In the middle of an overhaul and I'd like to figure out why my speedometer reads only half speed. The odometer stopped working but when it was going it also read half the miles.
it's been like this forever. I had the 'governor' replaced (i think this is what it was... under that round cap on the drivers side of the tranny) and it didn't didn't change anything.
Have read a bunch about gear ratios and stuff but still very unclear about what to do.

The transmission is a 4 speed r700 / the vehicle is a 1990 chevy g20 van with a 350 5.7

Thanks in advance

 

Anyone out there have any puzzle pieces to throw my way? still trying to figure this out.
Added info: There is no Speedometer cable it's electronic. just replaced the speed sensor too and there's no difference.
have also tried two other inst clusters and all of them read exactly half.

 

Ricky, does the vehicle contain a DRAC module, and have you ever replaced this? I have a DRAC in my 1993 Chevy G30 chassis (Class C RV on top of this chassis).

If ever replaced, it may not be calibrated to your tire size and rear end gear ratio, which can dramatically throw off your speedometer.

Best,

Paul Mc.

 

hey good question, i'm not sure - never heard of a DRAC, but what little i just googled is very intriguing.
also just seeing this now for some reason so sorry for the crickets.
I'm digging into the electrical today to solve a few other ongoing mysteries and i'll be looking for this module. there's a few i still haven't been able to ID.
This 1/2 speed thing has been ******* me since i got the van and since my original post I've pulled two other instrument clusters from the junk yard and it's changed nothing. My rear tires are now 17" with a bigger parameter and still reads half.

 

Check to make sure you've included the rear end gear ratio, which is part of the puzzle, along with tire size. I'm not an expert on this, but once I got a correctly calibrated DRAC, taking into consideration tire size and rear end gearing, mine works to within 4 MPH of real speed. That's good enough for me.

 

so thanks to @paulmc for asking about the DRAC module. I found mine, did some research and was considering having it sent off to be calibrated to work right but instead i pulled one out of a junk yard and threw it in for the heck of it. i had some other wiring challenges to overcome but once I did it worked perfectly. First test was on some 15" tires but I finally got a broken wheel fixed and put the 17"'s back on and, as expected, it's reading about 2-4mph slower than actual but I'm more than fine with that.

i've been trying to figure this out for the better part of a decade (on and off of course ((and mostly off)) and can't hardly believe i don't have think about this stupid issue anymore! it was probably the most inconsequential repair ever but was impossible to ignore when i drove the van. always reminding me that after way too many hour of research and tinkering that have only failed to figure it out! haha. thanks again PaulMC, spot on!