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Old 08-05-2003, 08:23 AM
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Join Date: Apr 2000
Location: Falls Church, VA
Posts: 5,316
Fuel Pump Relay Testing - Early 107's

On Sunday, the Wayward Woman ('81 380SL) turned over but refused to start. Very politely, in the garage. No noise from fuel pump. I jumped pins 1 & 2, and the pump ran, which would indicate a bad relay, but I decided to run through the tests in the manual to be sure. I thought I would write them up, since they are fairly straighforward, and there are conditions that could lead to replacing a relay when a wire or the TDI (ignition unit) is bad.

Note that this test is for cars with the older six-pin relay. The tests for the newer twelve-pin are different.

Remove relay.

1. Ignition off. Pin 2 - ground: 12V
2. Ignition on. Pin 3 - ground: 12V
3. Ignition off. Pin 2 - Pin 5 (ground): 12V
4. Check for RPM signal at Pin 4 with starter operating. I ran this test with my twenty-year-old Sears analog dwell/RPM tester, connecting the lead that would normally go to "-" on the coil to Pin 4.

Insert relay partially.

5. Check for voltage on the partially-inserted Pin 6 with the starter operating.

Corrective action for steps 1-3 is fix open circuit. Step 4 is to check for a bad connection from the TDI, and if none, replace the TDI (gulp!). Step 5 is to replace the relay.
Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe
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Old 01-23-2006, 05:23 PM
whunter's Avatar
Join Date: Dec 2003
Location: Metro Detroit, Michigan
Posts: 17,353
Thumbs up Thank you

Thank you.
Where is the relay?
A picture of location would be very nice.

This has been added to: ShopForum > Technical Information and Support > Do It Yourself Resources
Fuel Delivery:

ASE Master Mechanic

Prototype R&D/testing:
Thermal & Aerodynamic System Engineering (TASE) Senior vehicle instrumentation technician.
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Last edited by whunter; 01-23-2006 at 05:29 PM.
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