MCI Surge Tank Coolant Sensor

[ChuckMC8]

 On MC8, The Coolant level sensor in the surge tank, it has a " + " terminal, a "sender" terminal and a "test" terminal on the sender. Could someone explain how this switch functions? My bus has a bastard combination of the two schematics in the book. I just want the telltale light in the dash to flash if the water level drops below the sender in the surge tank. I don't need the test switch in the remote panel to work, I have a sight glass and can see the coolant level. Thanks Chuck Lott

Here's the info that gathered during the day today: If the tank is full, check for voltage at the positive post and signal post of the sensor..When the tank is full of coolant, the signal post should not have voltage.
So, applying this method (from Greyhound's Electrical Troubleshooting manual for MCI) sez my sender is defective. Looks like a new one is $65 or so...ouch. I've never seen a sender like this one, Its a Robertshaw brand. I'm curious as to the method the sender uses. Anyone explain?

[Nick Badame Refrig/ACC]

Hi Chuck,

I Don't know how MCI wired the MC8 But, I do know that most cars and trucks are wired Normally Open [NO]. The coolent level

keeps the circuit closed until the level drops below the sensor, Than releasing a holding relay to illuminate a light.

Hopefully one of our board members Knows the MC8 diagrams to better inform you!

Nick-

[Nick Badame Refrig/ACC]

Chuck,

Good pic!  It looks like you have three terminals there. I can only suspect that one terminal is common, and the other two will be N/O and N/C.

The only way to test it properly is with a Ohm meter, go through all three terminals and read the values, than again while the probe is under water.

It may be able to send current to 2 places [or circuts] depending on level of fluid!

It looks like a switch that opens and closes!  I could be wrong! I don't want to steer you in the wrong direction.

Nick-

[Sammy]

Chuck, based on the info from you and your Greyhound manual, here's what  seems to be happening:
1: Power comes from CB3 from the front bus bar, to stud 26 in the front J-box, to one side of your tell tale light, waiting for a ground to complete the circuit and illuminate the light.

2: Power is also waiting at your coolant sensor on the (+) terminal on wire 3A - (fed from rear bus bar)-- to energize the T & L relay.

3:Ground will be provided for the tell tale light thru the T & L relay N.O. contacts, only when the T & L relay gets energized from the signal terminal of your sensor - (wire 10A, to stud 55 in rear J box, becomes a yellow wire at the relay), and the contacts of the T & L relay close.(chassis ground is always on one side of the N.O contacts-white wire).

Make sure you don't have a tripped circuit breaker - #3 on front bus bar and T&L breaker on rear bus bar.
Make sure you have chassis ground on the white wire of the T&L relay too.


I am using the schematic : Low Coolant Level (Robertshaw) Model MC-8
                                                Effective Unit No. 31003
                                                      Date: April,1977
I can scan it and send it to you if you'd like. 

Hope this helps
Sammy 

[NJT5047]

Hi Sammy (and anyone else that understands these things),
Don't mean to burden you, but a friend with an 87 MCI is having a similar problem with a DDEC surge tank water level sender.  His stopped him on the highway.   He had a DD road service come out and dx the problem.  The DD guy jumped two of the three leads in the plug to the water level sensor.  The jumper was dislodged and now his bus won't continue to run unless the override switch is engaged.   According to the wiring diagram, the 3 leads are marked A, B, C, with A going to chassis ground, B is sensor (test?), and C is ignition hot.  Obviously cannot jump from A to C as that would be a dead short.  Reckon the jumper was from C to B?  Since I also have an identical 87 DDEC, really like to understand how the sensor works.  GLI used some DDECs and the sensors may all be similar.  Sounds like what Chuck described. 
To carry this one step further, the DDEC manual indicates two possible switches wired to the low water system...one is an overheat switch that sets, code "16" and the other is low water, code "43."  The DDEC manual shows identical leads, wired to the same DDEC pins.  I suspect that the code "16" works in some other manner (temp sender located in the LH head would be a good guess).  My NJT wiring diagram does not show any other switches connected to the low water system.  The low water sensor lead goes directly to the ECM.   A relay in the rear junction box controls the ig + to the sensor.   Any sorting out of this issue would be greatly apppreciated!  Chris's bus is setting code "43" and the sender has been replaced.  The wiring has been checked for continuity, correct pin out, and ground...still the thing may crank and run for a while, and then the problem starts again.  Jumping the sensor would rule out the sensor.  It worked before like that.  Any ideas? 
Thanks in advance, JR 

[Sammy]

Hi JR, your coaches are DDEC 2. They should have a CLSM - coolant level sensor module. DDEC shut the bus down when it detected a low coolant condition.I'd locate it and troubleshoot circuit again.If you have diagrams, scan them and send me copies. i'll troubleshoot and diagnose circuit for you as best as possible.I don't have any diagrams for your coaches.
Sammy 

[NJT5047]

