MAT Vs IAT

[geofffinlay]

Gentlemen,
I'm wiring up my 92 3800 S/C engine getting it ready to drop into my Lotus Super 7. I have an issue that maybe one of you can throw some light on. The only wiring diagram I currently have access to is a Mitchell. I may have the wrong diagram, but it's the only one available in On-Demand 5 for this car. It's certainly for a supercharged version as pin C9 (1724) Grey is for the Boost Solenoid Control. The stuff came out of a '92 Park Avenue.

Note that I have the PCM with about 12" of wire and the engine harness back to before the first main bulkhead. The rest of it is in recycler's heaven.

The Mitchell diagram shows a MAT sensor terminating at pin A7 (454) Black - Ground and pin B7 (472) Tan - MAT Sense Input. Unfortunately, this engine doesn't have a MAT. The closest thing to a MAT is a single wire sensor screwed into the manifold (close to the ECT) with a Green/White wire.

The Green/White wire matches the wire @ pin D9 -603 - Coolant Fan Request which according to the diagram should run to the A/C Press Fan Switch. I suspect this could be a simple temp switch to control the cooling fan/s should the engine temp rise when the A/C is not engaged. I can of course heat the switch if necessary to determine if that's the case.

When my friendly recycler delivered all of the goodies, he had the foresight to remove the IAT sensor and put it in the box with the other goodies, but neglected to remove the 2-wire connector for the IAT from the vehicle. I understand why the IAT wiring may not be included in the actual engine harness, but there certainly has to be a couple of pins in the PCM to deal with it.

I suspect that the IAT should actually replace the MAT and that the pin locations and wiring colours are correct on the diagram, except they should be an Intake Air Temp Sensor.

Can anyone shed any light?

Thanks

Geoff

[J Wikoff]

As long as MAT stands for manifold air temp, it has to mean the IAT. There was never an air temp sensor in the manifold. And if I remember right, one of the IAT wires in my 92 SSE is tan...

[geofffinlay]

J Wikoff, thanks for the response. I believe you are right. It doesn't make much sense to have both. I'll have a chance this week to look at a GM manual, but it would be nice to confirm it before then.

There is a sensor in the intake manifold, but it has just one wire and it looks like a Manifold Air Temp Sensor. Its connector has 2 spaces for terminals, but one of them isn't used.

Geoff

[J Wikoff]

I can only think of two sensors that plug into the LIM, both coolant temp sensors. One to the rear of the throttle body, and one below it. I don't know why the temp gauge and PCM can't use the same sensor. I think the one under the TB is the PCM-utilized sensor.

The only one wire sensors I can think of are the knock sensor(s).

[geofffinlay]

J Wikoff
You are quite correct of course re the 2 sensors. I'm pretty sure that the single wire sensor is related to either/or both the a/c & rad fan. The Mitchell diagram shows an a/c pressure fan switch running to D9 (603) with a Dark Green-White wire, which is exactly the colours on the second sensor. There is a reference on the diagram to another circuit/connection N6-C265 which I'll chase down this week. I'm pretty sure it's going to be what I already suspect and nothing to do with the actual intake air temp.

Thanks again for your input.

Geoff

[J Wikoff]

Hmm. I'm not very familiar with the ac pressure sensor.

[geofffinlay]

The a/c pressure sensor is usually fitted into the a/c accumulator, nowhere near the engine. It turns the a/c clutch on/off when a/c pressure is in/out of range. If the a/c system runs too low on refrigerant, a/c pressure will be too low so the clutch can't be engaged, thus protecting the system. The same circuit can be used to turn on the cooling fans at the same time as the a/c.

Since the PCM also has control of the cooling fans, there will be some kind of connection between the a/c system, the cooling fans, the PCM and the single wire sensor in the LIM that I'm working with.

I've already wired most of the engine at this point. I've wired the unknown sensor into the harness so that it will be attached to the PCM. There are also two terminals on the PCM to control hi & lo fan speed already. This circuit will likely never actually operate since the PCM will calculate fan operation anyway.

It would be interesting to heat the single wire sensor and see if it's just a simple on/off switch at a predetermined temp, unless anyone else already knows about this piece.

Geoff

[J Wikoff]

Where does it plug into the LIM?

[geofffinlay]

It's between cyl #8 & the throttle body base, on the same plane as the coolant sensor & directly under the fuel pressure regulator. Can't miss it.

Geoff

[J Wikoff]

Cylinder 8?!?! How much did you mod this engine? lol

Sounds like the coolant temp sensor that the gauge cluster uses. It kinda blocks good access to one of the LIM bolts, right? It should have two wires.

[geofffinlay]

I did say #8... that happens every so often - Senior Moment. Of course it's #6. The gauge cluster (IPC) would be fed by the BCM, which would get it's temp info from the PCM. The sensor in question has just one wire, but the connector has the facility for 2 wires, but not in this application. The original blanking plug for the empty side is still in place and visible. At this point I'm fairly sure that it's there to control the cooling fans, but not sure of the reason, since the PCM can do that.

The sensor is very close to one of the bolts but it wouldn't be much of an issue. I've had both of them out. I'll take some pics and post them.

I haven't done any mods to the engine itself at all. I've removed the alternator from the top of the engine and it's now on the other side, low down. I've added another pulley and now there is only one belt running the water pump, supercharger, and alternator. PS & A/C aren't necessary in this type of car and only add weight. I'll be building headers once the engine is in the car and the cooling system is going to be an adventure since the thermostat housing outlet now points to the rear. The air intake will also be an interesting challenge.

I'll be talking to a GM tech later this week and I'll get the straight goods. I'll be glad to let you know.

Geoff

[J Wikoff]

Maybe the Buick you pulled it out of is different than my 92, but my FSM (GM's manual) says it should be the coolant temp sensor that feeds only to the gauge cluster. The one under the TB feeds to the PCM. I just checked. The temp sensors are just the simple thermsistors (or whatever they are called) that vary resistance with temp, so two wires would be required for them to function and be read properly. So it must be a different function here.

[geofffinlay]

I should have a chance to look at at the FSM later this week. We've had a silly issue forever with small differences between US & Canadian cars. This may be one of those. I think you're correct in that it's for something other than a varying temp sensor.

It's quite possible to have a thermistor that only has one wire at the sensor. In that configuration, the ground side of the resistance is connected to the sensor housing. As resistance changes (with temperature), the powered feed wire (reference) is subjected to changing resistance values. Another resistor inside the PCM allows the voltage on the ref circuit to move through the full voltage range. That change is monitored by yet another circuit starting at the reference wire inside the PCM. The only difference between the two, is where the ground lands.

I don't think that's the case here though. If I can't get the info I need by the weekend, I'll set the sensor in a pan of water and heat it up to boiling. I already tried heating it a little to check for any resistance change. Of course that only takes me up to 212 degrees.

Are we having fun yet?

Geoff