by Richard Atwell
(c) Copyright 2005-2011


Behind the driver's kick panel on some buses (mainly CA models) is the operating time recorder (aka EGR Box) halfway down the speedometer cable . It is a mileage based counter that trips a switch at preset mileage intervals. When the switch closes a light on the dash comes on to remind you to go into the dealership to have your emissions system checked (we all know the EGR system rusts out since it's part of the exhaust and oxygen sensors have a limited life also).

Read the emissions notice in Owner's manual.

When the light comes, apart from being annoying, you may also fail your emission test if the tech believes it to be the check engine light (CEL).

To reset the EGR light remove the driver's side kick panel and press the white button on the box until you hear a loud click. If you only press the button half way without hearing a click the light will only stay off as long as you press the button. Some boxes have two button (more later).

You can reset the box at any mileage before the switch trips.


EGR part numbers EGR box side

I believe there were three versions of the EGR Box:

The large button always resets EGR and the smaller button if present resets CAT.


Here's a photo of a box from a 1978 bus.

EGR box back

Wiring terminals:

On 79 CA models, the CAT light is labeled OXS instead as a reminder to check the oxygen sensor. Earlier models only had a CAT but 79 CA had both. Each bulb (1.2w) is powered by the blue wire from the VR. When you turn on the key the light(s) should come on. When the alternator is engages the light will go off, unless the switch has tripped.

If you look at the wiring diagrams you'll find some variation between models:

Canadian models were free of all these hassles despite the EGR equipment on board thanks to differences in the regulations. I'm not sure about other countries but given the era I would guess this was a USA only requirement.


Unfortunately, just like the VDO odometer with it's plastic gears the EGR box can break similarly. The box itself is quite rugged but the rest of it is somewhat delicate like a clock movement. Any of the plastic tabs/teeth could become brittle and break off.

Electrically, little can go wrong except for the diode(s) that connect power to each light could conceivably burn out.

The more common problem is that the speedometer cable breaks and the two sections are both NLA.

The solution to reactivate your speedometer and odometer is to bypass the box completely with the non-EGR version of the speedo cable.


The box is a pass thru device for the speedometer cable. It uses familiar worm-gearing for reduction. Wheel rotations are input and the recorder trips when you hit a specified mileage.

EGR box gears reset buttons

EGR spring

The reduction gearing drives the two pawls which slowly crawl the teeth on the ratchet wheel. There is a cutout on the wheel which the pawls eventually fall into and pull the switch on the right closed.

When the rest button is pressed the pawls are pulled out of the cutout and the spring rotates the ratchet wheel back to the beginning which is controlled by a plastic stop.

There is space behind the ratchet wheel for a second wheel.

Mileage recorders:

So exactly when does the recorder trip? The exact moment depends on your tire diameter but let's say they are 26" tall. First we have to compute the reduction by multiplying all of the worn-gear ratios:

There are 73 ratchet wheel teeth from rest to the cutout:

31:1 x 16:1 x 20:1 x 16:1 x 73 = 11,586,560.

This is the reduction factor! Now we can determine when the box trips. First, how many inches are in a mile:

12"/ft and 5280 ft/mile = 63360"/mile

Now let's compute the tire circumference:

1 tire rotation: 26" x 3.141592 = 81.68"

The number of tire rotations are:

63360"/mile divided by 81.68"/rotation = 775.7 rotations

Now factor in the reduction by dividing the rotations into the gearing:

11,586,560 / 775.7 = 14936 miles.

So the EGR light triggers at 15k miles. The gear that drives the wheel for the CAT light has twice as many teeth so the CAT/OXS light would come on at 30k miles because the wheel is the same height and the cutout is also 73 teeth from the ratchet stop.


Wouldn't it be cool if you could use this notification system to remind you to change your oil?

  1. Change your oil, reset the box and make note of your mileage
  2. Drive 3,000 miles
  3. Remove the box, take the 4 screws off the back and mark where the pawls are engaging the ratchet wheel with a marker.
  4. Remove the circlip that holds on the wheel and make note of how the spring is wound.
  5. Add a 2nd cutout where you marked the wheel. It has to be similar to the original and you must make it wider clockwise from the mark for the pawl to catch correctly.

Reassemble and voila. Idea courtesy of Rob Taylor in Salt Lake (RAMVA).


09/22/05 - Created
09/08/11 - Fixed broken photos, added translate button, updated footer
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