BUSY BOLTERS Are you one?  The Shop Area
continues to pull in the most views on the Stovebolt. In August alone there were over 22,000 views in those 13 forums.
| | Click on image for the lowdown. 
====
| | |
0 members (),
557
guests, and
1
robot. | | Key: Admin,
Global Mod,
Mod | | | Forums66 Topics126,777 Posts1,039,270 Members48,100 | | Most Online2,175
Jul 21st, 2025 | | | | Joined: Feb 2001 Posts: 95 'Bolter | | 'Bolter Joined: Feb 2001 Posts: 95 | I recently purchased a '64 C60 with 292, SM420, Hydrovac brakes, and two-speed rear axle. I'd really like to get the speedometer working. I have searched for information to no avail.
The speedometer cable routes as follows: Speedometer to small aluminum looking piece (inside firewall between pedals) out the cab to the transmission. The small aluminum piece has a wire terminal with no wire connected to it. I assume this is to work in conjunction with the two speed rear axle so when shifted the speed will read correctly.
Does anyone have a wiring diagram for this small unit? It looks to be original GM and bolted the firewall
| | | | | Joined: Mar 2004 Posts: 6,189 'Bolter | | 'Bolter Joined: Mar 2004 Posts: 6,189 | The speedometer speed adaptor requires 12v to shift to HI range. Find the HI range wire going from the shifter switch to the electric shift motor on the rear and tap into it to power up the adaptor, that's it...the adaptor case is grounded to the fire wall. If you follow the wiring from the gear shift lever to the rear you will likely find where the tap was originally done somewhere under the floor. Tap back in with a new wire and route it up to the adaptor. Mike B  | | | | | Joined: Feb 2001 Posts: 95 'Bolter | | 'Bolter Joined: Feb 2001 Posts: 95 | Mike, thanks for the point in the right direction. However this truck has the vacuum actuated rear two speed axle and no switch or cable on the transmission shifter.
There is the remains of a cable coming off the original vacuum "switch" under the cab, so my axle remains in "High" all of the time.
Getting the rear axle to shift is also on my to do list. Any recommendations on the best way to accomplish this will be appreciated. | | | | | Joined: Jun 2011 Posts: 5,096 Crusing in the Passing Lane | | Crusing in the Passing Lane Joined: Jun 2011 Posts: 5,096 | Probably not ever need the low end of the 2 speed, so don't worry about it for now.
'37 GMC T-18 w/ DD 4-53T, RTO-610, 6231 aux., '95 GMC running gear, full disc brakes, power steering, 22.5 wheels and tires.
'47 GMC 1 ton w/ 302, NP-540, 4wd, full width Blazer front axle.
'54 GMC 630 w/ 503 gasser, 5 speed, ex fire truck, shortened WB 4', install 8' bed.
'55 GMC 370 w/270, 420 4 speed, grain, dump bed truck from ND. Works OK.
| | | | | Joined: Mar 2004 Posts: 6,189 'Bolter | | 'Bolter Joined: Mar 2004 Posts: 6,189 | I wonder is someone in the trucks past wanted to add the electric adaptor after the vacuum one attached to the back of the speedometer went bad. All you would need is a switch on the dash with 12 volt keyed power for manual use... Chevrolet always recomended trucks be parked in Low Range...had so oldtimers tell me the 2-speed rears could drop into neutral (between gears) and free wheel if the truck got bumped while parked. Hi Range is UP on the actuator on the rear housing and gravity could pull it down. I don't know it this is true or not, but I park mine in Low just in case it is. If I had a truck that would be in HI forever, I would somehow lock the rear shift linkage back at the rear so it couldn't move. I would hate to hit a pothole and have it fall into Low Range while I was traveling the country side... Mike B | | | | | Joined: Feb 2001 Posts: 95 'Bolter | | 'Bolter Joined: Feb 2001 Posts: 95 | I plan on fixing the two speed rear soon as the worn 292 doesn't like to move the truck with the dump bed loaded.
I removed my instrument cluster and there isn't a vacuum switch on the speedometer. I looked at another '64 C60 with two speed rear today and it has the same aluminum device bolted to the inside fire wall behind the pedals and no vacuum switch on the rear of the speedometer. The funny thing is, there was not a wire hooked to this one either! This one also had only a cable with a knob going from the vacuum switch up to the transmission shifter, no electric switch to operate the speedometer device could be seen.
Did GM make shop manuals for 60-66 Medium Duty trucks? All I have found are for the Light Duty trucks. | | | | | Joined: Mar 2004 Posts: 6,189 'Bolter | | 'Bolter Joined: Mar 2004 Posts: 6,189 | It would be nice to see a picture of the part in question... You need to start with the 1963 Truck Shop Manual (one book covers ALL trucks), that's the base line and then get the supplement for 1964 which will have new itens not found in the 63 book. Mike B
Last edited by Mike B; 10/16/2015 3:46 AM.
| | | | | Joined: Oct 2015 Posts: 7 New Guy | | New Guy Joined: Oct 2015 Posts: 7 | Good info here. I got lucky with mine and the speedometer works and I got the original manual with my 66. Good info in it but covers a lot of vehicles so is a bit vague in spots. Good luck and keep asking questions. Lots of good help here | | |
|
|
