Starting problems

But it wasn't the starter's fault...

I've had problems starting my car again. It had happened occasionally for a while: turn the key, fuel pump primed, turn another notch and all I got was a click. If I selected third gear and pushed the car a few centimeters then it would invariably start. But then... I got the click (luckily when the car was in the garage), and nothing would cajole it into starting.

I managed to get the starter out (I had to commit GBH against a spanner in order to get the top block-side nut undone), but it seemed to work fine when I gave it a direct 12V feed. Phil kindly offered to help the following evening so we took the starter apart, checking for dodgy connections. Everything looked fine so we lubricated it and put it back together. After an eventful refitting (which included a damaged connector on the lead to the starter, a blown fuel pump fuse, and being out of petrol) we got everything back together and the engine started. Curious... The only thing we did which might obviously have made a difference was the solenoid connector, and I really should have noticed that right at the start.

I was hopeful; the problem stayed away for a while, but then the dreaded click returned, again only occasionally. The wisdom on the Sevens list and Blatchat is that this problem is not down to the starter at all, but instead is due to a failed relay in the Multifunction Relay Unit (MFRU), situated under the ECU. The final straw was when it happened at the Bedford Autodrome track day. Luckily I had packed the trusty lead I'd made to hotwire the solenoid connector directly to 12V so I was able to get the car going. From this point it wouldn't start at all when warm unless I hotwired it.

I'd already bought the relay and other bits, but was thwarted by the current lack of a search feature on Blatchat [now reinstated]. I remembered that somebody had posted a recipe for fitting an external relay to bypass the MFRU, but I couldn't find it! Luckily Steve had the posting saved off, and offered to help with the fitting too.

This post by Chris W describes the problem. The relay modification referred to here is the one in Low Flying which adds an extra relay, rather than bypasses the one in the MFRU. I didn't want to do it that way because leaving a redundant component in circuit just seems to be asking for trouble later. I added the links.

OK guys - I believe I know what the real problem is and why the additional relay indeed solves the problem - I just didn't buy the high resistance wire theory so investigated further..........

.........are you sitting comfortably.....................

I opened up the MFRU unit (I've been like that since I was 5 years old - always taking things to pieces) and inside are 4 relays, whosewiring I traced. The relays are open types, ie: not enclosed in a casing, so you can see the mechanics of them clearly.

The 4 relays function as follows:

  1. Controlled by ECU
  2. Puts 12v on fuel pump if ignition and ECU are active
  3. not used
  4. Puts 12v on starter solenoid if starter button pushed and ECU active

So the red/white wire from the starter button applies 12v to the coil of relay no. 4 in the MFRU which closes and switches 12v on to the starter solenoid via the thickish brown/red wire coming out of the MFRU.

The point is that this relay will be switching about 10 amps and when it opens after you release the starter button there will be a nice juicy spark which will corrode the terminals. There is no spark suppression circuitry in the MFRU.

The other relays (ECU and fuel pump)are handling much smaller currents and so will be far less affected compared to this one. My belief is that this relay is probably not man-enough for the job of switching 10 Amps and the contacts inside are getting burnt causing a high resistance. My SV is only about 7 weeks old but there is already evidence of some corrosion on the contacts of this relay and not the others. Hence starting problems.

So by introducing the additional relay (30 amp type) the 10 Amp current for the solenoid is now handled by the new relay and the relay in the MFRU is only handling the current required to switch the 30 Amp relay (probably about 150milliamps). Therefore, no more sparking inside the MFRU and the additional relay's contacts are running well under maximum current, so no problem there either as the contacts are built to take up to 30 amps.

I'm not sure yet why it should be worse when hot but it's possible that the resistance of the solenoid decreases when hot and so it draws even more current making matters worse. I'll think about that a bit more. However, I can now understand why the new relay solves the problem and will be fitting one myself!!!!

Here's the account of how to do the relay mod properly (again by Chris W, and completely bypassing the MFRU for the starter).

The only issue I have is that this method of wiring in the additional relay still makes use of the MFRU relay which is not needed and causes the issue with the solenoid in the first place. Granted the MFRU relay is now handling only a tiny current but it is another unnecessary point of failure.

As the search facility is down, I will write it out again, but my preferred method is as follows: (this also does NOT need an in-line fuse) as it still utilises the ECU fuse 14, 30A which is there to protect the solenoid wiring anyway. Disconnect the negative of the battery while wiring.

  1. Leave the brown/red wire IN PLACE on the solenoid, but cut it off a couple of inches from the large connector on the MFRU or pull it out of the MFRU connector if you can. (if you cut it, tape up the end still connected to the MFRU). This takes the MFRU relay out of circuit.
  2. Connect the other (long) end of the brown/red, which you have just cut, to the new relay on terminal 87. Note there may be an "87" and an "87a" terminal on your relay. It is critical to use terminal 87. ie: do NOT use 87a.
  3. Connect terminal 30 of the new relay to any of the 3 thickish brown wires which come out of the large connector on the MFRU. Use of a scotchlok (tap) connector is the easiest as you don't have to cut anything. If you're not familiar with these, they are used to join two wires together without having to cut either wire. Available from any motoring accessory place or Maplin or VWP etc. - few pence each.
  4. Connect terminal 85 of the new relay to ground.
  5. Connect terminal 86 of the new relay to the red/white wire which enters the small connector on the MFRU. Again use a scotchlok connector to save having to cut into the wire.

(Although I have specified the terminal numbers above, in fact terminals 30 and 87 on the relay are interchangeable with each other - doesn't matter in this application which way round you wire them. The same applies to terminals 85 & 86 - they are interchangeable with each other).

Reconnect the negative and away you go.

And here's some extra info from Sid Spalding, who did the mod.

OK just to let you know how I got on. All was very straightforward as described with the following small exceptions: -

  1. On Step 3 there are actually four brown wires (well brown and some colour anyway) coming out of the MFRU. I choose one of two of the thickest, as described, which happen to have the same colour code on them.
  2. I positioned the relay just under the ECU which keeps all the cable lengths nice and short.
  3. I wasn't too certain on the scotchloks so I only used them on the low amp wire (red and white Step 5) and God thier brilliant.
  4. I used the one of the bolts on the bulkhead for ground (Step 4). Is this OK ? Not sure if it's sufficent ground as it's Ali.

So the result was that it works and seems to give the starter motor more power - i.e. it starts straight away rather than after 3 or 3 goes. Took it out on Saturday in very hot weather, stopped at the petrol station and it started no problem. Result !

I have had no failures to start since fitting the relay despite the uncommonly hot weather. Excellent! Thanks very much to Steve and Phil, and the currently nameless soul who posted the HOWTO to Blatchat.

14 August 2003: I was telling Phil, at the Club meeting last night, how well my car started now. "Come and see!", I said. "Click," said the solenoid. Bugger.

4 July 2004: A new battery seems to have cured the problems.



This page is http://www.strangely.org/diary/200308/starting.html. It was first published on Friday 8 August, 2003 and last updated on Thursday 14 October, 2004.