Hi guys,
got the following problem with this animal. It got an 7730 ECM.
When we depress the throttle the MAP value goes up for a split second and than drops from like 60 kpa at idle to 40 kpa at steady 2000 rpm.

Looking at my own 91 the kpa's go up when I hit throttle, from idle at 30 kpa to something like 50-60 kpa.

Seems like the values are reversed. How can that be???

We measured all wires from the ECM to the sensor. The ground wires from the CTS and MAP were at the wrong spot. Switched those but no difference.

This is an aftermarket set up from Arizona Speed and Marine.

Any ideas how we can get the MAP values in line?

Right now the engine starves gas, when accelerating.

Could it make a difference just reversing the ground wire and the 5 volt reference signal?
It says in the service manual the 3rd wire being the signal, should read 1.6 volts, unfortunately we don't get that, whatever we tried.

We tried 3 ECMS and 2 known good MAP sensors.

Help guys???

Thanks,
Axel

 

Sounds like the signal and ref are shorted together but not a full 5v.

 

Completely normal. When you first open the throttle the load goes up, which the MAP reports as 60 KPa. Once the engine RPM increases the load is lower, so is the MAP.

RBob.

 

Brain fart lol, low vacuum higher pressure. I thought you were saying the readings were bass ackwords.

 

I guess we have a small misunderstanding.
It's not a short blip on the throttle, but rather holding it steady at 2000 and the MAP value keeps falling and is holding at 40 kpa, whereas it's idling steady at 60 kpa.
2000 rpm sure is a harder load on the engine than idle, or not?

Just tried to switch ground and the 5 volt reference, unfortunately just 18 kpa steady without any changes. So that doesn't work.

No shorted wires, as we measured all from the sensor to ECM and tried all contact versus each other.

 

That can be normal with no load or even with light load. Free revving will go down in kpa value with rpm. There is no load at all in free rev neutral mode. Although most of my motors dont drop that much with rpm, its definately the direction it should go

Try this as well. Put in gear and brake stall it up if you can. As rpms climb load increases against converter. You should now see higher kpa readings as rpms go up

 

Load would be less free reving like that and with a cam that gets more efficient out of the idle rpm it could be correct.

Just to reconfirm the sensor wiring...
A4 goes to Pin C on MAP (+5V power)
F15 goes to Pin B on MAP (Sig out)
B6 goes to Pin A on MAP (Common/Gnd)

 

I was going to suggest same thing. May be coming "on cam" at that point and therefore more "efficient".