racedogg214

Ok yes, I have read all over this forum extensively and I have follow much of its advice. However I am having a really weird issue and wondering what could be triggering this or what can be done to eliminate it.

I have a 1987 C4 L98 with Doug Nash 4+3

To create manual control of the OD unit I have done the following: I have disconnected from the ECU and jumpered together A7 & C8 at the ECU harness.

I have also unplugged the OD relay and jumpered a couple wires on that relay to basically take the relay completely out of the equation. I should have full manual control at all times the way I am wired now. I have installed a switch on my dash that will initiate OD when I flip the switch.

This is all working famously EXCEPT....Once I get to over 60 mph on the highway the OD will come OUT of OD and into std 4th. If I slow back down to just under 50 mph then the OD will once again engage and let it run in OD 4th. At 50 mph or less and in all the gears the manual switch works like a charm for putting trans into OD and taking out of OD. This issue of the transmission coming out of OD on its own only happens at just over 60 mph. WTH??? Has anyone heard of such a thing? Is there something I am missing on my setup to have manual control of the OD unit? .

I am just completely at a loss. There is no reason it should be falling out of OD above 60 that I can think of. I have looked all over and cant find anything that seems to address this. Any help is sincerely appreciated.

Thanks much guys!

C409

..... Oil level and filter check on the OD unit ? .....

jv9999

It's likely losing pressure. There is a switch inside the OD until that disables it if that happens.

racedogg214

Yes they are perfect, in fact I just changed the filter and everything in the OD unit. All looked great. Nothing burnt smelling or anything. This one has been taken care of pretty well and apparently not abused at all.

racedogg214

I have heard of this switch and see it inside the OD unit. How do I verify if this switch is bad and needs replaced? Is it serviceable? Where can I get another one to replace it with?

Its also worth noting that at speeds less than 60 everything is fine. It seems weird that above 60 it would then lose pressure. If anything I would think it would have more pressure the faster you are moving.

I cant seem to find anyone that has one for sale anywhere. I would hate that 1 switch is keeping this corvette from being on the road.

jv9999

The switch is not serviceable and can only found used (with difficulty). If you jumper the white connector next to the drain plug to +12 and it still does it, than it's something inside the OD.
You can bypass the pressure switch to test it. As for how it can lose pressure at high speeds, it can either suck the pan dry or cavitate (suck air), either from being too low on fluid, the fluid foaming, or there being a leak in the intake path (filter).
Or it could be a valve body problem.

Billygoatkid

From what I remember when burning a chip, or changing the tune: There is a selection you can make that controls when and if it changes out of od. Its a look ahead based on speed. Have you done any modifications to the chip, other than what you said about jumper wire? I think the jumper helped you control the od , but to complete it you will either have to burn a new chip, or run a piggy back system to the computer like I have done so you can tune and tune again.
Mines a 88 4+3, with 3.54 rear.

racedogg214

I topped off the fluid today so it is not lack of fluid. When you say jumper the connection next the to drain plug, what am I jumping? the connection on the OD, or the connection the goes into the connector? I have no idea where i would find 12v+ down in that area. LOL

racedogg214

We have never touched the chip or changed the tune. The L98 is stock except for headers, and a heated O2 sensor. We initially started with it falling in and out of OD while on freeway. Hooked up to a scanner we found that it was opening and closing the loop. So we installed a heated 02 sensor and now it will stay in the closed loop. For a little while it worked fine. THen started doing it again. So then we jumped the ECU and combined wires at the OD relay and installed a manual switch. It works perfect up to 60 ish, then all of the sudden will drop out of OD despite be manually told to stay in OD. The chip has never been changed.

I do know that the speedo sensor is off by about 13 mpg, meaning at 80 on the stock dash you were really going about 67. That being said I didnt really care as I removed the stock dash and installed a custom gauge cluster replacement dash (looks badass). Speedo on new dash just calibrates manually.

I am so frustrated at this point ready to rip it out and put something else in it. However nothing is bolt plug and play swap. The only swap I found was a T5 but i do not know if that is strong enough. Just at a complete loss. What type of piggy back system do you run? or where would i burn a chip for this? Thanks again! BTW without the OD i am at 3000 rpm at 60mph so that is no good. LOL