Cold Start Advance Curcuit, coolant sensor. 1988 Heavy Duty 6.2L

[Mark Krieger]

Maybe someone can help. (I can provide photos if anyone is interested.)

I've got a retired 1988 USPS Parcel Delivery Van with just about 80,000 miles on it. The engine is the heavy duty version of the normally aspirated 6.2L diesel. The engine and all of the controls are more like a HUMVEE setup than a civilian vehicle with the exception of 12v operation versus 24v in the HUMVEE.

Screwed in to the outboard rear passenger side of the cylinder head is a coolant temperature sensor with a twin-blade connector. This sensor controls the cold start advance of the injector pump and also a selonoid that bumps up the idle speed slightly.

To make a long story short, my sensor is bad and I'm finding it a little bit difficult to find a replacement.

Can anybody help?


Also, I'd love to understand what the electrical function of the throttle position sensor is. My only guess is that it must do something for the glow plug controller because I can't figure out where else it could go.

Thanks in advance!

[gmctd]

That's why the information in your sig is important - lets us know the various particulars of your vehicle, incl transmission type -

3L60 and 3L80 would need a combination vacuum-modulator\kickdown switch - not a TPS

4L60 and E would need TPS for the emissions control ECM - is a TPS

Kalifornia vehicles would need TPS for the emissions ECM, even with manual trans - also a TPS

That is the fast-idle switch, not a sensor - try asking for a switch, they may be have better luck finding it - orange wire is switched IGN power, green wire is to HPCA and Fast Idle solenoid

[Mark Krieger]

That's why the information in your sig is important - lets us know the various particulars of your vehicle, incl transmission type -

3L60 and 3L80 would need a combination vacuum-modulator\kickdown switch - not a TPS

4L60 and E would need TPS for the emissions control ECM - is a TPS

Kalifornia vehicles would need TPS for the emissions ECM, even with manual trans - also a TPS

That is the fast-idle switch, not a sensor - try asking for a switch, they may be have better luck finding it - orange wire is switched IGN power, green wire is to HPCA and Fast Idle solenoid

Okay, I see your point about the signature and I'll figure out how to update it soon.

I looked at basically every sensor they offered for this enigne and this model year and found nothing that looked close. Switch -- sensor -- everything.

For now, let me say this: It's a 1988 normally aspirated heavy duty (non-emissions, non-EGR) 6.2L GM diesel mated to a Turbo-Hydramatic 400 (TH400) three-speed automatic transmission. There's no torque converter clutch and no ECM that I'm aware of. (What exactly does the ECM normally control? Does anyone have a schematic available?)

Attached is a photo showing the IP as viewed from the passenger side rear looking forward toward the radiator. I see the vacuum-modulator switch but I also see something that looks like a TPS with a pair of wires attached.

In the background (up and right of the TPS) is a coolant temperature switch that has been disabled and I'm not exactly sure why. I believe this is supposed to be for the Glow Plug controller, correct? The Glow Plug controller generally seems to operate normally but I could be missing some subtle clues as I've never seen how it works with the coolant sensor is connected.

Also attached is a photo of the switch controling the timing advance and fast idle. (I have not included a photo showing its original location.) this is the switch I'm trying to find!

Stamped on the internal end of the switch is the following: "29" and "C8" and "CS95"
Moulded in to the plastic on the exterior is the following "GM" and "1047"
Adjacent to one of the terminals is the following stamped in light blue ink or paint: "C"

[Mark Krieger]

After some late-night web surfing tonight I was able to answer my own question about the disabled Glow Plug Inhibit Switch. It seems that this was a deliberate modification on newer engines that someone must have implemented on mine.

It seems that hot starts (when the IP is hot) with worn IPs are sometimes a problem and the engine needs the glow plugs to start under these conditions.

Still looking for the HPCA switch...

[Robyn]

The TPS on this engine is a vacuum control that mimics engine vacuum using the onboard vacuum pump. The purpose is to send vacuum in the right proportions to throttle opening to the Modulator on the TH400
The shift points and feel are what this controls (Throttle Pressure)

As far as the two wires go they should be for the kick down (passing gear) Or forced down shift. The 400 has a small solenoid that controls the full throttle downshift.

The cold advance switch is a dealer item. (Ask for Cold advance switch)
Just tell the GM parts man you have a 1988 Chevy Suburban 3/4 ton 4x4 with the 6.2 and you need the cold advance switch.

If there is a problem have him show you the exploded view of the engine and you should be able to point it out.

All the 6.2's used this switch up until they went to the computer controls in around 90.
My 91 had a different setup.

