Could my ECM on my 84 with the 4 +3 cause to Od to not engage?

Thanks,

Hugh

 

Yes.

 

If the dash light light comes on, the ECM and ECM inputs are all working fine. The problem would be in the trans itself, or the output signal to the trans.

If the light doesn't come on, it's probably a bad switch or bad connections at the relay, or your coolant temp is reading below 176F. The ECM is rarely to blame.

 

The light comes on. The OD is doing it's own thing, sometimes it's on and sometimes not. My Mechanic has over 25 years in the business just opened his own shop. I work with his wife and so far he sounds like he knows what he is doing. He was the one who told me that the relay on the fire wall could be bad and that was before he saw the car.

If it is the OD I will drive it till it goes. It's just a toy for the summer weekends.

Any other thoughts?

Thanks,

Hugh

 

I'm assuming you mean the light is steady on everytime you hit the switch?

Print this post out and bring it to your mechanic. It doesn't really matter how long he's been in business if he doesn't have intimate experience with how the 4+3 operates.

There is nothing wrong with the relay. The OD indicator light is on the same circuit as the overdrive itself. The light going on means the ECM and all control inputs are working and trying to turn on the overdrive.

But the overdrive isn't getting the signal or building enough pressure to shift. This could be:

1. Pinched wire between relay and overdrive unit.
2. Bad connection at side of overdrive casing.
3. Low fluid level (pump sucking air).
4. Filter or grommet loose/damaged (pump sucking air).
5. Failing internal pressure switch.
6. Failing internal solenoid.
7. Mechanical failure of overdrive (In which case you're f*****)



If you're sure the fluid level is good, then diagnose it as follows...

Make sure trans is not in 1st gear.

Jumper ALDL pins A+B and put the car in diagnostic mode. This will force the overdrive circuit on and the indicator light should come on. (The cooling fan should also come on and the check engine light will flash during this.)

Then check for 12V at the plug on the side of the overdrive casing. it's the single wire one between the reverse switch and the speedo sensor.

If that has 12V, then you have to pull the overdrive pan. There is a low pressure safety switch in here. Jumper it and see if the overdrive solenoid clicks and moves. (If the piston is loose, it has no power or is dead.) Don't leave the solenoid powered to long or it will overheat. If it works, then the trans is not building up enough pressure or the pressure switch is bad. Removed the pressure switch and test it with compressed air. It should have continuity at 65 psi and open back up under 40 psi.

Also check the filter and grommet and pickup tube very carefully. If they aren't installed correctly the pump can suck air which makes the OD cut out.

If none of this fixes it, your overdrive is probably on its way out, and you should research your options for replacement or converting to something else.

 

Thanks for the info. I will show it to him He did mention somthing about hooking up a jumper wire.

Hugh

 

About the OD light, when I hit the switch the light does not come on untill the engine is hot. Is that the way it should work?

Thanks,

Hugh

 

The 84 OD unit won't operate with coolant temps under 176 degrees.

By any chance, do you have a 160 degree thermostat in the car??

 

Are you sure about the temperature? My '86 would engage the OD with the stock PROM at 120 degrees.

 

It varies by year. Later years are 122F. 85 is 140F.

84s will allow the OD on when coolant reaches 176F. One critical reason you don't want a low temp stat in a '84 4+3 car.

But if it trips out from low coolant temp, the dash light will go off. So this shouldn't apply to his problem in the first post.

 

CC, this is a question I'd probably have, as I'm sure the OP and possibly the mechanic, is: in one sentence you say "it isn't the relay", in another you say in #1, " a pinched wire between the relay and OD unit". My question is this....could not a bad, or sticky relay give the same results as a pinched wire between the relay and the OD unit? I've seen, and tested many a relay that tested good until it stuck (sticky relay) then it failed, at least momentarily...a relay is cheap..$10-15. why not replace that first to eliminate a cheap possibility? Have I missed something?

 

To be honest I have no idea what thermostat is in there and I really never looked at the temp when the od light came on. I do know it comes on after driving for a few miles.

