What PPM do you have the box set at?

 

The box is set for 4000ppm. Switches 6,7,9, and 11 are "closed". I guess I will try the additional grounds. By the way, the box does not have a high or low sensitivity input setting. There is a dip switch labeled number 12 that can be used with "weak" signal or something called the " Hall effect". I have tried activating it, but there is no change. By the way, the speedometer will sometimes move when the vehicle is stationary. Appreciate the tips, keep 'em coming. Again if there is someone that is competent with electronics and has had success with getting the box to function within a 100-150 miles from Meridian Mississippi I would be willing to make the trip.

 

I've had 4 cars with Cable-X's and never had this kind of problem from one of them. Are you sure you're getting a clean speed signal to the box? Do you have a scanner or tuning software that will read real time data that you can use to monitor the speed while driving to show what the PCM is putting out?

 

I agree with ls1nova71, using some scanning software to see what the PCM sees could help.

I know it sounds odd, but have you checked your wiring to ensure that nothing is shorted?

Is the speed output wire to the Cable-X box run near one or more of the coils or plug wires? This may cause interference. Are your VSS wires from the PCM to the transmission twisted? They should be from the factory to cancel unwanted extraneous noise.

Hopefully some of what either of us said helps.

 

Make sure in addition to the battery to engine block ground, the block to frame and the body had a ground, lots of people forget about the body ground! Id be sure you have the correct VSS wire connected up and that its not routed near any types of interference as noted above. Also get a scanner on it and monitor the VSS see if the PCM is seeing what your seeing.

When you say it pegs out, does it jump and come right back? Or is it pegging and staying there?

 

Thank for the help. After you guys kept mentioning a "clean signal", I noticed there were two butt connectors used to connect the signal wire from the PCM to the box. The guy who was doing some other work on the truck mentioned that butt connectors can cause resistance and shouldn't be used on a signal wire. He soldered the connections and put some heat shrink on the connections along with adding an additional ground from the battery to the body and voila! A functioning speedometer. The speed appears to be off by about 2mph at approximately 50 mph when comparing to a gps app on my iphone. I am willing to live with that. I assume that has to do with the tune with the tire size possibly slightly smaller. I didn't realize until recently that the same size tire from different manufacturers can vary by upto 1/2 an inch in diameter.

 

Glad to hear you got it working. If you're only off by 2mph at 50, then that's probably as close as you're going to get since you're working with a 40 year old speedometer that probably was never as accurate as a GPS is now days. My experience is you can get them close, but the speedo and odometer are usually never the same, meaning if you get the speed correct with your GPS, then the odometer will be slightly off. It's the cost of using original parts, but I for one prefer the look of stock gauges over aftermarket.