The glow inhibit switch was a boondoggle and just leave it bypassed.
The theory was that once the engine was at 125F or above that the GP was not needed. As engines wear and also depending on air temp and other factors some engines would get balky to start under slightly warm conditions.
Generally what would happen is the switch would go south and fail to close until the thing was dead cold or not at all.

Leave the Inhibit bypassed :D

Hope this helps

If you cant get the switch, PM me and I will get the part number for you

Robyn

[gmctd]

Ok, you're doin' good - appreciate the pics - show'n'tell rules.............

ECM is Emissions Control Module, for EGR, Fast Heat Warmup, HPCA, Fast Idle, etc - chokes the engine down B4 ECT comes up to operating temperature, also prevents TCC lockup in the 700R4\4L60 trans, if ya got one - which ya don't, even.

As Robyn explained, that not-a-TPS module is the Variable Vacuum Regulator and Kickdown switch for the T400

That sensor from the passenger-side head rear is, as Robyn also explained, the HPCA\Fast-idle temperature switch - opens at ~85-105deg ECT, iirc - allows advance and idle to normalize as engine warms up.

The HPCA solenoid is inside in the top of the IP , spade terminal\green wire, on the passenger side - top front terminal\large pink wire, is, of course, the Fuel Shutoff solenoid

Ditto on the glow inhibit switch - I kept my '89 6.2 setup working by guess and by gosh, then finally, in desperation, replaced it..........with a '95 EFI 6.5TD engine - that got the glow system workin' real good, I can tell ya that...............

[Mark Krieger]

This is great information everyone -- thank you. I'll plan on stopping at the dealer tomorrow and see if they can get me the switch. After removing it and fiddling around with it I was able to make it work again for a while.

With your help I was also able to verify the correct operation of the WOT transmission kick-down switch too. Since the transmission shift RPM seems to vary with throttle position in general, I'm pretty sure my vacuum pump and vacuum modulator are working too.

I'v included a photo of the truck and the engine compartment too. Maybe it'll be helpful to someone else.

I also found a couple Military service manuals for the HUMVEE engines that I'd be happy to share if anyone is interested.

More and more I feel like I know this setup as well as anyone can. I'm beginning to enjoy having this truck. Unfortunately, I lost a connecting rod bearing in my (gas) Suburban and I'm down 1 vehicle and some serious cash until I get it fixed. The stepvan continues to be my daily driver until I get the other truck working again.

I've had a for sale sign in the stepvan for a few weeks because I need cash to fix my other truck. Wouldn't you know it that when I've started to enjoy the stepvan, and it seemed like nobody else wanted it, someone saw me working on it today and offered to buy it. Blah...

[gmctd]

The hose off the top of the VR connector goes to the steel tubing then back to the vacuum modulator on the side of the T400

The hose off the bottom of the VR connector goes back to the vacuum pump, which appears to be missing - instead, you got the oil pump drive stub.

Does it have an electric vacuum pump?

[Robyn]

I am envious of all that accessability at the back of the engine. :D

The Pickups and Burbs are doable but not like that. Geeeeez you can even get at the tranny bolts. WOW :eek:

[Mark Krieger]

The hose off the top of the VR connector goes to the steel tubing then back to the vacuum modulator on the side of the T400

The hose off the bottom of the VR connector goes back to the vacuum pump, which appears to be missing - instead, you got the oil pump drive stub.

Does it have an electric vacuum pump?

Up unitl now I would have bet you it didn't have a vacuum pump because I hadn't seen it and hadn't understood all of the stuff hanging off of the IP.

I pulled the alternator off and there it was hiding quietly underneath -- the vacuum pump. It is belt driven off of the same belt as the alternator.

Can you believe it has dual batteries and only a 78 Amp alternator?

[Robyn]

You can upgrade the ALT easily to something that will do a far better job of keeping the juice flowing.

Talk to the local auto electric guys and I am sure they can help.

Robyn

[gmctd]

78amps is normal - only load after starting is the Fuel Shutoff solenoid, brake lites, turn signals - added load of headlites\taillites at nite - alternator just idles after recharging from the heavy starting load - 88-up new style maybe got the 105a due to stereos, ac, power windows, rear ac\heater, defog, lift pump, EFI, etc - 140a is available.

Easy upgrade, as suggested, but prolly not required in your low-load configuration

[Mark Krieger]

I am envious of all that accessability at the back of the engine. :D

The Pickups and Burbs are doable but not like that. Geeeeez you can even get at the tranny bolts. WOW :eek:

Don't be envious too long...

