Oil pressure indication problem with M1083A1 and M1088A1

[Mike Wryley]

I thought I posted this this am but do not see it, so apologize if redundant,

new to to this site so bear with me,

first of all, the engines have oil and run fine,
the m1083a1 has had intermittent oil pressure guage reading since I got it and the oil pressure and stop engine lights come on after start up, in the past the indication may start to work after about the third start,, and pressure reads nominal
went to Cat and bought a new sender, problem has changed slightly, now most of the time the pressure reads high at idle, increase rpm and it drops to zero,
not having a schematic, this suggests a broken wire or bad connection as the loom is built wrong in that area and it is very tight wrap for the sender plug.

my question for the experts is can the computer be causing this ? The m1088a1
also has a faulty oil pressure reading. The 1083 computer throws an error, something about scrambled memory, and the engine check light is always on,
but the truck runs great, and I am trying to iron out the little probs first.
thanks in advance

 

[sandcobra164]

First thing that we check on these trucks is the battery's. They are so sensitive and heavily electronic. If you jump one off that has bad battery's, it will do all kinds of crazy stuff. First thing it will do is the ¨No Charge¨ light which means it has disconnected the alternator from charging the batteries so they'll be dead no longer how long you leave it running. When it first starts up with low batteries, it will do the ¨Circus Wheel¨ meaning the indicator lights will come on and go off in a counter clockwise pattern while it is running. All of the gauges on an A1 are fed though the Computer on Board. If there is a bad battery, it's going to act silly. I have a similar problem on my M1089A1P2 Wrecker. Idling in the Motorpool, it will sometimes throw a ¨No Charge¨ light, voltage gauges will be reading low, but as soon as I put it in Drive, it will start to charge and both the 12V and 24V gauge will both jump into the normal range.

 

[Mike Wryley]

Sand cobra,
thanks for the heads up,,should I use a hydrometer to check the batteries ? Cranks great,
are you referring to the computer mounted to the side of the engine ? Or is there another one ?
i assume the Allison has its own computer ?
i was relieved when I saw that the '95 M1078 I got has no computer on the engine,
thanks mike

 

[Suprman]

I have gauges that go full left when my 97 m1088 is running. My volt meter always stays on regardless of switch position and if I remove all of the relays. I was thinking I had a short somewhere under the dash or a bad ground causing a potential somewhere it shouldn't be. Does the older lmtv trucks have some type of computer that controls the dash gauges? I didn't see anything in the tm. There are lots of not so mil spec connectors in the dash that show some corrosion from moisture and time.

 

[sandcobra164]

I'm not sure about the 97 model but I know the 12 model runs a computer for most everything but the door locks similar to everything else. They run a ¨Can Bus¨ system with computers to control most everything. In fact, I've probably got a computer issue on my wrecker. I had to hook to a HMMWV today and it will engage the PTO to run the hydraulics but it won't run the engine at high idle like it is supposed to. Once the PTO is engaged, the throttle pedal will not speed up the engine and the high idle button doesn't work as well. They both work without the PTO engaged. I sure hope I don't have to winch anything out because I need the engine running fast to make pressure for the underlift and winches. I'm sure we'll get it sorted out as my first stop is to a higher echelon of support this trip.

 

[Suprman]

I need to find a schematic diagram of the truck. Even if its something I can print in pieces and lay out. A p2p would be nice.

 

[Tater6]

Sand cobra when your no charge comes on does the oil pressure gauge stop working and the 24v stop. But 12v is fine. If so try unplugging the c.t.i.s harness that should do the trick. If it does not turn battery power off on the c.t.i.s harness you will see a little "a" and the next pin above to the left of the a see if you can move them around a little with a small object. Once done leave disconnected and turn truck back on see if it works. If it does you will need a new harness. Pain to replace the harness.

