Glow plug inhibit device

[Robyn]

Been noticing the glow plugs dont always come on in the 91 Burb when its warm.
The sucker will sometimes when hot and sometimes not.

Got to looking through a real nice GM wiring book on the R/V models
(Classics in the 88-91 Burbs, Blazers and crewcabs)
And after some careful studying I discovered that the glow plug controler has a circuit that is run through the ECM that shuts power off to the glow controller when the engine is warm.
Unfortunately when the engine is sort of warm this can cause a hard start condition.
I thought them suckers got smart and tossed the inhibit switch out.
The J engines dont have this feature and the glow will work any time the ignition is cycled on.
The C engines are running the inhibit relay as part of the computer.

I fail to see why they did this on just the C engines

Looking at this thing trying to figure out how to wire it up so the sucker will glow every time I turn the key on.
The 6.5 engines all glow every time you crank on the key.

Possibly modifying the coolant temp connections may fix this anomaly.
Just placing a temp sensor in the system and tossing it in the corner will allow the computer to see inputs but not be able to turn the stuff off and on. :confused:
The engine is controled via a DB2 so the only purpose for the computer is to power the Cold advance and fast idle on and to control when the egr functions.
Im gonna replace the intake with a J model manifold and open the epr and if/when the Banks goes on thats all going in the can anyway.

The 86 units had the CCA and FI controlled with just the little temp switch on the RH head. This opened when warm and shut off the advance and fast idle. Some models had the inhibit switch up top in the RH rear but that was easy to defeat with a short wire with two spades especially when it left you without glow on a cold engine :eek:


Trying to find an accurate wiring schematic of the 700R4 tranny for 91 has proven to be a PITA too.

I am getting ready to change the tranny out and would like to know just how they have the power flowing to the converter clutch and what the circuit in the tranny is supposed to look like.
My current tranny has a bad chatter in the TCC when it goes into 4th and you try to put power to it. Better when warm but not good. Time to fix it.


Too many years and miles on this old girl to have any idea on who has done what.
The tranny shifts later than it should unless you back off the throttle a bit. Makes me think that maybe someone used a gasser tranny in it.
The governors are different and the shift characteristics are somewhat different due to the torque and rpm differences.

[john8662]

HPCA Switch (fast idle and timing advance) is on the passenger's side rear head just like ANY 6.2L.

The inhibit switch is on the water coolant crossover pipe, passengers side as well, don't get the mixed up :rolleyes:

Not sure on the control of the controller, perhaps some resistance is in order.

Personally, mine works pretty good, I don't mind it not coming on.

My old-school 86 does the same thing.

For your transmission I'd suggest re-wiring the guts underneat the pan. From what I understand it'll just mean replacing the plug that goes inside the transmission case, and the routing of the wires.

The pins are in different places throughout the years. I THINK the 91 has more than one wire on this plug.

The 82-87 700's just had the 1 green wire, on one of the 4 pins.

But, which pin is is supposed to be?

Exactly!

[Robyn]

I was pretty sure the unit up top of the crossover was the coolant temp sensor????
The diagram calls out the "Cold advance, glow plug inhibit relay" and it shows it as being part of the computer accept the J engine which works like the older ones without the inhibit.

I have pretty well decided to set the 700 up with a single wire TCC solenoid and run it through a toggle switch on my auxilliary panel.
My tranny has the hydraulic valving to run the TCC already in it and it works fine.
I just want to be able to shut it off around town and also to stop the contant hunting/ on off on off crap that it does if you give it just a little power after it engages.
These systems were very fussy. This is one reason the TCC failed so often as they literally got beat to death with all the cycling.
I love the way the 4L80's lockup at about 50 and unless you romp them a lot they stay locked until you drop the speed back down.

My 700 will lockup in 2-3-4 and will stay locked in 2 under compression down to about 25 MPH.
The only caveat is that it wont drop out of lock when the tranny does a 3-2 downshift at about 30 mph under load.
Its a tad rough there too. The lockup will drop out in a 4-3 shift under hard throttle.

The engineering on the early units just left a little to be desired.