The penalty for all of the beautiful space results in 10 MPG fuel economy. (And six $350 tires slowly wearing out as you cruise along without any A/C or radio to enjoy!)

[Mark Krieger]

83-93 6.2L Diesel , Cold start switch GM P/N 10154649

I'll let you know if it is the right one after I receive it. (None in the whole county, had to order one.)

[Mark Krieger]

Gah...

That TPS looking thing on the right side of the IP with the wires hanging off and the vacuum hose on top is sure expensive!

I know that this has two functions -- both related to the transmission. The vacuum regulator valve is on top and sends variable vacuum to the transmission to help it decide when to shift. The switch is below and it closes at full throttle to tell the transmission to downshift.

So, I've noticed that the shift points change with throttle position and I'm sure the vacuum pump and valve work. But now I'm noticing that the transmission won't always downshift at full throttle.

The wires between the IP and the transmission look intact and I don't think it's the solenoid but I could be wrong.

AC Delco
P/N 23500822
VAC REG VLV
List Price: $136.36
Dealer Cost: $81.95

[gmctd]

Next step, B4 throwing parts at it, is to check for vacuum - should be 26" at pump at idle and 2000rpm - should be 26" everywhere that hose goes from the vac pump

[Mark Krieger]

Next step, B4 throwing parts at it, is to check for vacuum - should be 26" at pump at idle and 2000rpm - should be 26" everywhere that hose goes from the vac pump

I don't have a gauge right now but it seems to me that I can verify its operation by observing symptoms.

It seems to me that an inoperative pump would behave like wide-open throttle and postpone shifts until fairly high RPM no matter what I did with the accelerator pedal. An operating pump with an inoperative vacuum valve would cause the engine to tend to always shift at the same RPM without regard to throttle position -- probably fairly low RPM. The down-shift switch should force a down-shift with the accelerator floored.

I can tell the shift RPM is varying depending on throttle position. With light acceleration the transmission shifts at fairly low RPM and under hard acceleration it runs the RPMs up near redline before shifting. For this reason I think the vacuum pump and valve is operating just fine.

Some days I can floor it and get a good solid down-shift and others I'll get nothing. For this reason I think the down-shift switch circuit isn't working all the time. I can check out the down-shift switch with my volt meter and be pretty sure its bad before I replace it. I can also check the down-shift solenoid too because the problem could be at either end or the wiring in-between.

Am I understanding the way it should work correctly?

Before I spring for a new $100 part, I'll tear the old one apart and see if I can clean up the contacts and rejuvinate it myself.

[gmctd]

Likely it's the d\s solenoid in the trans - but first check the 2-terminal electrical connector on the case, driver's-side rear - internally, the solenoid connector goes intermittent as the plastic darkens from heat

[JeepSJ]

83-93 6.2L Diesel , Cold start switch GM P/N 10154649

I'll let you know if it is the right one after I receive it. (None in the whole county, had to order one.)


This is also available through Checker/Shucks/Kragen -
http://www.partsamerica.com/ProductDetail.aspx?MfrCode=GPS&MfrPartNumber=TSU152&PartType=865&PTSet=A

And through Napa -
http://www.napaonline.com/MasterPages/NOLMaster.aspx?PageId=470&LineCode=ECH&PartNumber=GPS200&Description=Diesel+Glow+Plug+Warning+Switch

[Mark Krieger]

This is also available through Checker/Shucks/Kragen -
http://www.partsamerica.com/ProductDetail.aspx?MfrCode=GPS&MfrPartNumber=TSU152&PartType=865&PTSet=A

And through Napa -
http://www.napaonline.com/MasterPages/NOLMaster.aspx?PageId=470&LineCode=ECH&PartNumber=GPS200&Description=Diesel+Glow+Plug+Warning+Switch


Thanks, but those are the wrong one. The picture on the first link shows a different connector and the second link is the wrong description. I think you've got the switch for the Glow Plug Controller and not the switch for the Cold Start Advance.


I got my switch last week and it matches the previous picture I posted exactly. I put it in this morning and it works beautifully. I get fast idle at startup and the timing is clearly advanced because I can hear the extra clatter until it gets warm..


I've had a for sale sign in the window of the truck for a while and I've had a few interested people who only offered $500 lesss than the price I was asking. (I'm sure they would have done the same if I had advertised $500 higher price!) I've started liking the truck the more I drive it, and on Friday I even took down the sign and decided to keep it.

Can you believe someone stopped while I was installing the switch today and asked me if it was for sale? I ended up selling the thing for my asking price. I need the money more than I need the truck, but now I'm kind of bummed out. That's the way it goes I guess...