I have a 1991 G30 series van (motorhome) with a 5.7L engine and auto tranny. The ECM went out about a year ago (TBI was firing randomly - lots of herky jerky whenever the ECM would get warm but no fault code). All I could get in order to get home was an 8747 ECM. I installed the EPROM from the origional 7747 ECM. Since then, the motorhome has run just fine with plenty of power (well....relatively speaking - since it's a motorhome) except that my fuel mileage is worse and has since dropped from 10mpg to 5mpg. I also had made some other changes - mostly opening up the exhaust around that time but I can't imagine what might have caused the fuel mileage change. Spark plugs, dist cap / rotor, ignition module, O2 sensor, aircleaner, and the fuel filter are all new. Could the fact that I have an 8747 in place of a 7747 be causing my fuel mileage problem?

Secondly, I notice four letters after a lot of the ECMs number listed (i.e. 1227747 AMUN) - what do these indicate and how important are they? I ~think~ this is an indication of the EPROM that is/was installed originally ... not sure.

Thanks in advance...

Jake

 

Considering the CODE and therefore PROM chips are not compatible with each other this does not suprise me. Do you not get a SES light flashig at all times.

What was the PROM code in the 8746?

I am also guessing that your MH has a TH400 transmission, if so, you will not be getting passing gear when you floor it with the stock coded 8746.

 

Actually, the code on my EPROM is AKWA (I used the other for refence since I hadn't removed the ECM to check yet) - can anyone verify which ECM that EPROM is intended to go in? It's possible I got things confused since I was in the middle of trying a lot of things to fix the problem (I definitely have an 8747 ECM at the moment but the original ECM went in as a core).

I do NOT have any SES light flashes or fault codes. Now that I think about it, I don't recall the transmission kicking down lately but I admitedly don't hammer it all that often because of the loads involved (and the fuel mileage)

I have a used 7747 coming to test and barring failure there, I'll probably start to monitor the O2 sensor voltage to see what kind of mixture it's running at various conditions.

 

I can tell you with 100% certainty the stock Van ECM will be a 7747. You say you have an 8746. You put the chip from the 7747 into the 8746, creating a miss-match.

You are also missing an IAT sensor which will affect mileage. Basically the system is in LIMP mode.

 

Thanks again for the help....In the thing now is an 8747 - you guys both said 8746

Before this drags out, here are the two in question (not sure if links will work with website). What have I done?

Showed as a match to what I had:
Advance Auto 7747

What I installed:
Advance Auto 8747

 

I put in the 7747 ECM that I bought off ebay and installed my AWKA eprom. Immediately the SES light was flashing when the ignition was turned on. I then put in the AMUN Eprom that came with the 7747 ECM origionally and the SES light no longer flashed. However, the van ran like crud and stumbled and would occasionally stall. I reinstalled the 8747 with the AWKA and it ran well (as usual). However, on my 140 mile trip, my fuel mileage came back to 10-11mpg. It must have been a bad connection to the ECM.

so originally the van must have had the 8747 ECM (and I got confused) with the AWKA Eprom. Also notable is that the EPROM from an 8747 is not compatible with a 7747 ECM.

 

The four letter code you see on the ECM case is the BCC (Braodcast Code) of the Eprom that was originally in the ECM I have software to look up these codes. When I get a chance later I will look up your BCC's and tell you what they are from and which ECM they belong to

 

AKWA comes back as belonging to a 1989 350 with a 3 speed automatic, using unleaded fuel and weighing more tham 10,000 lbs. It belongs in a 1228747 ECM

AMUN belong in a 1227747 ECM with a 305 motor with a 3 speed automatic in a 1988 1/2 or 3/4 ton van using unleaded fuel

There such a thing as a 1228747
1228747

Application:1989-90 truck 5.7 V8 TBI “K” L05
1991-92 truck 5.7 V8 TBI “K” L05 w/o 4L80E
1990 truck 7.4 V8 TBI “N” L19
1991 truck 7.4 V8 TBI “N” L19 w/ MT


1227747

Application:1987-91 truck 4.3 V6 TBI “Z” LB4 (exc. “S” & “T” body)
1990 truck 4.3 V6 TBI “Z” LB4 w/ MT (“S” & “T” body)
1990-91 truck 4.3 V6 TBI “B” LU2
1987-91 truck 5.0 V8 TBI “H” L03
1987-90 truck 5.7 V8 TBI “K” L05
1991-92 truck 5.7 V8 TBI “K” L05 w/o 4L80E</I>
1987-89 truck 7.4 V8 TBI “N” L19