Being able to just turn the thing off under pulling conditions and around town will really help the longevity issues.

One can also remove the lockup gear and replace the convertor with a non lockup unit and install a special valve to reroute lube oil.
This is a marvelous setup and still retains the overdrive and stuff.
The non lockup convertors are bigger and tougher but are $$$$$$ :eek:
So with my unit having few miles on it I will go with what I have already.

[ToddMeister]

Robyn,

Just put a jumper wire on the Glow Plug Inhibit Switch connector, then the glow plugs will always cycle. Back when I had my 86 6.2, I had to do this, the switch got flaky and sometimes wouldn't close when the engine was cold, no glow cycle, engine would not fire up. Good mosquito control, however.

[Robyn]

The issue on the 91 is that the controls are done with the computer.
I dont have a wiring schematic, I have a diagram book and it does not really tell me what wires are hot and when.
The 86 truck had the inhibit switch mounted at the right rear of the engine on top of the head and you could just defeat it there.

This sucker has a coolant temp switch and the whole 9 yards.
The 91's were the beginings of what we have today.

As far as I can tell the computer runs the cold advance/fast idle, glow plug controller power, EGR/EPR, Converter clutch and the speedo.
The fuel management and the timing and such are all still good old DB2.
D Max maverick and I have been discussing how to get around this issue.

The thing thats making me upset is that sometimes the glow plugs will glow at start even when the engine is hot and sometimes not when the engine is just barely warm.
The book says its not supposed to glow on a warm engine on the C engine. The J engine will glow hot of cold >>>>>>>>?????????/

Could be a coolant temp switch issue as the CCA and Idle are still controled with the little switch on the RH head.

Thanks

Robyn

[john8662]

The 6.2's of this origin are a little bit different.

The glow controller isn't just the glow controller on these rigs, it also has another relay that's used in conjunction with the normal controller.

You'll find another small black box on the firewall by the hydroboost.

Here is my guess of how this works...

Coolant sensor on crossover sends valve to PCM, if certain value the PCM sends voltage to the auxillary relay on the firewall, that in turn sends voltage to the pin on the glow controller (that's used for the inhibit switch on prev controllers), then turns on the controller, when other factors are present (like power).

You could defeat the whole thing by shunting the inhibit switch wire on the controller just like the diagram shows for converting over to the 85+ controller.

This should make it more like the J series.

I don't know if you'll get an DTC in the PCM for this or not...

*PCM, because it controls aspects of the engine and transmission "powertrain".

J

[Robyn]

Thats the ticket.
My wiring book calls out the aux relay but does not tell me where it is on the truck. It tells me where everything else is accept that relay.

I will go searching now and see if I can find it.
A real schematic that lets one see what the circuits actually do would really help.
I hate these diagram books. Better than nothing butttttttttttttt.

Thanks

Robyn

[john8662]

Here is where it resides...

it's not on the firewall, but on the fender, oops...

Close ?

http://www.thedieselpageforums.com/photopost/data/506/relay.jpg

[Robyn]

Yup, thats the little monster.

I spent some time yesterday under the hood searching and found it.

I also spent about an hour learning how to navigate through the wiring book I got for the truck.

Once I got that figured out finding the parts was easy.

I am still at a loss as to why the C engines have the relay and the J engines dont.
Both use the temp switch on the head to control the cold advance and the fast idle but the C has the inhibit function and the J does not.

Now what possible purpose can that serve??

Having the glow plugs cycle any time you turn on the key certainly wont hurt anything and if you have an engine that likes to get glowed when slightly warm it sure helps.

I think I have it figured out how the relay is wired but have not had time to test my theory yet.

There are two pink wires going to the thing (Both should be hot with Ign on) Of the other two it looks like one is hot with ign on and the other grounds through the computer to activate the relay and keep the glow system off.

Looks to me like wasted electronics and more stuff to fail. :confused:

[john8662]

Now, defeating it...

I have a method already stated, but, perhaps there is a way to calibrate it, via resistance from the sensor to the PCM.

Splice and dice, or splice and resist.

eh.

J