88 Auto-trans OD prob/quest?

[hh65flyer]

New to me 1988 pickup. 4WD, extended cab with auto trans and 3.0 V6. 156K. Truck is pretty basic, roll-up windows, etc. No tach.

The OD switch doesn't seem to do anything other than light-up the dash light. W/out a tach I'm having a hard time telling if it's shifting into OD. I don't believe it is. Is there an easy diagnostic/fix or common problem here? Search did not yield any real results for me.

Sorry I don't have an Owner's Manual or FSM. The only other odd thing seems to be the temp gauge barely moves. I was thinking perhaps the truck wasn't getting up to temp and thus no OD. But when I got home I hit the t-stat housing with the IR and it read 173.

Any help wold be greatly appreciated! Thanks!

 

[pappy]

From my 1985 owner's manual:

Pushing the button down turns OD on. Having the button out turns OD off.

"Overdrive switch: When the overdrive switch is turned off, the transmission will automatically upshift from the first to third gear but will not upshift to the overdrive gear at all."

And as you surmised:

"However, if the engine coolant is below 122*F, the transmission will not shift into the overdrive gear even with the overdrive switch on."

 

[hh65flyer]

What's interesting is there is a new, in the box, t-stat in the glovebox. I'm wondering if someone took out the t-stat to 'troubleshoot'. Regardless, you would think a 20+ mile drive home at 60-65 would heat up the engine beyond 122...especially with no OD! It's in the low 50's here today.

 

[Mudder]

I found out if I click my OD on at 50MPH then let the gas go I can feel it kick down. If not connect T1 and E1 and see if there's any codes for it.

 

[pappy]

Make sure you have the rubber o-ring gasket for the t-stat.

 

[hh65flyer]

Removed a gutted t-stat (surprise) and installed the new one from the glovebox. We now have OD!!

I'm a little concerned with what seems like a a high operating temp now though. The gauge reads about a needle-width over center once warmed-up and climbs another few measures at highway speed. The t-stat (Napa) is listed as 195 deg on the box. Is that appropriate?

I checked over the radiator with my thermal temp gun and I don't seem to have any dead spots. I need to research running temps and maybe the accuracy of the gauge. For now just thrilled to have the OD working!!

 

[pappy]

The factory gauges are notorious for not being correct. Mine doesn't even work which is why I have an Autometer. A 195* t-stat should be correct for your truck.

 

[Mudder]

Mine goes above middle then slowly climbs back down.