Thanks Sammy,
The buses have DDEC 1.   You are correct with the CLSM...it has a small module mounted near the DDEC ECM.  And the low water sensor is mounted immediately below the sight glass in the tank.   Chris may have a module failing...the module doesn't show up on any of my data.   Per my schematic, the sensor goes directly to the ECM.   The DDEC guide doesn't show the module either...just the sensor. 
Just for giggles, can the water sensor be defeated?  What signal does the center terminal of the 3 lead sensor that's in the GLI units send...or for that matter, what does an MUI low water sensor send from the center term?   I'd guess that the center tap sends a signal of some voltage (5V?) to the module...
Suppose I could crank mine and see what's present on the module...assuming the wiring can be sorted out. 
The schematics are about impossible to read...scanning on what I have to work with would about useless.  I may take the diagrams down to a local quickie print and have them enlarged and copied.  That would be a good start! 
DDEC 1s are sort of "bastard" DDECs.  Minimal info avail for them.  Not sure who used them other than NJT.  From all I read, they weren't problematic...however, they are getting old, and they are more complicated than a DDEC II.   Know of one guy that converted to DDEC II (or III) and had major issues with ATEC com link.  Reason enough for me to forget that idea.   If you come across any data regarding the CLSM, let me know!  Reckon I could also buy a sensor and apply voltage and ground and figure the thing out.  
Thanks, JR 

[Sammy]

JR, my buddy has a DDEC1 manual, I'll get it from him to troubleshoot your bus.
I need a couple of days - I'll let you knowwhen I have it.
Sammy

[niles500]

this is for DDEC II - but may help

http://www.tpub.com/content/trucktractor6x4/TM-9-2320-363-20-1/css/TM-9-2320-363-20-1_400.htm

http://www.tpub.com/content/trucktractor6x4/TM-9-2320-363-20-1/css/TM-9-2320-363-20-1_398.htm

http://www.tpub.com/content/trucktractor6x4/TM-9-2320-363-20-1/css/TM-9-2320-363-20-1_402.htm

http://www.tpub.com/content/trucktractor6x4/TM-9-2320-363-20-1/css/TM-9-2320-363-20-1_404.htm

http://www.tpub.com/content/trucktractor6x4/TM-9-2320-363-20-1/css/TM-9-2320-363-20-1_397.htm

http://www.tpub.com/content/trucktractor6x4/TM-9-2320-363-20-1/css/TM-9-2320-363-20-1_405.htm

http://www.tpub.com/content/trucktractor6x4/TM-9-2320-363-20-1/css/TM-9-2320-363-20-1_412.htm

http://www.tpub.com/content/trucktractor6x4/TM-9-2320-363-20-1/css/TM-9-2320-363-20-1_414.htm

http://www.tpub.com/content/trucktractor6x4/TM-9-2320-363-20-1/css/TM-9-2320-363-20-1_406.htm

http://www.tpub.com/content/trucktractor6x4/TM-9-2320-363-20-1/css/TM-9-2320-363-20-1_408.htm

[NJT5047]

Thanks Niles and Sammy,
The military site won't display in my computer???  Appears to be tons of info on it. All jumbled up.  Words on top of words.  May have the font size too large.   I've been there before and had the same problem.  Being not especially computer literate, I'm lost.  Cannot read it...jumbled up.  What I can read looks like copies of the DDEC diagnostic guide that I have. 
My bus is working...another identical unit is suffering from a code "43" and apparently the wiring has been checked, and a new sensor installed.   
The DDEC manual shows a resistance unit in the sensor with the center (test or signal) terminal tapped into the center of the resistance unit.  So some form of low voltage is present on the signal terminal.  Why would the 12V be sent to ground thru a resistor?   How does a "low water" condition modify the signal?   Be interrested in figuring that out without molesting my coach....afraid I'll stir up problems.
I'm going to see if Chris still has the low water sensor he pulled.  If I had that, this could be sorted out post haste.
Thanks, JR

[NJT5047]

Niles, I just adjusted the font size on the screen and that didn't help.  Looks like an "echo" of two different sized fonts. 
Curious.   I'm using IE and would assume that it would display almost anything worth seeing?  Wrong!
What you reckon?    JR   

[niles500]

Jr - I just checked - found same thing - never did that before -  appears to be over written with 2 different fonts - if you already have the manual then I guess you just have to follow the trouble shoot rundown - sorry it wasn't more helpful -

[NJT5047]

Thanks Niles...thought my computer was screwed!   
The DDEC manual doesn't get into signal values unless it's required for troubleshoot.   Sort of a minimalist approach.  There are probably better docs but got no idea where to find them.   I've got the orig MCI "Crusader" manual, NJT manual, DD engine rebuild and service manuals, and a DDEC 1 diag manual.   I've often had all three out sorting some dumb thing out.   
I'll post the outcome to the CLS conundrum.  Others with DDECs may find some benefit... low water, low oil pressure, and overheat will shut the coach down.  These are the only three failure modes that do that. 
Chris has been dinking around with this problem off and on for a year now.   
Thanks, JR

[ChuckMC8]

 The MCI coolant senser is $65.00 (new)........That's more than I want to spend to make the idiot light work.Does anyone know of another application that uses a low coolant light/ It just needs to make a signal when submerged, and be adaptable to 1/4" pipe threads.

[gumpy]

OK, here's how to handle the screwed up site....

Go to a link posted above. It'll bring up the screwed up image.

Right click somewhere on the screwed up image and select "Save picture as..."

Save it to your hard drive. It'll save as a jpg file.

Find it in a windows explorer window and open it with your jpg viewer.

Voila!



craig