Radio Suppression Relay
#21
04-04-2016, 12:18 AM
Just a recap in case some poor guys are going through the same drama …
Progressive symptoms of “Radio” relay
1. Loss of power going up long steep hills on freeway doing speed (more load on relay because pulses wider)
2. Long time to start
3. Sometimes won’t start
4. Once started sudden loss of power
5. Won’t start at all
6. Start engine with ‘startyoubastard’ but is unreliable once going
7. Hot-wiring cold start injector with dashboard push button keeps car limping along for a while but does not fix problem
8. Car dead
Oscilloscope trace on the injectors shows pulse, but you have to look closely to see the positive rail dipping slightly with each pulse. A two-channel scope on the two injector wires (green – positive rail, grey – from ECU) with shield to chassis explains what’s happening (see attachment).
Scope shows that the Radio Interference Suppression Relay (Volvo 899931 1323592-1, 12V 40A, located on some 1990’s Volvo 740’s on the water reservoir bracket) needs replacing. Super moderator Pierce has a list of where to get ahold of these enfants terrible (see above). And don’t ask about the name of the thing – it’s the elephant in the room
Progressive symptoms of “Radio” relay
1. Loss of power going up long steep hills on freeway doing speed (more load on relay because pulses wider)
2. Long time to start
3. Sometimes won’t start
4. Once started sudden loss of power
5. Won’t start at all
6. Start engine with ‘startyoubastard’ but is unreliable once going
7. Hot-wiring cold start injector with dashboard push button keeps car limping along for a while but does not fix problem
8. Car dead
Oscilloscope trace on the injectors shows pulse, but you have to look closely to see the positive rail dipping slightly with each pulse. A two-channel scope on the two injector wires (green – positive rail, grey – from ECU) with shield to chassis explains what’s happening (see attachment).
Scope shows that the Radio Interference Suppression Relay (Volvo 899931 1323592-1, 12V 40A, located on some 1990’s Volvo 740’s on the water reservoir bracket) needs replacing. Super moderator Pierce has a list of where to get ahold of these enfants terrible (see above). And don’t ask about the name of the thing – it’s the elephant in the room
#22
04-04-2016, 12:59 AM
note that many of those symptoms can happen from a variety of causes. I have had loss of power at speed and heavy throttle due to fuel starvation. hard starting can be lots of things, too.
dead car can be lots of things too, from the crank position sensor to the fuel pumps or fuel pump relay, to ignition problems, to a broken timing belt, to probably 47 things I'm not thinking of.
dead car can be lots of things too, from the crank position sensor to the fuel pumps or fuel pump relay, to ignition problems, to a broken timing belt, to probably 47 things I'm not thinking of.
#23
04-05-2016, 03:33 AM
#24
04-05-2016, 06:18 AM
I wouldn't go about using compressed air to "blow out" the fuel system. Instead I would try to systematically check out the fuel system. You seem to be focused on the radio suppression relay where the symptoms really look like a fuel delivery issue. Loss of power under load many times can be traced to a faulty in-tank fuel pump if your car has the Bosh LH 2.4 FI. If your car has the Rex Regina FI, then there is a single in-tank pump.
Second item to check would be the fuel pressure at the fuel rail. Verify you have the proper fuel pressure. You can borrow a fuel pressure tester at your local autozone.
Second item to check would be the fuel pressure at the fuel rail. Verify you have the proper fuel pressure. You can borrow a fuel pressure tester at your local autozone.
#25
04-05-2016, 12:10 PM
#26
05-07-2017, 07:26 PM
In the last few months on separate occasions(conveniently) I have replaced crank sensor, ignition control module, coolant temp sensor. Thought I was on top of it but still had occasional sputter and rare stalling. Was then thinking it could be just injector related, dumped some lucas in and was running like a dream on the freeway.
-Fuel filter was also replaced about a year ago.
All was well until last night on my birthday, after driving it all day it stalled a mile from my house.
Will be biking to the parts pickup for hopefully the last time in the morning for this part.
Seeing that I have owned the car 2.5 years and not replaced this, I guess I could call myself lucky in this case.
-93 940 233xxx Wagon
-I already had ordered this but wasnt able to pick it up before the place closed. Autohausaz has these bad boys for a good price if anyone is in need. (KAE Relay)
-Fuel filter was also replaced about a year ago.
All was well until last night on my birthday, after driving it all day it stalled a mile from my house.
