Three or four months ago I installed a Jet Accu-Speed in my 2000 Blazer (4.3 4x4) to correct for slightly smaller than stock tires. I did this because I use the truck for delivery and I want the speedometer and odometer to be correct. I installed it exactly as the directions showed, with the exception of the crimp joiner which I soldiered instead.

The calibration was correct and the speedometer read correctly up to about 25 MPH. Above that it would quiver erratically and fall to 0. As I drove the vehicle more through start and stop cycles, the speedometer would slowly read higher to about 60 MPH. If I shut off the vehicle, the problem would start back at 25. It's as though the vehicle had to "learn" the device each time I drove it.

I talked to Jet technical support and they asked to send the device back. I didn't hear from them for some time, until today when a new module arrived. They didn't include a note indicating if they found the device defective or not, or any suggestions for possible problems. It appeared to be a new device, but when I installed it, the same problem occurred.

the computer appear to work correctly for all the other functions and when the device is not connected, it works fine (just reads off).

Anyone with any experience with these?
Thoughts?

 

No experience with jet accu speed, although instead of purchasing that to correct your speedometer, odometer you could have just hooked a scan tool up to the blazer and changed it to the tire size you have and it would be calibrated, and if you do not have a scan tool you could rent 1 from like orielly's

 

Shoot, wish I'd know that before, but then again, I wished I'd joined here way sooner that I did

Thank man, we just got some O'Reilly auto stores around here. I'll run out tomorrow and check it out.

It's not a huge difference, but enough to put the reading about 5 MPH higher than it should be. I figured that has added around 1000 extra miles since I bought the tires. The stock tires are a "75" sidewall while the ones I found were only "70". I bought them because I needed tires and I got a good inside on these.

The most annoying thing for me is the slight jolt at the shift points. Some people don't feel it, but when the Jet tool worked, I could notice a much softer shift.

Thanks again.

Sam

 

I've never see a decrease from a 75 series to a 70 series give that much problems. Mine is cable driven and I wouldn't even notice a change. Now when I stepped up to 31s I seen a jump.


Personally I think the speedo cluster in your truck might be screwed.

 

Ok, so I called O'Reilly and Autozone and both only carried the code reader for rent or sale. I need to "reflash" the VCM. I tried some performance shops and finally the dealer service dept. The service manager was very cool and called GM to ask if they could reprogram the VCM. They would give him the authorization code, however, GM said that the only way they could do it is if the tires are an approved stock size and the vehicle is completely stock. This would cost in the neighborhood of $200.

So, sounds like I'll just wait until I have $500 and get the HP Tuners software/hardware, the Hypertech Hyperpac, or some such system that can program the computer with the tuning I want multiple times.

Just to be clear, Im not spending $500 dollars on a speedometer change, but I've looked into this for some time and would like to get into reprogramming the VCM.

The discrepancy I see is 70 displayed at a true 65. Not huge, but enough to add faux miles, some weird shift points, and make doing the math annoying.

Thanks everyone who contributed ideas and experience.

Sam

 

Just a quick update, I found the problem. I finally reinstalled a radio (two way amateur radio rig) that had been off for service. It has a voltmeter build in that is 100x better than the dash mounted turd. There are wild voltage spikes from the alternator everywhere from 15 to 11.5 volt. The Accu-speed ran directly off the battery, and that could very well have been the problem. I'm going to reconnect the new unit, and with the truck running, unplug the alternator. I suspect that it will work fine running off the battery.

In the past week or so, I've been getting flickering in the lights and a whine has developed in the speakers. It's time for the AD244 upgrade. (Thanks again to the Blazer forums, I found the mod and some good write-ups here)

I'll update again once the new AD244 is in.

Sam