924Board.org

Knz · Joined: 05 Mar 2014 Posts: 105 Location: Sparrow Bush, NY

Can anyone recommend a good (read not expensive lol) voltage regulator tool for testing relays? I'm trying to troubleshoot my fuel pump failure, but I don't want to blow a 100$ + relay, nor do I want to blow 100$+ on a testing tool since I don't do this regularly.

I was thinking of rigging something up with a 12v power adapter, but all the ones I have are either 1 or 2 amps. I've just been having trouble finding the input amperage needed for the relay.
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1980 931 (#185) for sale
1980 931 (#196) engine rebuild
1983 928S Project
1983 944 Summer Car
2002 BMW 525i Winter Car

jason c · Joined: 13 Jan 2014 Posts: 1018 Location: Nwi

You only need a multimeter (or test light in a pinch). Just check all the wires in the Connector for power & grounds. If you have supply power in (30), the trigger power and the ground but no power out to the pump, the relay is bad.

!tom · Posted: Sun May 04, 2014 4:06 am Post subject:

A relay is going to draw way less than 1 amp through its coil, so the power supplies you are looking at are more than sufficient.
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78 924 NA
5-lug

larchie · Joined: 19 Jun 2003 Posts: 297

If you have not already done so, check the fuel pump 16-amp fuse 2 on the additional fuse board under the dash.

Here's one way to test the circuit to the fuel pump. With the ignition off and the fuel pump relay removed, put a jumper wire with an in-line fuse to bridge terminals 87 (or sometimes labeled L13) and 30 (sometimes labeled L14). The fuel pump should run. So the jumper is taking place of the fuel relay; when the jumper is removed the fuel pump stops. If a working pump does not run then the fuel pump circuit must be tested. (More below.) To clean grounds: terminal 31 ground of Fuel Pump Relay is near the fuse/relay panel, and the fuel pump ground is near left side luggage rear light.

The jumper wire can be made from 16-gauge wire and an in-line fuse holder with a 16-amp fuse. I recommend making the jumper wire with a two pole toggle switch which will help prevent damage to the relay plate which can be caused by frequent insertion of the jumper wire.

Fused Jumper

Or if you can find the VW jumper tool US 4480/3, it's already built with a switch that after insertion in terminals 30 (from battery) and 87 (to pump) will switch on and off to control the fuel pump.

Zelenda Switched Jumper US 4480/3

You can get an inexpensive multimeter that will work for this purpose but the inexpensive digital meters are not really precise enough for other work, and I doubt that testing your fuel relay will be the only electrical tests you will do over your lifetime. Get the best multimeter you can afford -- here's some suggestions of recommended multimeters at different prices starting as low as US$10: Top 10 Best Multimeters 2014

To check the voltage at the relay plate with a multimeter, with the relay out, connect between terminal 30 (battery) and 31 (ground), and with the ignition on, you should see 12-volts.

Next, check voltage between terminal 15 (ignition coil) and terminal 31 (ground) and you should see 1 to 2-volts.

For further tests on the fuel pump relay itself, here's a thorough and helpful PDF troubleshooting procedure for a relay which also applies to the 924 fuel pump relay and incorporating specific electrical checks and provides more detail and better advice than my post: Troubleshooting a Nonfunctional Fuel Pump.

For the 924 fuel pump relay itself, if you want to test it, you should get resistances something like:

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Note:I don't think it is mentioned often enough that whenever electrical tests are done with the engine off but the ignition on, the positive wire from the alternator should be disconnected so that the alternator diodes are not damaged. Also if your model has breakerless ignition, disconnect the high tension cable from terminal 4 of the ignition coil and ground the cable so that the engine does not start if you run the ignition to prevent damage. Finally, all connections and disconnections in these procedures described above should be done with the ignition off.

Also, if the check is not done efficiently, to be on the safe side, it's best to pull the electric connections off the control pressure regulator and auxiliary air regulator if the engine has CIS, so that they do not heat up while testing.

Knz · Joined: 05 Mar 2014 Posts: 105 Location: Sparrow Bush, NY

Thanks far all the info larchie, I've had to put this all on the back burner while other stuff took precedence. I'm back at it now, got everything I need to test the relay.

I can't find 16 amp fuses anywhere tho! I checked the automotive shop, they only had 15s and 20s, and Radio shack was even less helpful. Any Ideas on where to go? Or am I just going to have to resort to ordering online?
_________________
1980 931 (#185) for sale
1980 931 (#196) engine rebuild
1983 928S Project
1983 944 Summer Car
2002 BMW 525i Winter Car

!tom · Posted: Thu May 29, 2014 12:38 pm Post subject:

Different styles of fuses come in different values.

A 15 amp fuse will be fine for your test.

The european style fuses come in 16 amp values, whereas blade type fuses come in 15 amp. You can rig up either one for this test.
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78 924 NA
5-lug

Knz · Joined: 05 Mar 2014 Posts: 105 Location: Sparrow Bush, NY

managed to find a 16 amp at advanced auto parts today. the pump checked out, it was the relay that was the problem.. just to find a replacement now, and I'll be back up and running!
_________________
1980 931 (#185) for sale
1980 931 (#196) engine rebuild
1983 928S Project
1983 944 Summer Car
2002 BMW 525i Winter Car

larchie · Joined: 19 Jun 2003 Posts: 297

Also worth considering for the glove-box is a small assortment of fuses or a German/European fuse kit for around $10 from ebay, amazon, or a Porsche, Audi, Mercedes or VW dealer. I think a variety of different Bussmann fuse assortments in plastic packages are available on the racks at most most auto supply stores -- at least they are in my local area.

Knz · Joined: 05 Mar 2014 Posts: 105 Location: Sparrow Bush, NY

yep, I got one of them at advanced the other day. SO now that I have a new used relay on the way, I've started thinking about what might have caused it to blow.

I
ve read that dieing fuel pumps will pull more amperage, blowing the relay again, Is there a way I can diagnose this? I'm thinking about possibly replacing the fuel pump, but my auto budget is non-existent atm since my DD sorely needs attention.
_________________
1980 931 (#185) for sale
1980 931 (#196) engine rebuild
1983 928S Project
1983 944 Summer Car
2002 BMW 525i Winter Car

larchie · Joined: 19 Jun 2003 Posts: 297

Generally as fuel pumps wear, you'll hear louder and louder buzzing. Sometimes when a fuel pump fails, a good hit will get it running again. Fuel relays often fail from sulfidation and oxidation of the silver contacts and/or fail from internal heat causing poor connections and can often be repaired by re-soldering. With ignition off, you can measure amperage by placing a bottle-fuse jumper between terminals 30 Battery and 87 Pump on the fuel relay board and put an auto-rated multimeter in series set to measure amperage. If your fuel delivery is within specification and the pump's sound is a normal soft buzz, the fuel pump is most likely O.K. If the fuel relay is relatively new, I don't think you need worry about it being damaged by increased amperage since the relay is protected by a fuse.

ggrecian · Joined: 20 Oct 2013 Posts: 29 Location: Pa

I am having a very similar problem and this post has been extremely helpful.i keep blowing dme relays...brand new one doesn't work for long before something goes wrong.the "jumper" method on Clarks garage works fine(I use the paperclip on the old relay).my fuel pump makes a very loud buzzing.extremely loud buzzing.so I hope this post is my trouble shooting answer! I received very little on rennslist and pelican parts forum.thank you much!!!!!!
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porschelover