 

[408chevelle]

I know this is an old thread, but I have a M1087A1 Exp van, It shows no oil pressure ( with oil pressure light illum), 24V gauge does not work and a No charge light. Everything else works, troubleshot with new IETM and it says replace engine ECM, didn't like that answer so trouble shot with older IETM and it said Replace MMDC (black vehicle data computer under drivers kickpanel. Replaced MMDC with known good one and no changes, still didn't want to replace ECM, checked all faults individually with no luck. went and traced all schematics couldn't find issues. stumbled upon this thread and disconnected CTIS ECU and everything works. Is there a specific wire that shorts and causes this that can be repaired without replacing harness? thanks

Sand cobra when your no charge comes on does the oil pressure gauge stop working and the 24v stop. But 12v is fine. If so try unplugging the c.t.i.s harness that should do the trick. If it does not turn battery power off on the c.t.i.s harness you will see a little "a" and the next pin above to the left of the a see if you can move them around a little with a small object. Once done leave disconnected and turn truck back on see if it works. If it does you will need a new harness. Pain to replace the harness.

 

[Suprman]

Let me start by saying I dont have much experience with the A1 trucks. I have had alot of alternator issues on first gen trucks though. The regulator leads seem to go bad often causing a low voltage situation. The first thing I would do is measure the battery voltage with the truck off and then with the truck running. You should have around 27-28 volts running. If its under 26 something is not right. All of these trucks have wiring gremlins especially grounds. I would check the alternator ground strap. Maybe sand where it bolts up to ensure good contact. Make sure batt connections are clean and tight. Power panel ground same thing. I have seen where radio equipment was removed some power and ground leads are loose. Under the spare tire is a polarity protect box. There are 2 versions the newer a1s have power and a charging control lead the older ones are just power in power out.

 

[408chevelle]

Before I had a chance to get started on it, they thought it was just a normal no charging issues, So Generator/ regulator were replaced, nothing different so then they replace LBCD/reverse polarity box and a set of batts. Still nothing. have correct voltage everywhere. once I started on it I noticed the oil light and gauge as well as the 24v gauge and the no charging light. so begin to check other stuff, obviously with the older trucks the oil pressure switch is what fires up the charging system with the alt excite so we started checking. with this having a Cat C7 its even more of a PITA. But with the help of CAT ET i was getting oil pressure readings on the computer which ment everything was working in that area, alt excite relay was working as well, and still nothing. no lights lit on the regulator and the led on the LBCD was not illum. after pulling the CTIS plug everything works, at least for 10 min then i get a no charge light again but all the gauges are back to working and the oil light is off. surprisingly there were no codes.

 

[Suprman]

Jump 24 volts from the fuel solenoid to the 24v regulator lead. Disconnect the ac lead on the regulator and remove the original 24 volt regulator lead when you do this. See if any change. Intermittent bad regulator leads are common in first gen trucks.

 

[Tater6]

Let me find my wiring schematics or an old harness. I do know that one of the wires should only have around 5 to 7v. This wire runs to all the diff. Ecms in the truck. And is spliced under the pdr box on passenger side (relay box). I belive 5 or 6 total splices. Give me about an hr and I should have the wire number and pin location on the ctis harness you can test.

 

[Tater6]

I would go back and chalk all wires of items replaced or swapped out. Also to answer you question about the wire to test it is wire #J1587 this wire should have I think max of 7v. This wire can be found on the test port 'F' when you hook up the test equipment. Also on the c.t.i.s harness at Port 'a' I think or what looks like an lower case a.

 

[408chevelle]

Just to try I had a couple other bad CTIS ECUs laying around so I tired them although they were known bad but it fixed the current issues but made others, each one did something different but neither had any of the same issues.

 

[Tater6]

Did you test the wire I just tested had 3.5v with truck off and power off but main power by battery box.

 

[408chevelle]

checked that wire just as you said and it had 2.2v

 

[Tater6]

So do you still have a no charge light on with the ctis harness unplugged. Or what other issues do you have with the truck.

 

[408chevelle]

With ctis unplugged everything goes back to normal for about 10 minutes then ill get a no charge, but everything else stays working fine

 

[Tater6]

When the no charge light comes on does the 2 lights on the regulator flash. If they do not check to see if tl35 the lug close to the front to see if it has 24 ish volts. If they do flash check the back lug for 14ish volts. Both tests with engine running.

 

[408chevelle]

With ctis pluged in the regulator nor the LBCD will light up. CTIS unplugged I will get everything lit up and both green for the first 10 min everything working as normal 14 and 24 then it will go to 1 solid orange on the 24 side for a bit, I always have constant 24 and on the AC lug is where it looses it.