upgrading to a 4l85e from a 4l80e

CanyonCrazy · Newbie Joined: May 16, 2008 Posts: 2

I have a 1993 K2500 suburban with the 4l80e transmission. I would like to upgrade to a new GM performance parts 4l85e transmission.

My questions are:
1) Will my PCM/TCM, designed for the 4l80e, control the 4l85e, or will I need a product such as your transmission controller to interface. I was told that in 1994, the 4l80e was upgraded to a different force motor which uses a higher frequency from the PCM/TCM. I suspect that the 4l85e uses this force motor. I was also told that the force motor can be pulsed over a wide range of frequencies, and that the lower frequency from earlier PCM/TCM will still work fine on the 1994 and later tranny. Not sure though.

2) Photographs of the 4l85e transmission show only one active speed sensor on the side of the tranny, toward the front. The rear speed sensor, just above the wiring harness plug, is blanked off with a dummy plug. How would I adapt either my PCM/TCM or your transmission controller, if needed, to one less speed sensor.

Any help that you can give will be greatly appreciated!

Russ · PCS Employee Joined: May 12, 2004 Posts: 286

You should be able to upgrade without any issues. You will need both speed sensors. Just make sure that the reluctor ring is inside the transmission before you purchase it. You can always take the speed sensor from your transmission and install it in the 85e. The force motor frequency should not be an issue.

Russ

CanyonCrazy · Newbie Joined: May 16, 2008 Posts: 2

Hi Russ,

Thank you for your help.

I just received the 4l85e transmission from GM perfromance parts. This is the 4x4 spec transmission, as I am putting it into my 1993 4x4 3/4 ton suburban (replacing the OE 4l80e). Sure enough, the transmission came equipped with the input speed sensor, but has a dummy plug in the output sensor port. After a quick trip to the local chevy dealer I was told that the 4x4 trucks equipped with the 4l80e used the input speed sensor on the transmission, and that the output speed sensor is mounted on the transfer case tailshaft, and that a dummy plug had "always been installed in the output sensor port". But on my '93 4x4 truck with 4l80e, there are a total of three speed sensors plugged into the wiring harness: the input speed sensor mounted on the drivers side front of the tranny, just behind the bellhousing; the output speed sensor mounted toward the rear of the tranny just above the wire harness plug; and finally the vehicle speed sensor mounted on the transfer case tailshaft. The new 4l85e tranny only has the input speed sensor, with a dummy plug in the output port, with no reluctor ring. I guess that GM phased out the output speed sensor on the 4x4 trucks at some point after '93, using the vehicle speed sensor from the tranfer case to take the place of the output speed sensor.

Do you have any idea what year this took place? I am faced with a problem: What to do with the plug for the output speed sensor which is used on the '93 year model, but apparantly phased out at some point after '93.

I hope to have the unit installed and on the road this weekend. Then I can see if there is any effect. I plan to put the output speed sensor from my 4l80e into the dummy port on the 4l85e, even though there is no reluctor ring, just to see if there are any trouble codes. If so, I may splice the vehicle speed sensor output (from the transfer case) into the output speed sensor wires to 'fool' the computer into thinking there is a functioning output speed sensor.

Anyway, If you can shed some light on this issue I'd appreciate it. My local GM dealer is giving me conflicting info, and I can't seem to find a similar thread on my internet searches. Maybe it is a non-issue, and the tranny will shift just fine without a functioning output speed sensor.

Thanks!

Russ · PCS Employee Joined: May 12, 2004 Posts: 286

I fairly certain your going to need the speed sensor. You could take the reluctor ring off your old transmission if you still have it. I checked our service manuals all the way up to 2001 and all the 4L80e equiped trucks have the speed sensors before and after the transfer case.

Russ