1985 maf/maf power relay/ecm problem?
Thread Starter
Heel & Toe
Joined: Jan 2002
Posts: 19
Likes: 0
From: Centennial CO
1985 maf/maf power relay/ecm problem?
Code 33 was being set with a four-month old Bosch rebuilt maf, limp-home mode resulted. A brand new Bosch/tpis high flow maf (gutted) set code 33 after the two-second delay. A discussion with tpis yielded the opinion the ecm is registering too-high an airflow. This does not make sense to me since the engine pumps a set amount of air at a given rpm. So how can the ecm tell there is more flow at idle? In other words, how does the ecm know there is a high-flow maf installed.
tpis also opined that there can be nothing wrong with a brand new maf, especially if the hot-wires look to be intact.
Doing non-scan diagnostics per Chevy TPI Fuel Injection Swapper's Guide, requires a "disconnect maf power relay, probe circuit 340 and 120 with a test light to ground." I understand the maf power relay is in or near the ecm.
How do I identify the relay?
How do I test the relay?
Am I correct on the ecm not being able to "sense" the high flow maf?
What solutions have worked to this problem?
The engine is a zz4 with Miniram, headers, tpis cat back and high-flow cat.
tpis also opined that there can be nothing wrong with a brand new maf, especially if the hot-wires look to be intact.
Doing non-scan diagnostics per Chevy TPI Fuel Injection Swapper's Guide, requires a "disconnect maf power relay, probe circuit 340 and 120 with a test light to ground." I understand the maf power relay is in or near the ecm.
How do I identify the relay?
How do I test the relay?
Am I correct on the ecm not being able to "sense" the high flow maf?
What solutions have worked to this problem?
The engine is a zz4 with Miniram, headers, tpis cat back and high-flow cat.
Team Owner
Joined: Dec 2000
Posts: 90,675
Likes: 304
From: SE NY
Cruise-In II Veteran
Re: 1985 maf/maf power relay/ecm problem? (L.E. Dezman)
I believe that your MAF differs from later years in that it has the burn-off module external to it.
Anyway, look behind the battery and on the firewall you should see two relays. The inner relay (closer to the engine) is the MAF burn-off relay; the other (outer) is the MAF Power relay.
You can probe the 120 circuit at pin D of the MAF Power relay; the 340 circuit at A of the MAF power relay.
That ref also says that if the burn off voltage is applied to the MAF with the engine running the code 33 will be set. You can check this by looking into the end of the MAF with the engine running; if the sensor wires glow bright red (as they do for 2sec after you turn the engine off) then the burn off signal IS being applied to the MAF. In that case probe pin F of the MAF burn off relay to see if it is at ground potential.
So, clear the codes, pull the MAF burn-off relay and start the engine. If no code 33 is set within the usual time replace the MAF burn off relay and/or wring out the control wiring to that relay, in particular the control signal on the blk wire to pin F (this line should only be pulled to ground for 2sec after the engine is shut off).
Hope these hints/tips help.
Anyway, look behind the battery and on the firewall you should see two relays. The inner relay (closer to the engine) is the MAF burn-off relay; the other (outer) is the MAF Power relay.
You can probe the 120 circuit at pin D of the MAF Power relay; the 340 circuit at A of the MAF power relay.
That ref also says that if the burn off voltage is applied to the MAF with the engine running the code 33 will be set. You can check this by looking into the end of the MAF with the engine running; if the sensor wires glow bright red (as they do for 2sec after you turn the engine off) then the burn off signal IS being applied to the MAF. In that case probe pin F of the MAF burn off relay to see if it is at ground potential.
So, clear the codes, pull the MAF burn-off relay and start the engine. If no code 33 is set within the usual time replace the MAF burn off relay and/or wring out the control wiring to that relay, in particular the control signal on the blk wire to pin F (this line should only be pulled to ground for 2sec after the engine is shut off).
Hope these hints/tips help.
Instructor
Joined: Aug 2002
Posts: 179
Likes: 1
From: Oxford AL
Re: 1985 maf/maf power relay/ecm problem? (L.E. Dezman)
I spent the afternoon chasing down error codes on my recently purchased '86. Code 33, 34 and (I think) 36. Followed the procedures in the service manual, but all it came up to was replace the MAF. I happened to notice that the O2 voltage was locked on 1.1 volts, even with the sensor disconnected. After running wiring for a while, I finally gave up and replaced the ECM. Now, and all is fine except the car is still wrecked :cry
Thread Starter
Heel & Toe
Joined: Jan 2002
Posts: 19
Likes: 0
From: Centennial CO
Re: 1985 maf/maf power relay/ecm problem? (L.E. Dezman)
Thanks for the information.
I have heard the 85 has either the maf burn-off or power relay near the ecm. I have also heard one of the relays is no longer made. Comments?
65Z01, I did not understand your last paragraph.
I have heard the 85 has either the maf burn-off or power relay near the ecm. I have also heard one of the relays is no longer made. Comments?
65Z01, I did not understand your last paragraph.
Team Owner
Joined: Dec 2000
Posts: 90,675
Likes: 304
From: SE NY
Cruise-In II Veteran
Re: 1985 maf/maf power relay/ecm problem? (L.E. Dezman)
If the burn-off power is being applied constantly to the MAF the wires will glow red; with the burn-off relay pulled this cannot happen, so isolates the problem to that relay or it's circuits.