Will be biking to the parts pickup for hopefully the last time in the morning for this part.
Seeing that I have owned the car 2.5 years and not replaced this, I guess I could call myself lucky in this case.
-93 940 233xxx Wagon
-I already had ordered this but wasnt able to pick it up before the place closed. Autohausaz has these bad boys for a good price if anyone is in need. (KAE Relay)
#27
05-07-2017, 07:32 PM
#28
06-19-2021, 09:48 PM
Radio Interference Suppression Relay – that’s got to be one of the most misleading descriptions ever! It’s also perfect for planned obsolescence – the car’s symptoms of intermittent fault, no starting, sudden loss of power and general unreliability are often put down to the car being old and obsolete, so it’s easy to think it’s time for a new car. It should be called the Fuel Injector relay – but you won’t find that description on any circuit diagram.
OK, the re-location of that relay away from the radio (where they apparently had problems with it) eliminates or minimises radio interference, but the relay itself doesn’t and can’t suppress radio interference, so it’s a total misnomer. Radio interference apparently was caused in the first place (see elsewhere in this thread) by moving all the relays in the 1990’s Volvo 740 directly under the radio behind the ashtray at the back of the fuse box where nobody can get at them. The wires carrying the injector pulse return load radiated interference into the AM. Radio interference suppression is not the relay’s function and does not describe what it does. The relay’s only function is to keep the engine going. It supplies the power to the fuel injectors. Take that away and you’ve got no car!
Apparently these fail regularly. Looking at the guts of the thing (see photo) no wonder! This relay, (Volvo 899931 1323592-1, 12V 40A) stuck to the water reservoir bracket on the right front on some 740’s, doesn’t look anything like a 40A relay! The contacts can’t even handle 3A on a regular basis!
The relay powers the + rail of the injector array (green wire on the injectors which are wired in parallel). They get activated by a minus pulse on the grey wire from the ECU. You could bare one of the green injector wires (roll the rubber back a bit) and stick a meter on it when trying to start the car. It should read about 12V.
To check the radio interference suppression relay (geez – I love that description), unplug it and measure the voltage of the yellow wire with green stripe that goes into the plug. It should read +12 V with the ignition on. Then plug the relay back in. If you don’t get 12V on the green injector wire with ignition on check the relay for defects.
It’s possible to fix these by carefully hacksawing the cover off near the base (see photo) and cleaning the point with abrasive paper. Mine read anything from 3 ohms to 50 ohms between the points with the relay on - the perfect intermittent fault. That went down to 0 ohm after cleaning.
Attachment 12881
Attachment 12882
Attachment 12883
OK, the re-location of that relay away from the radio (where they apparently had problems with it) eliminates or minimises radio interference, but the relay itself doesn’t and can’t suppress radio interference, so it’s a total misnomer. Radio interference apparently was caused in the first place (see elsewhere in this thread) by moving all the relays in the 1990’s Volvo 740 directly under the radio behind the ashtray at the back of the fuse box where nobody can get at them. The wires carrying the injector pulse return load radiated interference into the AM. Radio interference suppression is not the relay’s function and does not describe what it does. The relay’s only function is to keep the engine going. It supplies the power to the fuel injectors. Take that away and you’ve got no car!
Apparently these fail regularly. Looking at the guts of the thing (see photo) no wonder! This relay, (Volvo 899931 1323592-1, 12V 40A) stuck to the water reservoir bracket on the right front on some 740’s, doesn’t look anything like a 40A relay! The contacts can’t even handle 3A on a regular basis!
The relay powers the + rail of the injector array (green wire on the injectors which are wired in parallel). They get activated by a minus pulse on the grey wire from the ECU. You could bare one of the green injector wires (roll the rubber back a bit) and stick a meter on it when trying to start the car. It should read about 12V.
To check the radio interference suppression relay (geez – I love that description), unplug it and measure the voltage of the yellow wire with green stripe that goes into the plug. It should read +12 V with the ignition on. Then plug the relay back in. If you don’t get 12V on the green injector wire with ignition on check the relay for defects.
It’s possible to fix these by carefully hacksawing the cover off near the base (see photo) and cleaning the point with abrasive paper. Mine read anything from 3 ohms to 50 ohms between the points with the relay on - the perfect intermittent fault. That went down to 0 ohm after cleaning.
Attachment 12881
Attachment 12882
Attachment 12883
Old topic, but future reference for others searching as I did today.
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