Hugh

 

No. The dash light comes off the same pin on the relay. If the dash light works, the relay works. It's easy enough to swap with the fuel pump relay next to it, but won't help any in this case.


Same for the thermostat and coolant temp. If the dash light goes on, the coolant temp is satisfied.

 

CC, I thought the '84 light came from the OD Switch?

 

Nice Ray wish I understood it. Have not read a schematic in over 30 years, forgot most of what I learned.

Hugh

 

BY FAR the most informative explanation and proceedure on how to track down O/D problems.

i've tried almost everything on mine and i'm pretty sure either my pressure switch or my solenoid have gone bad in my 85 4+3. I've had the car for about 5 years and only recently really looked into the issue. last year the o/d would randomly turn on and off about 2x per second while driving down the road or highway and then eventually just not kick on at all.... i went over an old post and found a place that supplies the momentary switch for 15$ purchased that instead of the 180$ corvette specific resale'r normally charges before really taking the shifter apart and seeing what the problem was.

the previous owner took horrible care of the car and the connector @ the transmission was replaced due to damage, the relay's were all bad and when i took the shifter apart i found that my switch was perfect but one of the lead wires in the harness was cliped to a point where a small thread of plastic insulation was the only thing holding it together. thank you snap-on dvom!

anyways to make a long story short i shorted aldl pins a and b got an over drive light on dash and heard a click from engine compartment, got under car w/ friend in drivers seat holding pin and ready to put ignition to on checked for 12v @ the switch outside of the tranny and got 12v.

needless to say i'm positive it's either my solenoid or pressure switch. i also took the car for a 107mph run down a local interstate w/ no o/d engagement..... anyone got a solenoid or switch they want to ship and recieve some paypal for??? pm me or send me an e-mail!!

to the rest of u please please run these tests you will definately find your problem with a lil time and patience... and seriously i'm not patient and it took me all of 2 45min trips to the garage to pull the shifter apart and to test under the car. goodluck to everyone with this problem. in the meantime any idea's or parts that anyone might have to help my situation hit me up!!

 

I don't hold a candle to centralcoaster on this stuff, I'll say that up front, and I'm working off of memory, that being said...

I don't think the 85 used a momentary switch, putting that in kind of negates your observations. When people with later years are setting up to have overdrive on demand they want the 85 switch because it isn't momentary.

There are sensors for 1st gear, speed, and temperature and I don't know what all. Any bad sensor could cause the overdrive not to function, you described bad wiring situations...a short to any sensor driving down the highway could cause the in / out OD situation you describe.

If I recall correctly, to bypass the computer on an 85 you connect the wires to c7 and a7 to eachother, I believe one is brown and the other is black. Leave enough of a pigtail on the computer side so you can reconnect them in the future. Personally I don't see a reason to reconnect them, but no point in burning bridges. By doing this you will have overdrive whenever you want it, some common sense is required as you can now have OD in first gear (4+4 instead of 4+3) and without regard to temperature etc.

Any $2 on/off switch can control it. I put on an aftermarket shifter **** and mounted a switch in the console. I think the switch will appear to work backwards, open circut (off) = overdrive.

I believe the pressure switch is hardware not software so to speak, if it's bad bypassing the computer won't have an effect on what's happening.

Get more input from others and double check the wiring before proceeding if you decide to try this route, I'm pretty sure I have it correct, but I wouldn't go do it to my own vehicle again without double checking, so don't you either. LOL

Good luck.
Mike

 

"When people with later years are setting up to have overdrive on demand they want the 85 switch because it isn't momentary."

On the models with momentary switch the switch only switches the trigger signal to the ECM ; not the relay directly as it does on the '84.
Push once ON , push again OFF.

 

Hi I have an 85 which may be alike the 84 when it comes to OD.
Yes the ECM controls the OD. But there are some weak links in the chain.
The OD relay and the wiring to the OD solenoid. If the relay dont engage you will not have the OD Engaged sign at the dash. So check out the relay and that you get signals to the socket of the relay and that you have the right voltage at the OD solenoid. The connector there can also be bad