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Old 03-12-2022, 06:32 PM   #6
cleanflametrap
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Join Date: Sep 2009
Location: near baltimore
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Originally Posted by dl242gt View Post
There is a section on cleanflametrap web site that tells you what voltages he measured on good mass air sensors. I'd get your voltmeter out and check them referencing that article.
Anything I've got in that regard is specific to LH2.0 and not applicable to anything Volvo put in a 940 turbo. Even so, the measurements are for the AMM at rest, which is only one data point on a long curve, good for a bet on whether to spend $35 or leave it at the pick'n'pull yards I used to visit 15-20 years ago. They didn't accept returns.

I've always said the only way you can diagnose the AMM is by substitution with a known good one. Good advice was to test and keep an AMM and an ECU for every type of Volvo you own, but that advice is useless after the fact.

Best thing to do is look for false air; vacuum leaks, poor electrical connection, maybe even fuel pressure trouble, and if that fails, find someone with the same engine and fuel system and do some swapping. The resistance measurements purporting to read the hot wire that are suggested in the literature are next to useless.

I just fixed my daughter's car using the rest voltage as a hint the AMM went south, but wasn't confident until swapping in a spare fixed it, and only certain when re-installing the suspect unit broke it again.

First, after checking fuel pressure, I checked the battery supply to the AMM. This is an orange wire on a 91 240. Key on engine stalled.



Next, I check the output voltage the AMM produces without any air flow. On a 240, this is a white/red wire. A gentle tweak of the corrugated AMM hose produces a fluctuation indicating the AMM is responsive. By experience, I've learned to distrust the -016 AMM if it shows much less than 1.4V at rest.



Next, I check the output voltage in our 89 245 just to remind me where it ought to be:



My spare stash is getting low on tested units. Swapped one in -- symptoms gone. Swapped the old one back, symptoms return. By the way, the symptoms on her 91 (380K miles) were it wouldn't idle and only ran at all with a lot of footplay on the accelerator.
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